Symposium EP04 : Novel Photonic and Plasmonic Materials Enabling New Functionalities

Flat optical devices based on lithographically patterned sub-wavelength dielectric nano-structures provide precise control of optical wavefronts, and thus promise to revolutionize the field of free-space optics. I discuss our work on high contrast transmitarrays and reflectarrays composed of high index nano-posts located on top of low index substrates like silica glass or transparent polymers. Complete control of both phase and polarization is achieved at the level of single nano-post, which enables shaping of the optical wavefront with sub-wavelength spatial resolution. Using this nano-post platform, we demonstrate lenses, waveplates, polarizers, arbitrary beam splitters and holograms. Devices that provide multiple functionalities, like simultaneous polarization beam splitting and focusing are implemented. By embedding the metasurfaces in flexible substrates, conformal optical devices that decouple the geometrical shape and optical function are shown. Multiple flat optical elements are integrated in optical systems such as planar retro-reflectors and Fourier lens systems with applications in ultra-compact imaging systems. Applications in spectrometry and various types of microscopy are discussed.

Circularly polarized light (CPL) exhibits enantioselective interaction with chiral molecules, providing a pathway toward all-optical chiral resolution. Compared to existing chemical schemes like chiral chromatography, this all-optical approach could be rapid, economical, and readily adaptable to a wide variety of chiral molecules. CPL alone has been shown to achieve enantiomeric excesses only up to 2% while maintaining yield higher than 40%; however, high index dielectric nanoparticles can increase separation efficiencies. Such nanoparticles support both electric and magnetic Mie resonances, which locally increase the electromagnetic density of chirality, C, as well as the rate of differential absorption between enantiomers, g. Here, we investigate Si metasurfaces for all-optical chiral resolution. We consider a metasurface consisting of nanoscale cylinders and optimize C and g by varying the cylinder aspect ratio. First, using finite-difference time domain simulations, we simulate a square array of Si cylinders that are 100 nm high with a lattice parameter of 1000 nm. As we vary the radius of the disk from 170 to 360 nm, the electric dipolar mode redshifts relative to the magnetic mode, providing the opportunity to independently tune the phase of the electric and magnetic dipoles. Accordingly, we can achieve both electric and magnetic field enhancement as well as perfectly circular near-fields (a phase lag of pi/2 between electric and magnetic field), the ideal condition for enantioselective light absorption. Using this method, we demonstrate that a disk with radius 280 nm gives a 4-fold average enhancement in enantioselective absorption (g) at lambda=1363 nm. Importantly, this enhancement in g occurs over the entire cylindrical surface. Local enhancements up to 11x in g and 130x in C are also observed. Using the molecule camphor as an example, we show how this enhancement can give rise to 20% separation efficiencies with 40% yield over a large volume region. Moving to experiment, we fabricate metasurfaces using electron-beam lithography and disperse the chiral molecule helicene-diketoppyrrolopyrrolle in the near-field. This molecule shows strong, circularly dependent absorption at 592 nm and photoluminescence at 610 nm. We show how photoluminescence of this molecule tracks with our calculated enhancements in g. Our results pave the way for full chiral resolution based on absorption dependent photoionization or photolysis.

The development of scattering arrays of dielectric nanoparticles that can efficiently and directionally extract partially coherent radiation from high refractive index active materials poses significant challenges to traditional photonic approaches. In particular, the directional extraction of incoherent Lambertian radiation from dielectric interfaces, which is important to emerging lighting device applications, has not been systematically addressed so far. In our talk we experimentally address this relevant technological and scientific problem under realistic testing conditions by systematically studying directional emission enhancement from aperiodic Vogel arrays of dielectric nanopillars atop light emitting LED materials. We prepared luminescent materials with their surfaces functionalized by using transparent oxide nanopillars arranged in the isotropically scattering golden angle Vogel spiral geometry and experimentally quantified the enhanced light extraction and directionality via Fourier-space photoluminescence imaging and microscopy. A variety of patterns were fabricated with varying nanopillar diameter and mean particle separations. The samples were excited with a 405 nm pump laser and directional enhancement was experimentally demonstrated comparing Fourier-space images of patterned and unpatterned regions of the samples. A total extraction enhancement of up to x2 times can be obtained with an angular directionality of ±30°. Our data have been compared with both numerical (FDTD) and analytical modeling of kinematic light scattering for incoherent radiation. Finally, based on our efficient analytical model we will also discuss general engineering design rules that enable directional extraction and enhancement using aperiodic scattering arrays with isotropic scattering.

Light Detection and Ranging (LIDAR) has attracted significant interest for autonomous navigation, imaging, and sensing. In particular, there is demand for compact, non-mechanically steered components with low size, weight, and power consumption requirements. Current leading LIDAR solutions use one-dimensional (1D) or two-dimensional (2D) phased-array antennas to steer a coherent beam bi-directionally. Interestingly, the RF Radar literature contains lens-based beam steering solutions (e.g. Rotman lenses) that overcome major challenges found in phased arrays. Their photonic analogs have not yet been investigated and are a viable solution for optical beam steering. Herein, we demonstrate the first planar-lens-based beam steering device that functions at λ = 1550 ± 50 nm and is fabricated with all-dielectric materials. The planar lens is fabricated with 25 nm of amorphous silicon on top of 195 nm of silicon nitride that allows for the design of a lens with a wide field of view in-the-plane of the lens. A grating made with the same dielectric stack allows for out-of-plane coupling in the far-field. We demonstrate this 2D beam steering through theory, simulations, and experimentally fabricated photonic components. The total angular range of this lens-based approach is an azimuthal range of φ = 41.0° and polar range of θ = 12.0°. In addition, we suggest new lens designs with subwavelength features that could enable an even larger field-of-view. The materials and designed components are fully compatible with wafer-scale fabrication and should allow for optical beams with high output powers. Overall, this planar-lens-based approach opens a path towards chip-scale beam steering at low size, weight, power consumption, and cost.

We have investigated graphene-gold metasurfaces to enhance light-graphene interaction in the MIR region and additionally, we demonstrate a new class of electrically controlled digital metadevices.
In the first part, we will experimentally demonstrate that it is possible to increase and tune the optical transmission response of a graphene-based device substantially by applying less gate voltage compared to the back-gating methods via ionic liquid gating and nanoplasmonic antennas in the same device. Designing and utilizing V-shaped plasmonic structures enabled us to increase the interaction between the graphene layer and the incident field in the mid-infrared wavelengths. In this work, we have decided to use V-shaped plasmonic antennas. Among many candidates of plasmonic structures, “V”-shaped plasmonic nanoantennas have also proven useful in many applications including energy localization in nanosystems, unidirectional side scattering, and even sub-wavelength scale devices that can create abrupt phase changes and allows complete beam shaping. By using an ionic gating scheme, we are able to induce high electric fields near graphene to efficiently tune graphene's Fermi level and control its carrier concentration, therefore shifting the transmission response of nanoantennas and the response of the device.
In the second part, we will focus on the effect of the graphene supercapacitor. Graphene supercapacitor has been studied by different research groups for promising capacitor properties. In this work, we focused on its tuning effect. To understand the effect of the separation between graphene capacitor and metamaterial arrays on the performance of our metadevice, we measured near-field transmission spectrum in the forward direction at various separation values while sweeping the bias voltage between ±1.5 V. The fabricated active metadevices enable efficient control of both amplitude (>50 dB) and phase (>90°) of electromagnetic waves. In this hybrid system, graphene operates as a tunable Drude metal that controls the radiation of the passive metamaterials. Furthermore, by integrating individually addressable arrays of meta-devices, we demonstrate a new class of spatially varying digital metasurfaces where the local dielectric constant can
be reconfigured with applied bias voltages.

Rectification of GHz signals by diodes coupled to a surface of light-intensifying antennas has demonstrated optical-to-electrical (d.c.) power conversion efficiencies near 80%, an impressive achievement potentially useful for direct power beaming to charge batteries. In the infrared (IR) and visible, rectification occurs via quantum tunneling and/or hot-electron transport coupled to optically rectifying antennas, but IR rectennas are very challenging to investigate. Novel device concepts, including new metasurfaces, materials, and nanofabrication, must be employed. Because rectennas have absorption tuned by metamaterial dimensions, instead of by a band gap, they capable of light-harvesting and detecting broadband IR.

We designed microantenna arrays, consisting of plasmonic stripes that produce metamaterial and plasmonic resonances with near-perfect absorption [2], with teeth that induce a net asymmetric electric field driving electrons into the ground plane. The microrectenna is formed by these field-intensifying teeth and a vertical Metal-Insulator-Metal (MIM) diode (the metal-NbO_x-Nb). The barrier layer NbOx, with thicknesses 5 – 30 nm, was ALD-deposited on Nb. Au stripes and symmetry-breaking stripe-teeth arrays were patterned atop the NbO_x/Nb base layer with Ti and other adhesion layers. Ti adhesion layers did not provide sufficient asymmetry for rectifying diodes. Pt without an adhesion layer resulted in significantly asymmetric I-V curves as reported earlier [3], but required advanced patterning techniques since features are very small (hundreds of nm) and standard liftoff techniques don’t work well without good adhesion layers. We explore both front-side physical etching and forming the microrectenna arrays upside down on a Si handle wafer. In the latter case, IR ilumination would pass through the wafer, shifting resonances such as the stripe arrays, making data analysis more challenging and requiring multiple excitation wavelengths. Nevertheless, we obtained good agreement between calculated and observed antenna resonances, large MIM diode asymmetry well-described by our advanced model of MIM diode conduction and rectification [4]. The short-wave IR reflectivity, polarized along the teeth, exhibits multiple peaks, whose positions depend on the separation between the teeth. Au stripes were also fabricated directly on the substrate using unique, “single-shot” electron-beam patterning. We also analyze Al/Al₂O₃/Au microrectenna arrays and compare the resulting I-V curves under laser illumination with simulations. We compare the NbO_x-based diodes, coupled to the Pt microantenna array, under SWIR illumination with the model, and predict the rectified signal, measured using a lock-in amplifier.
.
[1] McSpadden, J. O., et. al., IEEE Trans. Micro. Theor Tech 46 (1998) 2053.
[2] Wu, C. et. al., Phys. Rev. B 84 (2011) 075102.
[3] Chin, M. L., et. al., J. Vac. Sci. Tech B 31 (2013) 051204.
[4] Osgood III, R. M., et. al., J. Vac. Sci. Tech 34 (2016) 51514.

Ultrafast optical science and technology depend on optical pulse shaping, which compliments pulse generation and characterization methods. Example applications for pulse shaping include pulse compression, dispersion compensation for fiber optic communications, coherent laser control of quantum mechanical processes, and spectrally selective nonlinear microscopy. Fourier-transform pulse shaping has emerged as the most successful and widely adopted technique, in which shaping is achieved by parallel modulation of spatially separated frequencies without requiring an ultrafast modulator. Here, we offer the first experimental demonstration of femtosecond pulse shaping using a centimeter-scale silicon metasurface acting as both amplitude and phase modulation mask. The deep-subwavelength silicon nanostructures, positioned with nanometer precision, provide accurate amplitude and phase modulations to each frequency component. Masks of this type offer a lower cost, larger size, higher resolution, high diffraction efficiency, high damage threshold method for controlling ultrafast pulses.
The experimental implementation is based on a Fourier-transform pulse shaping apparatus. A femtosecond optical pulse from a Ti:Sapphire oscillator covering the spectral range from 725 nm to 925 nm is angularly dispersed by the first grating and then focused by an off-axis parabolic mirror. A dielectric metasurface mask positioned at its focal plane can introduce amplitude and phase modulation to each spectral component. Our metasurface masks are composed of an array of polycrystalline silicon pillars resting on a fused-silica substrate. The rectangular pillars that form the metasurface are placed in a two-dimensional array. Each column of pillars along the y-axis introduces a designed phase to a narrow spectral range around a target wavelength. The cross-section of each pillar is chosen via an optimization algorithm to achieve the designed spectral phase shift while maintaining a large transmittance. The pillars provide a phase depth of 2π radians such that phase wrapping is necessary to impart the given phase function. As an example, we design a metasurface which imparts a pure quadratic phase (as a function of frequency). This design would generate enough dispersion to stretch a pulse from 15 fs to 40 fs. The measured spectral phase, group delay, and pulse shape applied by a representative, fabricated device match well with expected values. We have also achieved independent amplitude (through polarization) and phase shaping, as well as dynamic control, using metasurfaces.
In summary, we have designed, fabricated and tested an optical pulse shaper that uses a metasurface for spectral and amplitude phase modulation. The high precision with which metasurfaces can control polarization, amplitude, and phase point toward new, previously unrealizable applications in optical pulse shaping.

Current OLED displays rely on a circularly polarised (CP) filter to enhance contrast by trapping ambient light inside the display. However, this means that 50% of the randomly polarised light emitted from each OLED pixel never leaves the screen, halving display efficiency and operational lifetime. One way to overcome this is to create CP light emitting OLEDs (CP-OLEDs). A potential route to fabricate such devices is to induce chirality in a normally achiral electroluminescent polymer by blending it with a chiral small molecule. Our approach uses polyfluorene copolymers blended with an intrinsically chiral helicene dopant, which allows for CP-dependent applications while retaining much of the performance properties of the original polymer. Previously circularly polarised polymer emission has been achieved via thick cholesteric stacks of liquid crystalline polymers, where linearly polarised light becomes circularly polarised. To date, people have focused on the high film thickness regime to investigate the cholesteric stacking inside the active layer and the properties of the liquid crystal structure. Here, for the first time, we show that it is possible via film thickness to control the competitive effect between cholesteric packing and local chiral emission, the latter originating from an induced asymmetry in the polymer electronic and magnetic transition dipole moments. Remarkably, we observe handedness inversion in a system containing only one sign of chiral dopant. We compare how the chemical structure of the non-chiral polymer and post-deposition thermal processing impacts the chiroptical response of the resulting thin films and OLED devices in an effort to provide a set of design rules for future high-performance CP-OLEDs. We demonstrate a liquid-crystalline light emitting polymer with a record high induced circular dichroism. The circularly polarised photoluminescence and electroluminescence both show thickness-dependent CP emission and handedness inversion. In our system, the energy level alignment between dopant and host polymer ensures no charge trapping effects. As a result, the CP-OLED shows minimal change in the device performance compared to the original undoped OLED in terms of turn-on voltage and brightness. This provides new design rules for creating efficient circularly polarised light emitting diodes with high brightness and a high electroluminescent dissymmetry.

Inorganic materials with finite absorption bandwidths could find uses in many optoelectronic applications such as wavelength-selective photodetection, optical imaging, target recognition, chemical detection and solar energy harvesting. They would be of particular interest for multijunction photovoltaics to facilitate current-matching between different cells, by, for example, enabling one of the cells to absorb only in the infrared while maintaining transparency in the visible regime. However, in contrast to molecular materials that exhibit finite absorption bandwidths in the ultraviolet or visible wavelength ranges, inorganic semiconductors typically strongly absorb at all energies larger than their bandgaps.

Here, we introduce a new set of materials based on photonic band structure engineering in strongly absorbing media to demonstrate inorganic semiconductors with finite absorption bandwidths. Photonic crystals (PCs), materials with periodic variations in their refractive indices, are used to manipulate light flow in a wide range of dielectric structures; however, the extent to which features such as photonic bandgaps can be supported in absorbing materials is still an open question. We studied this question in order to use PCs to artificially induce spectral transparency windows in the visible range in infrared-responsive materials.

To study the effect of absorption on photonic band structure in periodically structured materials, we used the finite-difference time-domain method to simulate the well-studied 2D GaAs slab PC structure, which consists of a triangular lattice of air pillars in a semiconductor membrane, as a test-case. We gradually increased the value of the imaginary part of the refractive index while keeping the real part of the refractive index constant in our hypothetical material to study how adding absorption affects the band structure. We quantified how, as absorption in the medium increases, the number of photonic bands decreases, and the magnitude of the remaining bands and contrast in the photonic bandgap diminishes. However, the band structure is partially preserved even in the high-absorption case, with the low frequency photonic bands and associated bandgaps retaining approximately 40% of their magnitude when compared to the non-absorbing case.

Experimentally, we demonstrated spectral tuning via photonic band structure engineering in a strongly absorbing infrared-responsive thin film composed of PbS colloidal quantum dots (CQDs). Using nanosphere lithography to create the PC structure, our photonic material had both twice the transmittance at 400 nm and twice the absorption at 960 nm compared to a planar CQD film. Employing photonic engineering to tune the spectral responsivity in strongly absorbing media, as demonstrated in this study, could provide a viable pathway for optoelectronic applications requiring finite absorption bandwidths.

Dramatic advancements in areas such as electrical modulation, signal processing, and nonlinear frequency conversion have revitalized initiatives to develop more efficient nonlinear optical materials. In response, epsilon near zero (ENZ) media have emerged as a promising novel platform for nonlinear optical enhancement. However, ENZ materials operating in the near- and mid-infrared spectral ranges rely on metallic components with narrow optical permittivity tunability and high extinction losses. Likewise, they may require nano-manufacturing in three dimensions, which can severely limit device implementation and increase manufacturing costs. Heavily doped semiconductors are a common alternative. In particular, the transparent conductive oxide, Indium Tin Oxide (ITO) has been demonstrated to have a tunable ENZ condition in near-infrared spectral range. Using co-deposited RF magnetron sputtering, we have developed an alternative material to ITO thin films, Indium Silicon Oxide (ISO). Upon post-deposition annealing this thin film material is found to extend the tunable ENZ condition into the mid-infrared with lower extinction losses. Moreover, the material is fully Si compatible, and has excellent surface smoothness properties that promoted ease of scalability and device nanofabrication. We perform TEM and XRD measurements in order to correlate the structural and optical properties of this new material. In addition, we measure this material’s optical bandgaps and resistivity to determine and modulate charge carrier concentration. Our work significantly diversifies and expands the reach of ENZ medium and nonlinear optical materials in the infrared spectral range.

Unusually large nonlinear optical properties of conducting oxide and metallic nitride materials in their epsilon-near-zero (ENZ) region (i.e. the region where the real part of their dielectric permittivity approaches zero), have been reported recently [1-4]. Processes involving nonlinear frequency conversion can be benefited from these ENZ regions due to relaxed phase-matching criteria. Moreover, nonlinear optical processes which do not require phase-matching, such as nonlinear refraction and nonlinear absorption, can also be largely enhanced in the ENZ region. This giant enhancement can be attributed to the hot free-carrier dynamics resulting from ultrafast laser assisted heating. Since the response time of such nonlinear processes can be ~100 fs, these ENZ materials could open distinct functionalities to the path to revolutionary nanoscale nonlinear optics and ultrafast on-chip optical applications.

In this work, we report an efficient method to engineer the nonlinear refraction coefficients (n₂) and the nonlinear absorption coefficients (β₂) of Al-doped zinc oxide (AZO) ENZ thin films. The AZO films with ENZ wavelengths ranging between 1500-1650 nm are synthesized on fused silica substrates by the atomic layer deposition (ALD) technique [5]. The properties of the ALD AZO films such as film thicknesses and ENZ wavelengths can be precisely controlled by changing the deposition conditions (e.g., number of deposition cycles and doping level of Al). Nonlinear optical properties of the fabricated films are measured using Z-scan technique using an ultrafast laser peaked at wavelength λ_p=1550 nm. Measured n₂ and β₂ values of the films are evaluated upon fitting the experimental closed-aperture and open-aperture data respectively. We also observed that the nonlinear refraction and the nonlinear absorption strengths can be engineered by controlling the ZnO to Al dopant layer ratios in the deposited AZO films even if they have similar thicknesses. The tunability of the ENZ nonlinearity can be attributed to the efficient control of free-carrier densities. Also, we experimentally demonstrate enhancement of the effective nonlinear properties of the films due to excitation of ENZ modes at oblique incidence while illuminating with TM polarized light. Measured values as large as n₂≈10^-10 cm²/Watt and β₂≈-10^-6 cm/Watt are obtained with a 216 nm thick ALD-fabricated AZO film for TM polarized light incidence at 60^O. The results of this work provide an understanding about engineering nonlinear optical properties of AZO ENZ materials for nanophotonic applications.


1) M. Z. Alam et al., Science 352, 795–797 (2016).
2) L. Caspani et al., Phys. Rev. Lett. 116, 233901, (2016).
3) A. Capretti et al., Opt. Lett. 40, 1500–1503 (2015).
4) A. Capretti et al., ACS Photonics 2, 1584–1591 (2015).
5) A. Anopchenko et al., Mater. Res. Exp. 5, 014012 (2018).

This work is supported in part by the YFA Program from DARPA (grant number N66001-17-1-4047).

As a result of recent developments in nanofabrication techniques, the dimensions of metallic building blocks of plasmonic devices continue to shrink down to nanometer range thicknesses. The optical and electronic properties of ultra-thin plasmonic films are expected to have a strong dependence on the film thickness, composition, strain, and local dielectric environment, as well as an increased sensitivity to external optical and electrical perturbation. Consequently, unlike their bulk counterparts which have properties that are challenging to tailor, the optical responses of atomically thin plasmonic materials can be engineered by precise control of their thickness, composition, and the electronic and structural properties of the substrate and superstrate. This unique tailorability establishes ultra-thin plasmonic films as an attractive material for the design of tailorable and dynamically switchable metasurfaces. Due to their epitaxial growth on lattice matched substrates, TiN is an ideal material to investigate the tailorable properties of plasmonic films with thicknesses of just a few monolayers.

MXenes, a class of two-dimensional (2D) nanomaterials formed of transition metal carbides and carbon nitrides, are yet another promising material platform for tailorable plasmonic metamaterials. They have the general chemical form M_n+1X_nT_x, where ‘M’ is a transitional metal, ‘X’ is either C or N, and ‘T’ represents a surface functional group (O, -OH or -F) and are chemically synthesized from a corresponding layered ternary carbides or nitrides phase known as MAX (M_n+1AX_n) phases. Until now, MXenes have been widely explored in a variety of applications, such as electromagnetic shielding and SERS. However, investigations of MXenes in the context of nanophotonics and plasmonics have been limited leading to this current exploration of MXenes as building blocks for plasmonic and metamaterial devices. Here, we present our investigations on optimizing the optical response of ultra-thin TiN and MXenes for plasmonic metasurfaces.

For what applications are plasmonic materials better than all-dielectric materials, and vice versa? Or 2D materials versus their bulk counterparts? How does the requisite bandwidth affect materials selection? Known material-based fundamental limits to optical response work at one of two extremes – a single frequency, or all-frequency sum rules – with no incorporation of bandwidth and no useful quantitative measure for all-dielectric approaches. Here, we use the complex-analytic properties of certain optical-response functions in conjunction with novel energy-conservation constraints to derive fundamental limits to near-field optical response for any material, over any bandwidth. We show that certain canonical geometries can approach the bounds at specific frequencies, while at many others there is significant opportunity for structured materials to surpass them by orders of magnitude. We map out a frequency-bandwidth phase space in which we identify optimal materials among plasmonic, all-dielectric, and 2D-material candidates, and we put forward a quantitative material figure of merit to stimulate new-materials discovery.

Materials such as W, TiN, and SrRuO₃ (SRO) have been suggested as promising alternatives to Au and Ag in plasmonic applications owing to their refractory properties. However, investigation of the reproducibility of the optical properties after thermal cycling at high operational temperatures is so far lacking. Here, thin films of W, Mo, Ti, TiN, TiON, Ag, Au, and SrRuO3 are investigated to assess their viability for robust refractory plasmonic applications. Films ranging in thickness from 50 - 180 nm are deposited on MgO and Si substrates by RF magnetron sputtering and, in the case of SrRuO₃, pulsed laser deposition, prior to characterisation by means of AFM, XRD, spectroscopic ellipsometry, and DC resistivity. Measurements are conducted before and after annealing in air at temperatures ranging from 300 - 1000° C for one hour, to establish the maximum cycling temperature and potential longevity at temperature for each material. It is found that SrRuO₃ retains metallic behaviour after annealing at 800° C, however, importantly, the optical properties of TiN and TiON are degraded as a result of oxidation. Nevertheless, both TiN and TiON may be better suited than Au or SRO for high temperature applications operating under vacuum conditions.

New materials for conventional photonic devices are predicated on high refractive index contrast for enabling high performance. Fabrication of nanophotonic devices also relies on the ease of pattern formation through lithographic templating and/or etching. Layered transition metal dichalcogenides are an area of intense focus due to their emergent properties at the monolayer limit. Bulk transition metal dichalcogenides possess very high refractive index. One convenient synthetic route for large area coverage of transition metal dichalcogenides is the deposition of transition metal oxides, e.g. MoO₃ and WO₃, and subsequent conversion into transition metal dichalcogenides at elevated temperatures in the presence of reactive chalcogenide species.

In this presentation, we demonstrate a scalable method for producing high refractive index WS₂ layers by sulfidation of WO₃ synthesized via atomic layer deposition (ALD). High index of refraction is achievable through multiple synthetic procedures. With careful control of synthesis conditions, we can produce very high refractive index WS₂ thin films and conformal coatings by short high temperature (800°C) or prolonged moderate temperature (650°C) annealing in hydrogen sulfide. Although this process yields highly polycrystalline films with grain sizes on the order of 10 nm or less, the optical constants are in agreement with those reported for single crystal bulk 3R-WS₂. The conformal nanocrystalline thin films demonstrate a surprisingly high index of refraction (n > 3.9), and structural fidelity compatible with lithographically defined features down to ~10 nm. Subsequently, we demonstrate three photonic structures - first, a two-dimensional hole array made possible by patterning and etching an ALD WO₃ thin film before conversion, second, an analogue of the 2D hole array first patterned into fused silica before conformal coating and conversion, and third, a three-dimensional inverse opal photonic crystal made by conformal coating of a self-assembled polystyrene bead template. These results can be trivially extended to other transition metal dichalcogenides, thus opening new opportunities for photonic devices based on high refractive index materials.

High temperature stability, chemical stability, low surface energy and mechanical robustness, combined with a zero-crossover wavelength in the visible region make titanium nitride (TiN) a promising plasmonics material. In this work we demonstrate the heteroepitaxial growth of TiN on (0001)-Al₂O₃, (001) MgO, (0001)- LiNbO₃ substrates using controllably-unbalanced reactive magnetron sputtering. Additionally, we discuss the relationship between TiN crystalline quality and optical properties. Coupled x-ray diffraction (XRD) of our TiN show high quality epitaxial growth on all three substrates, however, further structural characterization reveals differences in crystal defects, strain and surface morphology based on substrate crystal structure and lattice mismatch. Pendellosung fringes on the (111)-TiN diffraction peak for the coupled XRD of TiN on c-plane sapphire and LiNbO3 indicate uniform d-spacing and a pristine interface. Pole figure XRD show 6-fold symmetry for the TiN grown on sapphire, indicating the presence of stacking faults. These domains, which are further evident in atomic force microscopy (AFM) of the TiN surface, are attributed to different stacking within the TiN domains. XRD of the TiN on MgO show Pendellosung fringes on the (001)-TiN diffraction peak. Cross-hatching features similar to those on the bare MgO substrate seen in the AFM of the TiN surface. Variable angle spectroscopic ellipsometry (VASE) shows that TiN behaves metallic on all substrates with a zero crossover wavelength between 470 nm and 490 nm. Differences in the real (ε₁) and imaginary ( ε₂) permittivity for TiN on the different substrates are also seen in the VASE measurements. From the ellipsometry measuremend we calclate aquality factor (QLSPR = - ε₁/ε₂ ) of 3.34, 3.9 and 4.2 for TiN on LiNbO₃, MgO and Al₂O₃ respectively, for a wavelength of 1550 nm. The differences in ε₁ and ε₂ become more pronounced at longer wavelengths.

Steel is an alloy primarily formed by iron, carbon, and chromium that have transformed our society. Today, it is implemented in applications spanning from building construction to surgical tools. This disruptive material results from alloying, which provides superior mechanical properties. In analogy to steel, we propose the alloying of metals (Ag, Au, Cu, Al), which can enable the development of optical materials with unprecedented permittivity values, not found in its pure counterparts. Our paradigm overcomes the limitation imposed by the pre-defined permittivity of metals [1]. We demonstrate he unique near- and far-field optical properties of these alloys for energy harvesting devices [2], and how they are correlated with the alloys band structure [3]. Further, we show perfect absorbers using Al-Cu/semiconductor with near-unity (> 99%) and omnidirectional absorption in the visible and NIR range of the spectrum, formed by a simple dual-layer thin film stack [4]. Beyond coin-age metals, we introduce the use of earth-abundant materials for the realization of devices with on-demand response, including reconfigurability and transient behavior.

[1] ACS Photonics 3, 507 (2016) – COVER.
[2] Adv. Optical Materials 6, 1800218 (2018).
[3] Adv. Optical Materials 5, 1600568 (2017) – COVER.
[4] Adv. Optical Materials 6, 1700830 (2018). Inside COVER

Fano resonances are made of asymmetry components, which are either of different sizes or various shapes. In optical range, these kinds of nanostructure have strict requirements both for the design and the fabrication of each unit cell. Here, we put forward an alloy-made metasurface, which is composed of two kind of unit cells with different ingredients. In such a case, the properties of each counterpart can be adjusted gradually by changing the ratio of the alloy, which gives us a way to shift the phase of the metasurface, inducing the shift of asymmetry.

Metallic structures on insulators are essential components in advanced electronic and nano-optical systems. Their electronic and optical properties are closely tied to their crystal quality, due to the strong dependence of carrier transport and band structure on defects and grain boundaries. Here we report a general method for creating patterned single crystal metal microstructures on amorphous insulating substrates, using liquid phase epitaxy. In this process, the patterned metal microstructures are encapsulated in an insulating crucible, together with a small seed of a differing material. The system is heated to temperatures above the metal melting point, followed by cooling and metal crystallization. During the heating process, the metal and seed form a high melting point solid solution, which directs liquid epitaxial metal growth. High yield of single crystal metal with different sizes is confirmed with electron backscatter diffraction images, after removing the insulating crucible. Unexpectedly, the metal microstructures crystallize with the direction normal to the plane of the film. This platform can readily extend to the growth of bi-crystals, by specifying two seed structures at each end of the metal stripe, thereby enabling the detailed study of single grain boundaries in microscale devices. This platform technology will enable the large scale integration of high performance plasmonic and electronic nanosystems.

We present a facile, low-cost and cleanroom-free technique for the fabrication of metamaterials using computer-aided 3D printing, metal deposition and wet etching. This hybrid approach enables realization of complex 2D and 3D geometries, which require multiple steps in conventional lithography. As an example we show innovative Mushroom-type metamaterials which consist of metamaterial unit cells standing on a thin pedestal. Such metamaterials are first designed in 3D CAD software (SolidWorks) and the entire 3D structure is first printed using a stereolithography (SLA) based printer (Formlabs Incorporated; Somerville, MA, USA). The printed device is then washed with Isopropyl alcohol (IPA) and distilled water. Then the device is fully cured with Form cure (by Formlabs). We use two approaches for metal deposition of the metal resonators on the top surface of 3D printed design. In our first approach, we stamp the top surface of the mushroom-type printed device with silver paste (AG-510 Silver conductive ink, Applied Ink Solutions; Westborough, MA, USA).We measure the transmission spectrum of the device with continuous wave THz spectrometer (Toptica Photonics; Munich, Germany). Unit cells show disk geometry with 250μm and periodicity of 1mm and the height of the pedestals are 8mm. The silver stamped metamaterial shows resonant frequency at 222GHz matching with the simulation results. In our second approach, we sputter the printed device with 100nm of gold by NSC-3000 Magnetron sputter tool. Then we use gold etchant type TFA (TRANSENE Company, Incorporated; Danvers, MA, USA) to etch away gold from the whole device except the top surface of the disk resonators. Plasma treatment was used before etching so that the gold etchant would flow easily in between the pedestals of the mushroom-type metamaterials. The gold sputtered metamaterial shows a resonant frequency in 248GHz matching with the simulation results. We also characterized variability in fabrication by measuring the surface area of disks. The variability ranges from 235μm to 270μm showing a Gaussian distribution centered at 250μm. The coated areas by stamping method show high variability in the coated surface area with similar Gaussian distribution. We also extended our fabrication method for different types of metamaterials to show its versatility. As a second design we also implemented split ring resonator (SRR) geometry in a mushroom-type structure. The transmission spectrum of SRR shows a resonant frequency at 55.87GHz matching with the simulation result. In summary, our hybrid approach combining 3D printing, metal deposition and wet etching enables low cost realization of complex 3D metamaterials with unique electromagnetic functionalities.

Conductive transition metal nitrides (TMNs) have recently been suggested as viable alternatives to the noble metals gold and silver for plasmonic applications. Ceramic TMNs offer increased thermal stability [1] whilst also providing broad spectral tunability and CMOS compatibility. One such material, niobium nitride (NbN), has previously been suggested as a possible candidate for plasmonic devices at visible and near ultraviolet wavelengths, despite increased electronic losses [2].
We present an experimental investigation of the growth mechanism and optical properties of reactively sputtered NbN thin films grown at a range of deposition temperatures. Thin films of NbN have been demonstrated to display tunable double epsilon-near-zero behaviour, the mechanism of which will be discussed.
Furthermore, in order to assess the viability of TMNs for inclusion within future plasmonic and optoelectronic devices it is essential to understand their optical properties and thermal stability when patterned on the nanoscale. We present an investigation of the plasmonic performance of NbN nanoparticles and nanodisc arrays fabricated by colloidal lithography and e-beam lithography.

[1] Wells, M. P. et al. Temperature stability of thin film refractory plasmonic materials. Opt. Express 26, 15726 (2018).
[2] Patsalas, P. et al. Conductive nitrides: Growth principles, optical and electronic properties, and their perspectives in photonics and plasmonics. Mater. Sci. Eng. R Reports 123, 1–55 (2018).

Terahertz metamaterials (MMs) have shown their powerful capabilities in manipulating THz waves. By tailoring the unit cell’s properties and optimizing their arrangements, different functionalities such as filtering, absorption, focusing, and anomalous reflection of THz waves have been achieved. In the approaching to multi-functional THz MMs, reconfiguration at the unit cell level is of important. As one type of reconfigurable MMs, digital and programmable metamaterials have been experimentally validated in the microwave range, but not yet available in the THz regime. Therefore, finding the tuning mechanism at the unit cell level to achieve at least a 180° phase difference (for 1-bit) is the key for THz digital and programmable metamaterials. Once the digital states have been realized and controlled in a programmable manner, THz digital and programmable MMs can be realized.

Digital microfluidics has been developed recently based upon the manipulation of microdroplets. The size, spacing and flowing speed of microdroplets can be precisely controlled. Microdroplets in variety of materials, such as water, oil and liquid metal, can be generated and manipulated. More importantly, digital microfluidics possesses the capability to determine the locations of microdroplet within a small region of the device. This technique gives us an idea to design a very specific microfluidics device that integrates with carefully patterned microchannels to form an array of MM unit cells for THz manipulation. Channel in each unit cell then can be actively filled or unfilled by liquid material to obtain two digital states with a 180° phase difference, representing a reconfigurable operation of a 1-bit MMs unit cell.

In this paper, the idea of creating THz digital metamaterials using digital microfluidics technologies is investigated. A reflective-type 1-bit THz digital metamaterial based on a digital microfluidic system is designed for proof-of-concept purpose. A T-junction microfluidic setup is used to generate liquid metal microdroplets “train”. Next, the microdroplets train is delivered to the THz MM area with 9×9 subarays, where each subarray contains 3×3 unit cells. By determining the size of each droplet and the spacing between them, each subarray can be set to be filled or unfilled with liquid metal for two digital states. It is worth noting that, the patterns of microdroplets train can be quickly and precisely created and modified by adjusting the pressures over the T-junction. Numerical simulation results show a ~180° phase difference for the two digital states within a 67% bandwidth. Radar cross-section (RCS) reduction is also demonstrated by optimizing the coding sequence of the microfluidic-based THz digital MM, giving a minimum 20 dB RCS reduction in the ±45° phi cuts. Methods and materials used in fabricating the THz microfluidics chip are discussed in the last part.

Various optical technologies for discrimination of biochemical molecules have been actively studied and steadily developed for medical diagnosis, pharmaceutics, mutagenesis, phylogenies, and so on. In particular, terahertz (THz) electromagnetic waves have shown substantial promise for such applications since intrinsic vibrational, rotational, inter-molecular, and intra-molecular modes of many small molecules exist at broad THz frequency regime. Despite of such availability, it has been quite limited to obtain a reasonable signal from diluted molecule samples directly, since absorption cross section of the molecule is too small at THz frequency range. To increase sensing efficiency and absorption cross section of such target molecules, various types of nano metamaterial structure based sensing platform have been introduced [1, 2, 3]. Here, we performed THz time-domain spectroscopy (THz-TDS) using nanoscale metamaterial based sensing chips for discrimination and detection of small molecules including various types of nucleotides. This enables us to detect even very small quantities of sugar molecules sensitively and even selectrively. Moreover, several extremely pathogenic types of viruses were also discriminated in terms of the THz optical characteristics [4]. Finally, by combining of high quality factor of resonance structure and suspended graphene mono layer can provide unprecedentedly increased sensitivity in sensing of such small biomolecules even with enormous size ratio between THz wavelength and DNA nucleotide. Our suggested THz transmission model modified by THz field enhancement at nano-slot resonances and molecular plate capacitor model show an excellent fitting curve to the measured THz transmission changes in terms of used DNA quantity. With this report, THz spectroscopy research can go beyond the absorption cross-section limit with enormously enhanced sensitivity. Two new major areas of research might emerge with our study:

1) Ultrasensitive label-free type sensing of biomolecules even in aqueous state
2) Contact-free investigation of electro-optic characteristics for suspended 2D materials

[1] H.-R. Park et al., “Colossal Absorption of Molecules Inside Single Terahertz Nanoantennas,” Nano Letters 13, 1782-1786 (2013).
[2] D.-K. Lee et al., Highly Sensitive and Selective Sugar Detection by Terahertz Nano-Antennas,” Sci. Rep. 5, 15459 (2015).
[3] M. A. Seo et al., “Terahertz field enhancement by a metallic nano slit operating beyond the skin-depth limit,” Nat Photon 3, 152-156 (2009).
[4] D.-K. Lee et al., “Nano metamaterials for ultrasensitive Terahertz biosensing,” Sci. Rep. 7, 8146 (2017).

Acknowledgements
This research was supported by the Global Frontier Program CAMM-2016M3A6B3936653, and KIST intramural grants (2E28280 and 2V05880).

Metal-semiconductor metamaterials make it possible to enhance the light-matter interaction, due to resonant absorption and scattering of light by plasmon excitations in a system of metallic nanoparticles embedded into a semiconductor medium. The condition of such a resonance is Re(ε_m + 2ε_s) = 0, where ε_m and ε_s are the permittivities of the semiconductor and metal components of the metamaterial, respectively. The goal of this work was to develop and investigate new metamaterials based on the AlGaAs semiconductor matrix containing random or periodic systems of metallic AsSb nanoinclusions.
The 1 µm thick AlGaAs films were grown by molecular beam epitaxy at low temperature (200^oC) on semi-insulating GaAs substrates with (001) orientation. The concentration of the Al was 30%. The low growth temperature provided incorporation of the excess As into the epilayer in the form of antisite defects As_Ga with concentration of 1%. During the grown process, AlGaAs epilayer was additionally doped with isovalent Sb impurity. The samples were subjected to annealing at temperatures of 400–700^oC, which provided self-organization of the metallic AsSb nanoinclusions in the bulk of the AlGaAs epilayer. The volume fraction of the nanoinclusions was 0.17%. This value was determined by transmission electron microscopy and via measuring the concentration of arsenic antisite defects in the as-grown sample. Periodically arranged systems of layers of AsSb nanoinclusions were formed via periodic δ-doping of AlGaAs epilayer with Sb. The distance between δ-layers was 100 nm while their number was 12 and 24. The post-growth annealing at temperatures of 400–700^oC provided formation of the periodic systems of layers of AsSb nanoparticles near the δ-layers of Sb.
The experimental study of AsSb-AlGaAs random system for various post-growth annealing temperatures was carried out by the investigation of optical extinction coefficient spectra. We observed significant absorption and scattering of light in the random system of plasmonic AsSb nanoinclusions in the transparency window of the AlGaAs matrix. However, the plasmon resonance peak corresponds to the energy within the fundamental absorption band of AlGaAs.
Investigation of the periodic system of AsSb-AlGaAs lasyers was carried out by optical reflection spectroscopy for light incidence angles up to 85° for s- and p- polarizations. In optical reflection spectra we observed resonant reflection peaks, accompanied by satellite oscillations. The magnitudes of the resonant peaks increased with the increase of average size of AsSb nanoinclusions and reached 19% and 31% for the 12 and 24 AsSb-AlGaAs periods, respectively. Resonance wavelengths corresponded to the expected values in accordance with the Bragg law. We relate these significant reflection magnitudes to the high dielectric contrast between the AsSb nanoinclusion layers and surrounding semiconductor medium, which is caused by the proximity of the plasmon and Bragg resonances.

Chiral plasmonic nanostructures have attracted great research interest due to their strong light-matter interaction in the subwavelength scale, leading to strong chiral optical response potentially applicable for enantiomer sensing, imaging or data communication. Active tuning the chiral optical response of chiral plasmonic devices can be realized by modulating the plasmonic modes at the nanoscale. Several methods have been proposed to effectively switch the optical chirality of such devices, including using phase change materials^[1], DNA origami^[2], mechanical deformation^[3] and magneto-optical effects^[4]. However to achieve efficient chirality control is still a challenge. Here, we demonstrate switching of the extrinsic chirality of an achiral plasmonic metasurface at glancing incidence angles by using low loss magnetic oxide thin films. Upon switching the applied magnetic field direction, the metasurface shows sign reversal of the far field optical chirality in a wavelength range of 800 to 1100 nm. The device is composed of a thin film stack of perforated Au/Ce:YIG/YIG/SiO₂/TiN multilayers fabricated by sputtering and pulsed laser deposition (PLD). The periodic nanohole structure is fabricated by polystyrene (PS) sphere self-assembly in large areas. We experimentally demonstrate the circular dichroism (CD) of 0.15 rad induced by extrinsic chirality at an incident angle of 65°. Under an applied magnetic field of -3500 Oe to +3500 Oe out-of-plane, the far field CD can be switched with a dynamic range of -0.1 to 0.32 rad. Modal simulation also revealed a significant superchiral field modulation in the near field, which is about one-order of magnitude higher compared to previous work of magnetic chiral nanostructures. The effective modulation of optical chirality in our devices is due to a much higher field intensity enhancement in low loss magneto-optical oxide thin films. Our device provides a new method to realize efficient tuning of chiral metasurfaces using applied magnetic fields.

Nonuniform graded-pattern metasurfaces as flat and compact alternatives of conventional bulky optical elements have attracted immense interest with the opportunity of miniaturization and highly-densed integration in photonic/optical devices. So far, lack of reconfigurability and real-time phase/amplitude modulation has been a tremendous challenge that prevents realization of active spatial light modulation and dynamic beam steering. To surmount this obstacle, metasurfaces hybridized with functional materials can offer an ideal platform to create ultracompact modulators which enable a strong light-matter interaction and allow for introducing dynamic spatially-varying optical properties. Although various kinds of dynamic controllable paradigms have been introduced, they suffer from low reflection efficiency and limited phase shift due to high dissipative loss, short interaction length, and ineffective physical mechanism (e.g., hybridization with plasmonic nanoantennas with single on-resonance operation).
In order to attain highly efficient tunable optical design with simultaneous high reflection level and wide phase shift, a non-volatile optically controllable metasurface consisted of geometrically fixed chalcogenide nanobars is theoretically investigated where the refractive index of each nanobar can be effectively tuned under assumption of multi-level partial crystallization and structural amorphous-to-crystalline transition. This can be accomplished through careful selection of a state-of-art tunable phase-change material (PCM) coupled with a unique physical mechanism. We utilize a recently emerged PCM called Ge₂Sb₂Se₄Te₁ (GSST) with a strong resonant bonding, large bandgap, and low carrier concentration leading to large index contrast between amorphous and crystalline states (△n≈1.8) and low optical loss (△k≈0.4) compared to the classical GST alloys. In addition, the GSST nanobars as all-dielectric building blocks can support both the magnetic and electric resonances whose spectral positions and strength can be governed by varying the crystallization level. This gives us the possibility to take into account the contribution of both supported Mie-type dipolar modes and operate at the middle of them (off-resonance operation). By preventing from the concurrence of high field confinement and large extinction coefficient inside the PCM integrated element (known as the origin of an inevitable efficiency degradation), significantly large phase modulation of 270° is achieved while the reflection efficiency is not only high but also exhibits minimal changes between 0.6 and 0.8 for all the intermediate crystallization states. As a potential application, we leverage from phased-array concept and progressive phase-delay of nanoantennas in an array to achieve continuous steering ability when each GSST nanobar is individually crystallized.

Optical metasurface is an artificial optical interface capable of controlling the flow of light by bending, scattering, absorbing or filtering electromagnetic waves [1]. Metasurface is usually composed of periodic metallic components for their sub-wavelength scale response, but high refractive index based dielectric metasurface using Mie scattering is also desirable to integrate with dielectric circuit as well as the view of optic loss [2]. On the other hand, two-dimensional (2D) transition-metal dichalcogenides have emerged as a new class of active medium with atomic layer thickness [3]. 2D exciton-polaritonic state can play an important role in many optical properties such as enhancing emitting, lasing and modulating of light. However, light coupling such a dielectric metasurface with 2D material is still in lack of research.
Here we report such a system in consisting of MoS₂ and dielectric metasurface to engineer light-matter coupling. The MoS₂ sample was grown by chemical vapor deposition (CVD) method. The nature of MoS₂ was confirmed by both of AFM and Raman characterization, where by the thickness of typical ~5nm. The multi-layer MoS₂ exhibit exciton emission centering at 1.95 eV, with narrow line width of 60 meV. Dielectric metasuface was precisely designed by amorphous silicon nanoblock in order to have first-order Mie scattering corresponds to the magnetic dipole in a range of MoS2 exciton emission. To enter the magnetic mode induced strong coupling regime, we investigate scattering spectra with various coupling strength by detuning its quality factor and distance of each layer. The spectral evolution of the composite system shows a spectral mode splitting phenomenon to depend on the energy detuning based on Si-nanoblock geometry. To understand the strong coupling effect properly, coupled mode theory is utilized, which can describe splitting energy of the coupled system. Calculated spectrum corresponding the 72meV (Si-nanoblock width=150nm, height=150nm, thickness =100nm) splitting energy shows the strong coupling condition above the vacuum Rabi splitting criterion. This strong coupled state of the composite system occurs when the energy exchanges between the MoS₂ and Si-nanoblock is faster than their individual energy dissipation rate.
As a result, we have systematically studied the geometry dependence of coupled state, demonstrating the validity of strong coupled state of the composite system. The occurrence and utilization of the strong coupling condition can be tailored by proper structural design and its related harmonic mode condition. These results could provide important clues to integrate 2D material on a chip level circuit, and also quantum optics devices.
Reference
[1] Yu, Nanfang, et al. Science 334.6054 (2011)
[2] Yi, Yoonsik, et al. Applied Physics Express 9.4 (2016)
[3] Mak, Kin Fai, et al. Physical review letters 105.13 (2010)

Non-resonant metasurfaces were investigated as potential light trapping structures in perovskite thin film solar cells. The short-circuit current and energy conversion efficiency of thin-film single and tandem solar cells can be improved by light trapping while lowering the material consumption. Light trapping is commonly achieved by texturing the solar cells or at least texturing the contacts of solar cells. In the first part of the study periodically arranged metal oxide nanowire arrays were realized by electrodeposition. The zinc oxide nanowires were prepared by templated growth through a mask of photoresist. The subwavelength large zinc oxide nanowires can be used as fundamental building blocks in controlling the phase of the incident light. In a second part of the study, optical simulations were used to design non-resonant metasurfaces with optical properties comparable to commonly used light trapping structures like pyramids or inverted pyramids. A comparison of the metasurfaces with the pyramid textured perovskite solar cells, exhibiting almost identical quantum efficiencies and short circuit currents. For thick perovskite solar cells, the incoupling of light in the solar cell is improved, and the short-circuit current and energy conversion efficiency is increased by approx. 15%. For thin perovskite solar cells the incident light is diffracted and scattered, and gain in the short-circuit current and energy conversion efficiency of 20-30% is observed. Guidelines for the design of the non-resonant metal oxide metasurfaces will be provided.

Acknowledgment: This work is financially supported by The Hong Kong Ph.D. Fellowship Scheme supported by the Hong Kong RGC.

Metasurfaces are nanostructured interfaces designed to induce abrupt changes to light wavefronts and polarization by means of arrays of sub-wavelength scatterers acting as optical antennas [1]. This tailoring largely depends on the size and shape of the scatterers, as well as on the choice of material, all of which can be designed to a high degree using modern fabrication techniques. Such flat metasurfaces offer a vast and exciting playground for both fundamental studies related to light-matter interaction at the nanoscale, as well as technological opportunities for designing a new generation of flat optical components.

Given the large freedom in design, appropriate characterization methods and tools need to be developed, thus enabling access to various optical parameters for such metasurfaces. Photoelastic modulators (PEM) are typically employed to quantify optical effects, such as linear and circular dichroism/birefringence. However, we have observed that conventional PEM-based methodologies that are typically employed for determining such quantities [2] can be affected by artifacts and spurious effects, particularly when dealing with anisotropic and chiral metasurface architectures. We will present and discuss advantages, limitations, and different analysis methods that can still facilitate access to certain properties of such metasurfaces using PEM-based characterization techniques.

References
[1] N. Yu and F. Capasso, Nat. Mater. 13, 139 (2014).
[2] T. Oakberg, Linear & Circular Dichroism, Hinds Instruments, http://www.hindsinstruments.com/wp-content/uploads/Linear-Circular_Dichroism.pdf.

Optical phased arrays are important ingredients for beam forming in sensing, imaging and ranging applications. Electrically tunable metasurfaces comprised of elements where phase and/or amplitude may be addressed individually offer a highly integrated and versatile platform for beam shaping. However, addressing electrically a large number of metasurface elements may seem a substantial practical limitation. Here, we report a novel architecture of control for phased array antennas and metasurfaces. In contrast to previous schemes utilized for beam forming in phased array antennas and metasurfaces, our design involves a reduced number of control elements, is efficient and compact.
A typical metasurface employed for beam forming contains periodic arrays of emitters and/or scatters, each having its own phase and amplitude. Coherent superposition of the fields scattered by each of the emitters forms a narrow beam, direction of which may be controlled by an applied voltage. However, current architectures assume individual control (e.g., by an applied voltage) of each of the scatter. While for a small number of elements this architecture works well, with increase of the number of array elements the control becomes complex and power inefficient. For instance, a square array with N elements on the side would require N² of control voltages. Hence, for large aperture arrays, e.g., with 1000 elements on the side – that are used at optical frequencies, 10⁶ control elements are needed.
In contrast here we propose a different control scheme. Beam forming relies on a fundamental relation of independent phase progression along two orthogonal directions (e.g., x and y) and a constant phase gradient along these directions. It is this superposition of phase gradients along orthogonal planes that allows us to implement a perimeter control architecture. Control signals are applied along the orthogonal axes on the perimeter of the array superimpose at any given element inside the array. For the case of a linear voltage phase relation this voltage superposition directly corresponds to phase superposition. Such a control scheme in contrast to a previously mentioned one requires only 2N controls. We note that our architecture may also utilize nonlinear gradients and phase relations with a proper design. Our approach is generalizable to different frequency domains, including optics, infrared, terahertz, microwave and RF, as well as acoustics.

Nonlinear nanophotonics is a fast-developing research field with a high potential of applications with new types of metamaterials, nanoantennas, lasers and versatile photonic devices. Nanophotonics is usually associated with metallic nanostructures and nanoparticles due to their optoelectronic and plasmonic properties. Despite successful applications of such kind of structures, it is well known that plasmonics suffer from ohmic losses, heating and incompatibility with standard CMOS technology. However, recent research follows a new route of light manipulation at the nanoscale by employing optically induced Mie resonances in dielectric and semiconductor materials with high refractive indexes, for example silicon or germanium, as an alternative to metallic nanostructures to overcome these issues.
In our work, we utilized the electromagnetic properties of III-V semiconductor nanowires to design building blocks for nonlinear all-dielectric metamaterials and devices. We developed a new concept of fabrication standing disk nanoantennas by slicing epitaxially grown nanowires with focused ion beam milling. This method allows us to create nanodisk configurations on any substrate and it significantly expands the possible applications of III−V disk nanoantennas.
In the fabricated standing disks from GaAs/AlGaAs nanowires, we studied nonradiating current distributions, called anapoles modes. The nonradiating current distributions have two essential features that make them very promising structures for photonic devices: first, lack of energy losses and second invisibility to the propagating electromagnetic field. We demonstrated in III-V standing disk nanoantennas the suppression of the far field radiation and strong electromagnetic field confinement inside the disk structure that lead to the strong enhancement of the second harmonic generation. We have experimentally shown the drop in the linear scattering in the region of 800-900 nm and simultaneous enhancement around 5000 of the second harmonic generation from these disks in comparison with a layer of AlGaAs. For the theoretical analysis we developed a full model that allows us to calculate linear scattering, second harmonic generation and perform the multipole expansion of the electromagnetic field inside studied disks. Using this multipole expansion in both spherical and Cartesian coordinates, we confirm that the demonstrated nonradiating configurations are anapoles.
In the future, we belive that this approach of nanowires slicing can form a new branch of nanotechnology to fabricate different chains of disk and rod nanoantennas on different substrates. Such type of chains could become a core element for lasers and sensors with low power consumption based on lossless nonlinear nanophotonic components supporting anapoles and compatible with CMOS technologies.

Nanoplasmonic sensing is a very active and diverse field with a wide variety of applications in chemistry, biomolecular and materials science [1]. Optically resonant molecular systems often display what is called a strong coupling to the nanophotonic systems. This is primarily explored for the nanophotonics active control and in the studies of quantum optics [2]. At the same time, the strong coupling of the molecular resonances to the nanoplasmonic antennas has not been addressed to follow the light-induced molecular processes. Here we combine an exemplary molecular photo-switch, from the spiropyran photochromic family, with anisotropic nanoplasmonic antennas to earn the monitoring tool for the light-activated processes using molecular and nanoplasmonic resonances strong coupling regime [3]. We follow the reversible photo-isomerization of the spiropyran photoswitch from the spiro form to the merocyanine form by tuning in the nanoplasmon antenna to the excitonic state of the merocyanine form (at 570 nm), prompting the formation of a hybrid excitonic-plasmonic state. Our anisotropic nanoantenna provides two polarization-dependent spectrally separated resonances in the visible region, allowing for separate monitoring of the plasmon-exciton strong coupling and the conventional enhanced optical near-field refractive index sensing. This system uncovers a new modality in polaritonic chemistry and optical label-free monitoring of the photo-activated processes and can find applications in photocatalysis, biosensing and in hybrid molecular-nanoantenna actively modulated systems.

[1] M. I. Stockman, Science 348, 287 (2015); A. Dmitriev (Ed.), Nanoplasmonic Sensors, Springer NY (2012).
[2] Yoshie, T. et al. Nature 432, 200 (2004) ; Kasprzak, J. et al. Nature Mater. 9, 304 (2010); Reinhard, A. et al. Nature Photon. 6, 93 (2012).
[3] Hedayati, M. K. et al. Adv. Optical Mater. 2, 705 (2014).

Two-dimensional (2D) materials, such as graphene and hexagonal Boron Nitride (h-BN), have been attracting the eyes from the materials research society, especially for their photonic and electronic applications. In addition to their own 2D material functionality, in recent years 2D materials are also being investigate as a template to engineer the growth of 3D photonic structures and devices. Here, we show that the drastic change in surface energy on single-layer 2D materials can be effectively applied to engineer self-assembled 3D photonic nanostructures, including Sn-SnO core-shell nanoneedles and Sn nanodot arrays. The wetting angle analysis reveals that the 2D layer, though only with atomic-scale thickness, can already modify the surface properties of the substrate significantly. For example, the fused quartz (SiO2) substrate is originally hydrophilic (~40° wetting angle of water), while a single-layer graphene (SLG) lattice is enough to makes it hydrophobic (~100° wetting angle of water). In contrast, h-BN seems to be wetting-transparent to the surface, i.e. it retains the original surface properties of the substrate. It’s observed that such a surface properties variation leads to clear geometric differences of the Sn and Sn-SnO core-shell nanostructures grown on the top, and thus greatly influences the optical properties at 2D interfaces. The Sn nanostructure on SLG/quartz always tends to be more continuous than that on quartz directly; while the Sn nanostructure on h-BN/quartz and quartz directly show identical geometries. We derived the effective refractive index change of 3D Sn and Sn-SnO core-shell nanostructures on SLG vs. on silica, which can be further engineered to achieve other novel 3D photonic structures on SLG utilizing 2D materials as a template. An example is the highly effective light trapping in 2D materials contributed by the 3D nanophotonic structures, which has been revealed by direct optical absorption measurement and confirm by strongly field-enhanced Raman signals from the 2D materials. With Sn-SnO core-shell nanoneedles, the absolute absorption of SLG can reach up to 17% from the wavelength of 500 nm to 1200 nm; with Sn nanodot arrays, the absolute absorption of SLG can be up to 20% from the wavelength of 500 nm to 4500 nm. Furthermore, these structures can also be removed from the SLG and transferred to other substrates due to the weak Van der Waals bonding at the interface. Such a facile 2D-templated fabrication of 3D nanostructures offers another handle to optimize the self-assembly for nanophotonics and optoelectronics devices, e.g. photon management in 2D photonic devices.

Adhesion layers are often needed in the fabrication of metallic nanostructures. For example, Ti and Cr layers facilitate the adhesion of Au nanostructures on various dielectric and semiconductor substrates, which is essential for many applications such as plasmonics, metamaterials, and nanoelectronics. However, despite providing mechanical stability, adhesion layers sometimes have detrimental effects in certain applications. It has been reported that in plasmonics, metallic adhesion layers for Au nanostructures cause damping of surface plasmon resonances, and lead to a decreased field enhancement and plasmon lifetime. Hence, there has been a quest for fabricating Au nanostructures with an alternative adhesion layer or even without an adhesion layer. In our previous work, we have shown Au nanoparticle arrays can be made free from adhesion layers. Here we systematically investigate the effect of adhesion layers on Au nanostructures made from electron beam lithography and a wet lift-off process.
In this work, Au nanoparticle arrays and nano-gratings were fabricated via electron beam lithography on a silicon or Au-coated silicon substrate. A single layer PMMA resist was exposed and developed. Electron beam evaporation deposited the Au nanostructures with or without a Ti adhesion layer. Wet lift-off was performed in hot NMP without sonication. The samples were gently rinsed intermittently with flowing NMP.
Experiments showed Au nanostructure adhesion was affected by several factors, including nanostructure size and geometry, lithographic dosage, and the substrate being used. Au nanoparticle arrays have an almost perfect yield even without an adhesion layer. Hence, an adhesion layer is not indispensable in the fabrication of isolated Au nanostructures. However, a higher lithographic dosage is required to achieve a high yield in the absence of an adhesion layer, which shows the adhesion layer increases the tolerance to residual resist scum after development. As for Au nano-gratings, the adhesion layer is not necessary for short grating lines, but is essential for achieving a high yield for long grating lines. Longer grating lines could have more defects and cracks at the Au-substrate interface, leading to a greater vulnerability to delamination. Furthermore, we find an adhesion layer is also necessary when making Au nano-gratings on top of a Au-coated substrate. This observation indicates the adhesion layer is needed not only for the adhesion between the metallic nanostructures and the substrate, but also for the adhesion between the metallic overlayer to be removed and the resist layer. The latter adhesion is crucial for removing unwanted metal pieces from the samples in the wet lift-off process.

Halide perovskites are inexpensive and easily processable next generation semiconductors. We here demonstrate perovskite solid-state confinement in nanoporous oxide matrices as a general strategy to control the size of the nanocrystallites (<10 nm) in the strong quantum size effect region. Photoluminescence tuning between near infrared and ultraviolet is achieved by manipulating the size of perovskite crystals through confinement in nanoporous alumina (npAAO) or silicon (npSi) scaffolds [1].
Our novel method of nanocrystalline perovskites preparation within a porous oxide matrix results in device-relevant structure that requires no colloidal stabilization. Low-voltage LEDs with narrow, blue-shifted emission fabricated with perovskite nanocrystallites confined within npAAO thin films support the general concept for next-generation photonic devices. The template-controlled size of the perovskite crystals is quantified in npSi with microfocus high-energy X-ray depth profiling in transmission geometry, verifying the growth of perovskite nanocrystals throughout the entire thickness of the nanoporous films. We study in detail exciton recombination, exciton-phonon interactions and energy trap states in confined and bulk semiconductor films using low temperature photoluminescence spectroscopy down to 3.8 Kelvin.
Further areas of application include photon detectors, (polarized) electroluminescent devices, single-photon sources and metasurfaces. Future developments will include increasing the efficiency of the LEDs, exploring their applications in flexible devices and in depth study of the fundamental properties of the confined structures.

[1] S. Demchyshyn, J. Roemer, H. Groiss, H. Heilbrunner, C. Ulbricht, D. Apaydin, A. Boehm, U. Ruett, F. Bertram, G. Hesser, M. Scharber, N. S. Sariciftci, B. Nickel, S. Bauer, E. D. Glowacki and M. Kaltenbrunner, “Confining Metal-Halide Perovskites in Nanoporous Thin Films”, Science Advances 3 (8), e1700738 (2017).

The design and fabrication of electromagnetic metamaterials have been extensively studied given their unique engineered electromagnetic properties. A broad array of applications has been enabled by the development of metamaterials such as invisibility cloaking and perfect absorbers, among others. In order to achieve IR absorption, subwavelength structures are required on the nanometer scale. Diatoms, a type of photosynthetic microalgae, feature silica exoskeleton structures that enclose their cytoplasm. The abundant hierarchical micro and nanopores on the diatom frustules have been employed in a range of bio-inspired and bio-mimetic studies. In this study, we mimicked the cribrum pores (diameter ~224 nm) on diatom frustules in order to design a hierarchical gold nanoresonator array that can be used as a near-IR metamaterial absorber. The design was validated with preliminary simulations and measurements, which demonstrate the potential for future applications in IR sensing and thermal emitter applications.

Diatoms (Coscinodiscus Species) were initially cleaned with concentrated sulfuric acid to remove organic contaminations. The extracted frustules were subsequently washed with DI water. Pore sizes and pore distributions on the frustules were measured and measurement data were used to design and fabricate the nanodisk arrays. An n-type 100 silicon wafer was used as the substrate, on which a 200 nm gold layer was deposited as the ground plane and 200 nm of SiNx was deposited as the spacer. On top of it, the bio-inspired nanodisk arrays were deposited through e-beam lithography (EBL) and a subsequent lift-off process. Finite difference time domain (FDTD) analysis was conducted to study the proposed structure’s characteristics. A Bruker Vertex 70v Fourier transform IR (FTIR) spectroscope was used to obtain the reflectance of the structure.

The abundant micro- and nanopores are seen on both sides of the diatom frustule. Specifically, we are interested in mimicking the cribrum pore sizes, and corresponding measurements were conducted. The nanodisks’ size and hierarchical arrangements mirrored the cribrum pores on the frustules. The simulation result demonstrates that there is a strong absorption peak in the IR range. We found that the nanodisks are resonant at the peak frequency and the surface electric field patterns show a clear dipole resonance. Subsequently, we evaluated the frequency peak shift as a function of nanodisk diameter. We demonstrate that the absorption becomes stronger as the disk diameter increases, and the peak shifts towards longer wavelengths. The measurements of the structures show similar tendency of the peak shift as a function of disk diameter. The disagreement between the simulation and experimental measurements may be a result of material property mismatches. The results demonstrate a potential use of the hierarchical patterns as a novel route to design and fabrication of metamaterial absorbers.

Optical metasurfaces have provided a great flexibility in shaping the wavefront of light by engineering the phase response of constituent building blocks. In the conventional metasurfaces, the required phase profile is achieved through changing structural parameters of a resonant element or rotation of a half-wave plate element around its axis. Recently, an extensive effort has been put into post-fabrication tuning of geometrically-fixed metasurfaces by incorporating different mechanisms to tune the spectral shift of the resonance and modulate the phase response of the elements in real-time. Despite the real-time tunability of these actively controlled metasurfaces, their operation has been mostly studied in the quasi-static case where the temporal variations are disregarded.
Introducing time-modulation in a metasurface leads to generation of higher-order frequency harmonics and breaks the reciprocity constraint. In this work, we demonstrate that time-modulated metasurfaces can extend the degree of light manipulation and wavefront engineering through a non-resonant modulation-induced phase shift. It is rigorously established that the light picks up a dispersionless phase shift regardless of incident angle and polarization, upon undergoing frequency conversion in a time-modulated metasurface which is determined by the modulation phase delay and the order of generated frequency harmonic. The efficiency of frequency conversion is independent of modulation phase delay and merely depends on the modulation depth and resonant characteristics of the metasurface building blocks, with the efficiency being maximal near the resonance, and decreasing away from the resonance. This creates a new design paradigm which allows for realization of tunable spatial phase discontinuties with 2π span in the wavefront of higher-order frequency harmonics for a wide range of frequencies and incident angles.
We demonstrate the applicability of the proposed design approach to a realistic time-modulated metasurface in the teraherz frequency regime implemented using graphene-wrapped silicon microwires electrically biased with radio-frequency signals. Several different functionalities such as tunable beam steering and focusing of frequency harmonics are investigated and the broadband wide-angle performance of the metasurface in manipulation of higher-order frequency harmonics is verified. The proposed paradigm is a departure from the previous metasurface design rules and enables realization of multifunctional tunable metasurfaces with wide angular and frequency bandwidth.

Transient technology is deemed as a paramount breakthrough for its particular functionality that can be implemented at a specific time and then totally dissolved. Hyperbolic metamaterials (HMMs) with high wave-vector modes for negative refraction or with high photonic density of states to robustly enhance the quantum transformation efficiency represent one of the emerging key elements for generating not-yet realized optoelectronics devices. However, HMMs has not been explored for implementing in transient technology. Here we show the first attempt to integrate transient technology with HMMs, i.e., transient HMMs, composed of multilayers of water-soluble and bio-compatible polymer and metal. We demonstrate that our newly designed transient HMMs can also possess high-k modes and high photonic density of states, which enables to dramatically enhance the light emitter covered on top of HMMs. We show that these transient HMMs devices loss their functionalities after immersing into deionized water within 5 min. Moreover, when the transient HMMs are integrated with a flexible substrate, the device exhibits an excellent mechanical stability for more than 3000 bending cycles. We anticipate that the transient HMMs developed here can serve as a versatile platform to advance transient technology for a wide range of application, including solid state lighting, optical communication, and wearable optoelectronic devices, etc.

Dielectric and plasmonic metasurfaces require the integration of materials with accurate control over position, size and shape for high optical efficiency[1,2]. They are typically fabricated by the well-established lithographic or chemical processes. Hence, it remains difficult to scale to large-area and unconventional substrates. Here, we propose the template dewetting[3-5] of thin chalcogenide glasses[6,7] as a novel approach to self-organize a variety of large index contrast all-dielectric metasurfaces[8]. Given the right dewetting time-temperature settings, initial film thickness and underlying pattern, the breakup of the film can occur at prescribed locations resulting in nano-objects of tunable position and sizes.This control over the dewetting of chalcogenide glasses paves the way towards simple fabrication route of advanced 2D and quasi 3D photonic devices over large area, flexible and stretchable substrates.Low processing temperatures enable large-scale use of rigid but also unconventional flexible and stretchable substrates.This bears a particular importance as lithographic processes today are limited by cost, and also in terms of area and substrate rigidity.The structures we realize are shown to have a very large and tunable absorbance in the visible range opening up possibilities for photo-detecting applications.Such structures also enable strong electromagnetic field confinement, and will be shown to have a variety of application in sensing, light management and second harmonic generation.By dewetting increasingly thick layers, inter-particle gap down to 10 nm could be achieved in such metasurfaces. This leads to a Fano resonance in transmission, due to the interference between a sharp electric gap mode and a broad magnetic mode due to the particles in the lattice. We will demonstrate in particular monolayer protein detection with high sensitivity, with an LOD down to around 0.5mg/ml.
References
[1] Manuel Decker etal., “Resonant dielectric nanostructures: a low-loss platform for functional nanophotonics”, J. Opt. 103001(2016)
[2] Manuel Decker, etal., “High-Efficiency Dielectric Huygens’ Surfaces” , Adv. Optical Mater.3, 813-820 (2015)
[3] K. Donghyun, etal., “Solid-state dewetting of patterned thin films”, Appl. Phys. Letters.95,251903 (2009)
[4] A. Le Bris etal., “Self-organized ordered silver nanoparticle arrays obtained by solid state dewetting”, Appl. Phys. Letters105,203102 (2014)
[5] Ye, J. and Thompson, C. V., Templated Solid-State Dewetting to Controllably Produce Complex Patterns. Adv. Mater.,1567(2011).
[6] L. Li, etal., “Integrated flexible chalcogenide glass photonic devices”L, Nature Photonics 8,643(2014).
[7] T. Das Gupta etal.“Template assisted dewetting of optical glasses for large area, flexible and stretchable all dielectric metasurfaces”, CLEO STh1I. 5(2018).
[8] T. Das Gupta, etal. “Self-assembly of nanostructured glass metasurfaces via templated fluid instabilities”, manuscript submitted to Nature Nanotechnology

Chirality refers to the structural property of an object that cannot be superposed onto its mirror image. Due to its universal existence in nature, chirality has attracted immense research interest with important applications in spectroscopy, sensing, imaging, and pharmaceutical synthesis. However, limited by the small electromagnetic interaction volume, the chiroptical response of natural materials is usually very weak and thus difficult to be detected with high sensitivity. The advent of metamaterials offers an elegant and effective solution to this problem. So far, various intrinsic and extrinsic chiral metamaterials have been demonstrated.¹ Although a set of symmetry requirements derived from Jones matrices can guide the design of chiral metamaterials,^2,3 these guidelines are insufficient when we want to quantitatively design a metamaterial structure given a desired chiral response, or even to simply predict the trend in chiral response as the structure transforms. Currently, the prediction task heavily relies on iterative, time-consuming numerical simulations to solve Maxwell’s equations on a case-by-case basis, while the retrieval task remains extremely challenging with no general close-form solutions.

Inspired by the remarkable progress and success of the deeping learning technology in processing images, speech and videos, very recently we developed a purpose-designed deep learning architecture to automatically model and optimize three-dimensional chiral metamaterials.⁴ Different from numerical optimization approaches, data-driven methods based on deep learning can represent and generalize complex functions or data, to uncover unknown relations among a huge number of variables. The proposed deep-learning model is composed of two bidirectional neural networks aiming to solve three basic tasks simultaneously. In the forward modeling, it is a fast prototyping tool with high accuracy comparable to numerical simulations, to predict the full optical responses of a chiral structure. On the other hand, given the full optical responses, the network can be used to retrieve the geometric parameters of the chiral meta-atom to solve the inverse problem. Moreover, starting from some basic requirements on the frequency, amplitude and polarity of the circular dichroism resonance, the deep model can produce suitable geometric parameters of the meta-atom. Our results demonstrate that such a data-driven model can be applied as a very powerful tool in studying complicated light-matter interactions, and accelerating the on-demand design of novel nanophotonic devices, systems and architectures for real-world applications.

References
1. Wang, Z., Cheng, F., Winsor, T. & Liu, Y, Nanotechnology 27, 412001 (2016).
2. Wang, Z., Jia, H., Yao, K., Cai, W., Chen, H. & Liu, Y. ACS Photonics 3, 2096 (2016).
3. Kang, L. et al., Nano Letters 17, 7102 (2017).
4. Ma, W., Cheng, F. & Liu, Y, ACS Nano (DOI: 10.1021/acsnano.8b03569), online publication (2018).

Phase-change materials, such as Ge₂Sb₂Te₅ (GST), exhibit drastic changes of their optical and electrical properties, which is related to different bonding mechanisms in their respective crystalline and amorphous states. This is not only already used in non-volatile memristors and optical data storage devices, but is also of enormous interest for optical metasurfaces. Other than applying a heat stimulus, such as thermally, optically or electrically, to induce the change of state, we demonstrate that ion irradiation can be used to trigger the phase change from crystalline to amorphous utilizing the creation of irradiation defects.
First, we performed homogenous ion irradiation of thin GST layers. Crystalline and amorphous phases coexist after low ion fluence irradiations, because of the statistical nature of ion irradiation. Thus, homogeneously irradiated GST has gradually tunable optical properties with increasing ion fluence, and thus behaves like a disordered metamaterial. This new degree of freedeom will open new horizons: the combination of conventional metasurfaces made of noble metal antennas with GST as a switchable dielectric will thus be expanded by gradually tunable optical responses of such systems.
Additionally, we used a focused ion beam system to irradiate a thin GST layer in defined areas with structure sizes down to 100 nm, which is below the diffraction limit of direct laser writing. The local triggering of the phase change in sub-wavelength dimensions can be used to design metasurfaces operating in the near-IR. Because of the simplicity of our approach, we can easily create optical elements that are reconfigurable and inherently flat. We will demonstrate the design and characterization compared with FDTD simulations of our optical devices, e.g. reflective polarizers, Fresnel zone plates and beam steerers.

Hyperbolic metamaterials (HMMs) [1] consisting of alternating dielectric and metal layers are playing a key role in the field of nanophotonics due to its wide range of applications in super resolution [2], bio sensing [3,4], enhancing spontaneous decay rate [5]. To realize HMMs in the visible to near-infrared regime of operation, gold is a practical plasmonic material due to its good optical properties and high chemical stability. In these wavelength range, the thickness of the metal layer is generally between 10 and 20 nm [6,7]. However, gold does not adhere well onto oxides or Si, thereby leading to discontinuous and rough films for thicknesses below 15 nm. This imposes the need of an adhesion layer, most commonly Ti or Cr. Recently our group has shown that silane based adhesion layer, (3-Aminopropyl) trimethoxysilane (APTMS) eliminates the need for metallic adhesion layers and it allows the existence of surface plasmon modes with properties similar to the theoretically predicted calculations [8]. In this work, we have successfully fabricated and characterized structures composed of Au and alumina multilayers with up to 10 layers of Au. The gold and alumina layers were 10 nm thick, thus allowing for hyperbolic dispersion in the visible to near-infrared range. APTMS adhesion layer was used on each interface between Au and alumina to provide better adhesion and therefore to obtain high quality smooth layers. The Au and alumina layers were fabricated using sputtering and atomic layer deposition techniques, respectively. The use of these techniques helps to obtain a high quality HMMs with a RMS roughness of 0.80 nm after the tenth period. Using these structures, we show that the effective medium approximation (EMA) can be used for structures with as little as 4 periods. The optical characterization shows very good agreement with the theoretically predicted EMA after 4^th period and onwards. As a quantitative measure, we calculated the mean squared derivation (MSD) of the measured reflectance compared to the EMA predicted one. After the 4^th period, the MSD is below 4%.
1. A. Poddubny, et al, Nature Photonics 7, 948 (2013).
2. Z. Liu et al, Science 315, 1686 (2007).
3. K. V. Sreekanth et al, Nature Materials 15, 621–7 (2016).
4. E. Shkondin et al, ACS Applied Nano Materials 1, 1212 (2018).
5. D. Lu et al, Nature Nanotechnology 9, 48–53 (2014).
6. K. V. Sreekanth et al, Scientific Reports 3, 3291 (2013).
7. J. Kim et al, Optics Express 20, 8100 (2012).
8. J. Sukham et al, ACS Applied Materials Interfaces 9, 25049–25056 (2017).

Metasurface microlens is one of the emerging planar micro-optical devices with great potential in a variety of applications. Among this field, high refractive index contrast nanostructures show great advantages in areas such as spectral modification of optical wave front. These advantages have enabled the microlens to be designed in a scale ranging from micrometers to several millimeters with focus size less than a micrometer. Based on the subwavelength nature of phase shifters, the physical realization of the microlens relies on nanofabrication techniques. To be specific, it can be either fabricated through reactive ion etching (top-down process) or deposition then lift-off (bottom-up process). Despite the difference in the process details, both of the two techniques have one common challenge that it is extremely difficult to fabricate structures with a high aspect ratio (e.g., a ratio higher than 5:1 thickness versus width). For planar grating lens that is designed under linearly polarized incidence (e.g., utilization of propagation phase, not Pancharatnam–Berry phase), the different phase shift is created by altering the effective refractive index. To achieve enough phase shift in visible wavelength with limited thickness (e.g., low aspect ratio), researchers have looked into high refractive index materials such as GaN and TiO2. While high refractive index materials are enabling phase shifters more phase shift with comparatively low aspect ratio, their transmission may experience a significant drop. To design an optimum planar microlens, one should either increase the phase coverage of low-index material nanostructures or increase the transmission of high-index material nanostructures. While the most effective way of increasing phase coverage of nanostructures is to increase its thickness (increase single-mode propagation length inside the structure). This idea may not be practical, as we already approaching the limit of current nanofabrication with 80 nm of feature size and 400 nm of thickness. Upon realizing this, we have to consider another direction as mentioned previously, which is to increase the transmission of high-index material nanostructures.

In this work, we have discussed possible transmission enhancement mechanisms and their feasibilities with the application of planar microlens. In addition, we have proposed the subwavelength-tapered nanostructure to enhance the transmission of TiO2 nanostructure as phase shifters of metasurface lens under the linearly polarized light. The enhancement effect is then numerically demonstrated.

Metasurface lens is one of emerging planar nanophotonic devices that promises unprecedented control of the light at nano scale and have potential applications in highly multidisciplinary fields including imaging, sensing, spectroscopy and photovoltaics. Typically, for concentrating micro grating lens utilizing 0th diffraction mode, a high index contrast grating is used as individual phase shifter to satisfy the lens focusing requirements, as it can provide relatively large phase shift from 0 to 2π so that the light focusing condition can be met. However, the high index contrast gratings give rise to a significant chromatic behavior and achieving achromatic focusing over certain bandwidth turns out to be very challenging. The achromatic focusing capability is critical for a variety of applications, as light sources (e.g., light emitting diodes) or the signal (e.g., photoluminescence and fluorescence signals) has a substantial bandwidth, especially in the visible wavelength range. For refractive lens such as Fresnel lens, achromatic behavior requires hybrid design to compensate for chromatic loss, thus significantly increase the device’s complexity and cost. For emerging micro lens designed based on subwavelength phase shift units, metalens based on circularly polarized incidence is reported with achromatic behavior achieved through special dispersion control of Pancharatnam-Berry phase elements. While for linearly polarized incidence, only several wavelengths can be achieved in telecommunication range, lens with a notable focal shift or very narrow bandwidth has been achieved.

In this work, we have demonstrated the achromatic all-dielectric metasurface lens covering the whole visible wavelength based on relatively low index contrast gratings. Supported by the unique chromatic phase shift of polymer nano structure, we are able to demonstrate a broadband subwavelength achromatic micro lens which can cover 250 nm of visible bandwidths (from 435 nm to 685 nm) with focal shift less than 5% with linearly polarized incidence. Our work is a critical step further to achieve the promises made by the flat metasurface lens that is comparable with image qualities obtained by the commercial objective.

High-refractive-index (HRI) dielectric metasurfaces have attracted a lot of attention recently. Silicon is one of feasible HRI materials that has been widely used in solar cells, photonic waveguides, and photon detectors. However, the band-gap ~ 1 eV makes the quantum efficiency of silicon low at near-infrared (NIR) wavelengths. In this work, a high absorptance device is proposed and realized by using amorphous silicon nanoantenna arrays (a-Si NA arrays) that suppress backward and forward scattering with engineered lattice resonance with Kerker effect. The overlap of electric dipole and magnetic dipole resonances is experimentally demonstrated. The absorptance of a-Si NA arrays increases 3-fold in the near-infrared (NIR) range in comparison to unpatterned silicon films. Nonradiating a-Si NA arrays can achieve high absorptance with a small resonance bandwidth (Q = 11.89) at wavelength 785 nm.

Due to its wide-bandwidth and transparency to many optically opaque dielectric materials, Terahertz (THz) wave plays a key role in the areas of telecommunications and national security. Moreover, THz technology is also critical for spectroscopic applications in the areas of chemistry, biology, and material science by its ability to measure optical properties (both amplitude and phase) over a broad spectral range simultaneously. However, despite significant progress in the generation and detection of THz radiation over the last couple of decades, one of the current bottlenecks has been the availability of high-efficiency and compact free-space optical elements for spatial manipulation of THz wavefront. As an example, one of the most common (and possibly the only) way to very tightly focus a THz beam is still using bulky hyper-hemispherical Si lenses which also inconveniently requires samples to be in contact with the lens surface. Only in the last few years, there has been significant developments in the ability to arbitrarily manipulate the spatial properties of optical beams in the visible and near-infrared regime using wavelength-scale thickness flat-optical elements, referred to as “metasurfaces”. Metasurfaces are typically composed of laterally shaped nanoscale optical elements (either metallic or dielectric) arranged in a sub-wavelength lattice, and are able impart spatially variant phase (amplitude) modulation to an optical wave-front. They can thereby achieve almost arbitrary control over wave-front shaping (analogous to refractive optics), however only with sub-wavelength-scale-thickness elements. Similar efforts have recently been extended to the THz frequency regime, although high operational efficiencies can only be achieved with reflection-type devices. Both low-efficiency and device operation in the reflection-mode of these devices remains a concern since most practical applications require a transmission geometry and high throughput efficiency.

In this work, we discuss design, fabrication, and characterization of all-dielectric THz metasurfaces exhibiting high throughput efficiencies and operating in the transmission mode. We utilize commercially available high resistivity silicon and fused silica wafers, as well as standard semiconductor manufacturing techniques to fabricate metasurfaces with high-throughput efficiencies, making our approach both practical and readily accessible to other researchers and industry users. As a proof of concept to demonstrate the versatility of our approach, we show high numerical aperture (NA ~ 0.9) focusing of THz radiation as well as generation of cylindrical vector THz beams with metasurfaces.

Tunability and switchability are essential in a variety of photonic devices, from switchable mirrors to color filters. One avenue to achieve tunability is to fabricate the device with metal hydrides, which often display dramatic changes in optical properties upon hydrogenation. While some of these metals, such as palladium, have been well studied, many other promising materials have only been characterized over a limited optical range and lack direct in situ measurements of hydrogen loading, limiting their potential applications. In this work, we present such a systematic study of the dynamically tunable optical properties of Pd, Mg, Zr, Ti, and V throughout hydrogenation with a wavelength range of 250 - 1690 nm. These measurements were performed in an environmental chamber, which combines mass measurements via a quartz crystal microbalance with ellipsometric measurements in up to 7 bar of hydrogen gas, allowing us to determine the optical properties during hydrogen loading. Using these measured optical properties, we demonstrate these metals’ applicability by showing structures that have five orders of magnitude change in reflectivity, resonance shifts of >200 nm, and relative transmission switching of more than a factor of 30. We further investigate the added tunability that annealing gives to Ti and its hydride and the loading and optical property hysteresis of Pd during a series of hydrogen cycles.

Graphene is a two-dimensional (2D) material with intriguing electrical and optical properties for infrared photonic devices. However, single layer graphene (SLG) suffers from very limited absolute optical absorption, i.e. 2.3% for free standing SLG [1] in air and 1.4% for SLG on fused quartz (SiO2) [2], which significantly limits its efficiency as photonic devices. Here, a novel strategy involving oxide-capped ultrahigh refractive index semi-metal (n~8-9) hybrid nanostructures is proposed to address this challenge. This hybrid nanostructure is prepared by two stages. Firstly, a layer of self-assembled Sn nanodot arrays is thermally evaporated onto SLG/quartz, thanks to the low melting point of Sn (504 K) compared with other metals [3]. The Sn nanodot arrays itself does already introduce a significant enhanced absorption of the underneath SLG, i.e. up to 20% absolute absorption of SLG in the Near-Infrared (NIR) regime. Such an efficient light trapping effect of SLG is contributed by the unique optical properties of the Sn nanostructures – an ultrahigh refractive index of n=8-9 in a very broad wavelength rage of λ=1600-5000 nm and meanwhile a relatively low extinction coefficient. This allows Sn nanostructures to optimize the optical performance between high index dielectric and low index metal plasmonic light trapping strategies and therefore opens the door for the investigation of ultrahigh refractive index semimetal nanostructures such as Bi and Sb. Secondly, a layer of GeO2 is thermally evaporated onto the Sn nanodot arrays. Morphologically, the GeO2 evaporation modifies the Sn nanodot arrays to a nano-flower structure. The thickness of GeO2 has been well controlled to optimize the optical properties. With GeO2/Sn hybrid nanostructure on the top, electrically and under the visible light, SLG shows up to 10x photocurrent compared with that of pristine SLG under the same optical excitation at 650 nm wavelength. This is promising especially considering that only 1.4x photocurrent enhancement is achieved from Sn nanodots/SLG structures without the hybrid structure induced by the GeO2 cap. Such a facile and highly effective photon management technique is a great choice for the efficiency enhancement of graphene-based photonics and optoelectronics.

[1] Nair, R. R.; Blake P.; et al. Science, 2008, Vol. 320, Issue 5881, 1308.
[2] Spinelli, P.; Hebbink, M.; et al. Nano Letters, 2011, 11, 1760-1765.
[3] P. J. Smith, Chemistry of Tin, 2nd Edition. Springer Science + Business Media, University College, London, UK, 1998.

By combining plasmonic nanoparticle “antennas” with reactive species, such as catalytic nanoparticles, oxides, even isolated atomic species, a range of chemical reactions can be driven by light instead of conventional thermal excitation. The plasmonic photocatalytic version of several important reactions will be described. In addition to driving endothermic processes, this approach takes advantage of the plasmonic catalyst properties to enhance reaction specificity and to lower reaction barriers.

Photothermal therapy (PTT) using plasmonic nanoparticles for cancer treatment is on the verge of clinical application. Titanium nitride nanoparticles[1] are a promising alternative photothermal agents compared to commonly used gold-based systems: Besides lower materials costs by a factor of 10⁴, TiN also exhibits a tunable localized surface plasmon resonance in the near-infrared (NIR) region for spherical particles in contrast to gold, where more complex structures are required to shift the LSPR from visible wavelengths to the NIR.[2]

Little is known, however, of the relationship between TiN particle characteristics and their optical properties in colloidal systems. Here, a detailed study on the synthesis of TiN nanoparticles by nitridation of TiO₂ and their use as PTT agents is reported. Special emphasis is laid on the oxygen content on the particle surface and bulk, which dictates the TiN optical properties. Colloidal suspensions were studied through UV-Vis and with NIR laser irradiation and correlated to particle characteristics. Higher nitridation temperatures and longer residence times are beneficial for increased NIR light absorption, while too high temperatures lead to aggregation of particles and deteriorated optical properties. This was overcome by the use of SiO₂-coated TiO₂ nanostructures as a starting material: The resulting SiO₂-coated TiN particles exhibited increased plasmonic properties compared to bare TiN, which was attributed to reduced plasmonic coupling effects. The optimized SiO₂-coated TiN had a photothermal efficiency of 58.5% and mass extinction coefficient of 31.5 Lg^-1cm^-1, outperforming commercial gold nanoshells that are used in clinical trials. Finally, the potential of SiO₂-coated TiN for PTT was successfully demonstrated in vitro by controllably killing HeLa cells.

[1]: Guler U, Naik GV, Boltasseva A, Shalaev VM, Kildishev V. Performance analysis of nitride laternative plasmonic materials for localized surface plasmon applications. Applied Physics B. 107, 2012, 285-291
[2]: Sotiriou GA, Starsich F, Dasargyri A, Wurnig MC, Krumeich F, Boss A, Leroux JC, Pratsinis SE. Photothermal Killing of Cancer Cells by the Controlled Plasmonic Coupling of Silica-Coated Au/Fe₂O₃ Nanoaggregates. Advanced Functional Materials. 24, 2014, 2818-2827.

Polaritons, bosonic quasiparticles that result from strong photon-exciton coupling inside microcavities, have shown unique non-linear quantum phenomena such as Bose-Einstein condensation, superfluidity, and enhanced conductivity. Less well-known but equally intriguing is the strong coupling of molecular vibrations with resonantly-matched cavity modes, also referred to as vibrational strong coupling (VSC). The delocalized nature of hybrid polaritonic states opens up a new exciting route for modifying a material’s physical and chemical character including chemical reactivity. In this talk, we first demonstrate the strong relationship between cavity mode profile and spatial distribution of vibrational absorbers by systematically varying the location of a slab of vibrational absorbers within a Fabry-Perot microcavity. Both experiment and modeling showed a strong dependence of vacuum Rabi splitting on molecular distribution within the cavity, suggesting the possibility of controlling coupling strength, and potentially chemical reactivity, of a given region in the cavity through modification of the cavity field profile. We also examine cavities containing two remotely located slabs of vibrational absorbers that jointly couple to the cavity field and thus increase vacuum Rabi splitting by a factor of ~1.3. Finally, we examine a model reaction in pursuit of chemical reaction rates that can be modulated by VSC in a Fabry-Perot microcavity. Our results will further extend the knowledge of cavity-modified material properties, particularly chemical reactivity, which will have important implications for chemical synthesis and catalysis.

[1] Ahn, W., Vurgaftman, I., Dunkelberger, A. D., Owrutsky, J. C., and Simpkins, B. S. “Vibrational Strong Coupling Controlled by Spatial Distribution of Molecules within the Optical Cavity” ACS Photonics, 2018, 5, 158 – 166.

A collective excitation of localized surface plasmon resonance (LSPR) has been studied extensively on 2D crystalline sheet composed of metallic nanoparticles [1]. The particle sheets are fabricated by self-assembly at air-water interface and deposited on solid substrates by Langmuir-Schaefer method. Both the experimental and the FDTD simulation data revealed a unique optical property of the 2D sheet, where the homogeneously coupled LSPR in 2D sheet exhibits not only a significant red-shift of LSPR band but also an additional amplification of electric field at the interface. Recently, a drastic reflection color change of Ag and Au nanoparticle multilayers was found on metal substrates[2, 3]. This phenomenon originates from the peak splitting of extinction spectra due to the electromagnetically induced transparency (EIT), occurring specifically on metal substrates [4]. The layer number dependent light confinement was critical, which resulted in such strong colors.Here the Ag nanosheets act as a nano-plasmonic metamaterial light absorber with a large oscillator strength. These optical data were well reproduced by the calculation with the Transfer-Matrix method by employing the effective medium approximation. This EIT related phenomenon was applied to colorimetric sensing devices, to detect a molecular level of reaction by naked eyes[5]. The 2D crystalline sheet was also an effective tool for high sensitive, high resolution fluorescence imaging of nanointerface [6]. The test experiments of actin filaments-labeled rat basophilic leukemia cells (RBL-2H3) and Paxilin-labeled NIH3T3 cells revealed high axial and lateral resolution even under a regular epifluorescence microscope, which held even better quality compared with the images taken under total internal reflection fluorescence (TIRF) microscopy [7, 8]. This non-scanning type, high speed imaging method will be quite useful to study dynamics of biomolecules.

[1] M. Toma, et al., Phys. Chem. Chem. Phys. 12, 14749 (2012).
[2] K. Okamoto, et al., Plasmonics, 8, 581 (2013).
[3] A. Yoshida, et al., Langmuir, 28, 17153 (2012).
[4] K. Okamoto, et al., Sci. Rep. 6, 36165 (2016).
[5] S. Shinohara, et al., Phys. Chem. Chem. Phys.17, 18606 (2015).
[6] E. Usukura, et al., Appl. Phys. Lett., 104, 121906 (2014).
[7] S. Masuda, et al., Sci. Rep. 7, 3720 (2017).
[8] E. Usukura, et al., PLOS ONE, 12, e0189708 (2017).

Heat-to-electricity conversion using solid-state devices is technologically important for electricity generation in remote locations, and waste heat recovery. In thermophotovoltaics, photons radiated from a hot thermal emitter are absorbed by a photovoltaic cell, exciting electron-hole pairs and generating electricity. A very hot thermal emitter is usually required for thermophotovoltaics to operate well. However, the temperature of the emitter is typically relatively low (<1000 K) such as in waste heat recovery, severely limiting the power output. Recent theories have proposed that when the gap size between the emitter and cell is much smaller than the thermal wavelength, the power output can potentially be dramatically enhanced by photon tunneling. Demonstration of such near field thermophotovoltaics has been elusive due to the difficulty in achieving a nanoscale gap between parallel surfaces of the emitter and the cell. In this talk we will describe how we experimentally demonstrated a 40 fold enhancement in power output, compared to the far-field, when the gap size between the emitter and the PV cell is reduced from ~10 µm to ~60 nm. Further, we will describe the dependence of performance on the temperature of the thermal emitter, and the bandgap of the PV cell. Our experimental results are supported by modeling based on fluctuational electrodynamics. Our work allows for systematically studying near field thermophotovoltaics towards high performance.

Solid-state infrared-to-visible photonic upconversion is a promising technique for improving the efficiency of optoelectronic devices such as photovoltaics and image sensors by generating additional above-band gap photons from sub-gap ones that otherwise cannot be harvested. Previous works have demonstrated upconversion of near-IR (850 nm – 1010 nm) to visible (peaked at 610 nm) light utilizing PbS quantum dot absorbers and rubrene doped with 0.5 volume% of dibenzotetraphenylperiflanthene (DBP) as an annihilator/emitter layer.¹
However, one of the primary limitations to realizing efficient devices based on this scheme is the low absorption of thin-film quantum dots in the near-infrared, typically less than 0.5%. In prior work, a 5-fold enhancement of PbS absorption (to ~1.5%) was demonstrated by introducing a Ag back-reflector, enabling an upconversion efficiency of 1.6% as compared to 0.51% without the mirror².
In this work, we extend these studies to achieve unity absorptance in the PbS quantum dot layer. We design a nanophotonic structure based on a critically coupled TiO₂ photonic crystal slab backed by a SiO₂/SiN_x Distributed Bragg Reflector (DBR) to enhance the quantum dot absorption at 960 nm by ~200-fold. We assemble our upconversion structure of 5 nm PbS quantum dots/50 nm rubrene:DBP on a 70 nm thick TiO₂ photonic crystal slab infilled with spin-on glass (SiO₂). A pitch of 565 nm and hole diameter of 264 nm results in critical coupling to a guided mode in the TiO₂ slab, thereby enhancing the absorption into the PbS layer. By adding an 8-pair lossless DBR of quarter-wavelength SiO₂/SiN_x (163 nm/123 nm), transmission is fully suppressed, allowing us to realize unity absorption into only the quantum dot layer in simulation. We additionally note that by designing a purely dielectric structure to enhance the QD absorption, reabsorption of the visible-wavelength light emitted by the rubrene:DBP is minimized. This will allow us to significantly improve the overall upconversion efficiency.


References:
1. Wu, M. F.; Congreve, D. N.; Wilson, M. W. B.; Jean, J.; Geva, N.; Welborn, M.; Van Voorhis, T.; Bulovic, V.; Bawendi, M. G.; Baldo, M. A. Nat Photonics 2016, 10, (1), 31-34.
2. Wu, M. F.; Jean, J.; Bulovic, V.; Baldo, M. A. Appl Phys Lett 2017, 110, (21).

The advanced electronic devices such as portable and wearable devices have been widely spread, and the size of these handheld devices have decreased considerably while their power density has increased, therefore raising concerns about self-heating of devices. The undesired heat generation in the small electronics increases the surface temperature compared to skin temperature, resulting in thermal damage to epidermis layer. In addition to skin burn issues, the functionalities of the devices are deteriorated regarding operation speed, working time, and battery life. For the thermal management of these devices, a thin metal layer has been used as a heat sink element based on heat conduction and convection. Although such metallic film provides an excellent heat dissipation capability due to the high thermal conductivity, the thermal reduction path exists only convection effect with air.
Here, we propose the strategy of a high efficient metallic heat sink (HE-MHS) by introducing additional thermal reduction route, i.e., thermal radiation, therefore it boosts the cooling capacity remarkably. The overall configuration consists of a thin-metallic heat sink element and the engineered photonic structures that selectively emit the thermal infrared wave in atmospheric window (i.e., 8 – 13 μm). Aluminum was selected as the candidate of metallic layer for heat sink, with 30 μm-thick thickness allowing flexibility. The selective emitter (SE), which is photonic structure emitting thermal infrared wave, was formed on the metal film and composed of SiN_x, SiO₂, and polydimethylsiloxane (PDMS). The simulated and measured emissivities were over 70% on average value in the wavelength of 8 – 13 μm.
The cooling performance was characterized with the handcrafted heater was used. The maximum surface temperature was increased depending on the supplied current to the heater, varying as 0.30, 0.35, 0.40, and 0.45 A, and the resulting temperatures were measured as follows: 1) the heater without the HE-MHS registered 42.3, 52.0, 55.1, and 64.7°C, respectively; 2) heater with the HE-MHS registered 28.1, 30.4, 32.6, and 35.6°C, respectively. This experimental result proves that the passive radiation-based cooling strategy is effective to suppress the heating of small electronics under solar irradiation.

Passive daytime radiative cooling (PDRC) is a highly promising and sustainable way to cool structures ranging from buildings to vehicles, reduce carbon-footprints, and mitigate heat-island effects. Research over the years have yielded many designs for PDRC. On one end are common cool-roof paints, which, while inexpensive and easy to apply on surfaces, lack the high solar reflectance (R) and low long-wave infrared emittance (ε) required for PDRC. At the other end are multilayer photonic structures and metamaterials, which are efficient, but require silver mirrors to reflect sunlight and come as sophisticated and expensive sheets that are difficult to apply on rooftops. Therefore, a radiative cooler with performance comparable to such designs, but with the convenience of paints, remains sought after.

In our presentation, we will describe a simple, inexpensive and solution-based method for fabricating structured polymer coatings with an exceptional PDRC capability. Coatings fabricated by the process are hierarchically porous, which, due to the polymer's lossless behavior in the solar wavelengths and intrinsic emittance in the thermal wavelengths, yields remarkably high, substrate-independent hemispherical R (as high as 0.99) and hemispherical ε (as high as 0.97). The values are among the highest known in literature, and allow even completely exposed coatings to achieve sub-ambient temperature drops of ~6°C and cooling powers of ~96 W m^-2 under strong solar intensities of 890 and 750 W m^-2 respectively. The optical performance surpasses those of state-of-the-art PDRC designs, while the technique offers a paint-like applicability on diverse surfaces by techniques like painting, spraying and dip-coating. Furthermore, the process is compatible with a range of polymers, and can also incorporate dyes to achieve a desirable balance between color and radiative cooling. The performance of the coatings, along with the versatility of the technique, makes it attractive as a practical way to achieve high performance PDRC.

[1] Raman, A. P.; Anoma, M. A.; Zhu, L.; Rephaeli, E. and and Fan, S.; 2014, “Passive Radiative Cooling below Ambient Air Temperature under Direct Sunlight,” Nature, 515(7528), pp. 540–4.
[2] Gentle, A. and Smith, G.; 2015, “A Subambient Open Roof Surface under the Mid-Summer Sun,”Advanced Science, 2(9).
[3] Zhai, Y.; Ma, Y.; David, S. N.; Zhao, D.; Lou, R.; Tan, G.; Yang, R. and Yin, X.; 2017, “Scalable-Manufactured Randomized Glass-Polymer Hybrid Metamaterial for Daytime Radiative Cooling,” Science, aai7899.
[4] Mandal, J.; Fu, Y.; Overvig, A.; Jia, M.; Sun, K.; Shi, N.; Zhou, H.; Xiao, X.; Yu, N.; Yang, Y.; 2018, “Hierarchically Porous Polymer Coatings for Highly Efficient Passive Daytime Radiative Cooling,” Science, Early Release (2018).

Recent studies have shown that the radiative heat transfer rate between surfaces separated by nanometer gaps can greatly exceed Planck’s blackbody limit due to evanescent modes which are present in the near-field, i.e. gap separations below the peak thermal wavelength. However, achieving dramatic enhancements in radiative heat transfer in the far-field, when surfaces are separated by gaps much larger than the peak thermal wavelength, has not been explored experimentally. Here, we present experimental work demonstrating that the radiative heat transfer rate between appropriately designed nanostructures can exceed the blackbody limit by two orders of magnitude in the far-field. Our experimental platform is comprised of aligned dielectric nano-membranes with embedded thermometers that allow the radiative heat transfer rate between the nano-membranes to be quantified. Large enhancements in heat transfer compared to the blackbody limit were observed for membranes with sub-wavelength thickness over a wide temperature range. The observed heat transfer rates are found to be in good agreement with our calculations based on the framework of fluctuational electrodynamics, which provide additional insights into the far-field radiative heat transfer between nanoscale objects in terms of enhanced emissions and absorption cross-sections in the infrared. This work highlights the potential of employing nano-membranes for controlling thermal absorption, and may have important ramifications for thermal to electric energy conversion.

Phonon-polariton materials have been largely overlooked as the material of choice to control infrared light-matter interactions as in bulk form they are near-perfect reflectors. However, recently they have emerged as one of the most prominent candidates for infrared “plasmonics” showing all the capabilities for light-confinement and enhancement as noble metals do in the visible spectrum. In particular, just as noble metal nanoparticles support particle plasmons for visible frequencies, sub-micron-sized particles made of a phonon-polariton material support particle phonon-polaritons which are localized resonances for infrared light. The operational frequency for such particle phonon-polaritons depends on the phonon-polariton material used and lies within its corresponding reststrahlen band in the broader infrared spectrum.

We show here that a judiciously designed micropyramid array comprising SiC, exhibits a highly asymmetric cascaded coupling effect between such particle phonon-polaritons corresponding to each structural block [1]. We demonstrate how this asymmetric cascaded coupling effect leads to enhancement of thermal emission in one direction and its suppression in the opposite propagation direction [1,2]. We show that this highly asymmetric thermal emission can exist over a broad spectral range and over a wide angular range, of more than 45 deg. from normal incidence. In other words, this SiC platform behaves as a highly efficient forward thermal emitter [3]. This behavior is highly relevant to many application domains of infrared photonics such as one-way THz/infrared sources, thermal management and passive radiative cooling devices.

[1] G. C. R. Devarapu and S. Foteinopoulou, Broadband Near-Unidirectional Absorption Enabled by Phonon-Polariton Resonances in SiC Micropyramid Arrays, Phys. Rev. Appl. 7, 034001 (2017).
[2] (Invited) S. Foteinopoulou and G. C. R. Devarapu, Perfectly asymmetric reflection enables unidirectional emission in a phonon-polariton (Reststrahlen-band) material platform, Published in: 2017 19th International Conference on Transparent Optical Networks (ICTON).

[3] G. C. R. Devarapu and S. Foteinopoulou, Highly-enhanced forward emission with a SiC micropyramid array in the reststrahlen band, in preparation.

We present Si-based optical metasurfaces with suitably engineered spatial dispersion that perform mathematical operations on optical input fields in the visible and near-infrared spectral range. The nanopatterned surfaces, made by electron beam lithography and etching, are designed to sustain propagating leaky modes that interfere with a Fabry-Perot continuum to form Fano resonances with a line shape that creates a transfer function in Fourier space corresponding to first or second derivative. We present analytical theory, numerical modeling and experimental data using a broad range of input fields. Second-derivative metasurfaces create efficient edge detection with applications in image recognition as we will show. An outlook will be given for the emerging potential of analog optical computing using optical metasurfaces.

Optical fiber is a well-established and efficient way to guide and manipulate light¹. Although optical fiber is efficient for transmitting light, its functionality is limited by the dielectric properties of the constituent materials. The light exiting an optical fiber is typically diverging. Furthermore, the numerical aperture is determined by the refractive index of the fiber materials. Thus, the light intensity decreases significantly upon exiting the fiber. The emergence of metasurfaces provide the opportunity to tailor light’s properties for advanced light manipulation and to develop novel optical applications. Production of a specific phase profile using spatially-varied nano-antenna elements allow metasurfaces to control the wavefront of the light enabling novel ultrathin optical components such as flat lenses (i.e., metalens) ^2-3. In this work, we experimentally demonstrated an ultrathin optical metalens, cascaded on the facet of large mode area photonics crystal fiber (LMA-PCF), that enables light focusing upon the fiber in the telecommunication regime.
Our PCF metalens is designed to have a focal length of 30 µm and numerical aperture of 0.37 at operating wavelength of 1550 nm when fabricated on the core of LMA-PCF with diameter of 25±1 µm. A rectangular groove in the gold nano-layer is employed as a unit element (dimension: 489 nm × 134 nm; period: 602 nm). The large core area of PCF allows more fabricated unit elements (total 1261) on the core while maintaining single mode guiding, thus providing a smooth phase profile. To fabricate the PCF metalens, we first deposit a 40nm thick gold layer on the end facet of LMA-PCF by magneton sputtering and fabricate designed pattern by focused ion beam milling. To verify the focusing effect, we capture the mode profiles along the propagation direction with a z-scan setup that consisting of objective lens, quarter waveplate, linear polarizer, and infrared camera. The right-handed circularly polarized light is launched to the PCF metalens and the left-handed circularly polarized output component is collected. Output light focusing is observed at the focal length of ~ 25.3 to 28 µm with wavelength between 1500 -1630 nm, which is close our designed focal length of 30 µm. The full-width at half-maximum of our observed focus spot ranges between 2.4 to 2.6 µm. Our maximum operating efficiency is 16.4%, which approaches the theoretically-predicted level for flat metasurfaces. Our integration of metalens and optical fiber will be significant in the miniaturizing of optical fiber devices with multi-functionalities and pave the way for in-fiber optical imaging and sensing applications.
1. Philip Russell, "Photonic crystal fibers." Science299, 358-362 (2003).
2. S. Wang, et al, " Broadband achromatic optical metasurface devices". Nature communications 8(1), 187 (2017).
3. N. Yu, et al. "Flat optics with designer metasurfaces." Nature materials13, 139–150 (2014).

Emerging nanophotonic applications often rely on the use of 3D nano-structured materials, which require intricate low-throughput fabrication, are costly, and limited to the microscopic device footprints. I will discuss our work on the development of smart multi-functional nanophotonic structures, which can be fabricated by high-throughput techniques without the need for nano-patterning. These include single- and multiple-wavelength super-absorbers with flat surfaces designed via topological engineering of the layered material bulk (1, 2), flexible polymer-based materials for solar energy harvesting and passive thermoregulation (3–5), and meso-porous network metamaterials for solar-thermal energy harvesting, colorimetric sensing and security (6).

1. Y. Tsurimaki et al., Topological engineering of interfacial optical Tamm states for highly-sensitive near-singular-phase optical detection. ACS Photonics. 5, 929–938 (2018).
2. S. V Boriskina, Y. Tsurimaki, Sensitive singular-phase optical detection without phase measurements with Tamm plasmons. J. Phys. Condens. Matter. 30, 224003 (2018).
3. J. K. Tong et al., Infrared-transparent visible-opaque fabrics for wearable personal thermal management. ACS Photonics. 2, 769–778 (2015).
4. S. V Boriskina, H. Zandavi, B. Song, Y. Huang, G. Chen, Heat is the new light. Opt. Photonics News. 28, 26–33 (2017).
5. S. V. Boriskina et al., Heat meets light on the nanoscale. Nanophotonics. 5, 134–160 (2016).
6. A. Ruiz-Clavijo et al., ACS Photonics, doi:10.1021/acsphotonics.7b01569 (2018).

We have been exploring how the “extreme” light-matter interaction can be achieved using specially designed materials and metastructures. Several scenarios have been investigated, including near-zero-index photonic platforms, extreme quantum optics, geometry-independent “flexible” resonant cavities, ENZ electric-dipole levitation, coherent thermal emission, photonic bound states in the continuum, and more. Moreover, we have been working on materials platforms that would achieve functionalities useful for informatics, e.g., materials that “do the math”, structures that “solve equations”, etc. In a separate paradigm, we have also investigating how 4-dimensional metamaterials can give us more functionalities. I will discuss our most recent results for some of the above topics.

Evanescent waves are oscillating fields with energy spatially concentrated in the vicinity of an illuminated or emitting object. Evanescent waves are widely exist in nature, ranging from the non-radiative emission of nano emitters (e.g., quantum dots, N-V centers, and fluorescent dyes), to optical surface waves such as surface plasmon polaritons, and to near-field optical waves induced by moving charged particles. Evanescent waves carry rich non-radiative energy and important information about sub-wavelength features of the object. They have been extensively studied in many areas, such as super-resolution imaging/lithography, biomedical sensing, electron-driven light sources and energy harvesting, thanks to their unique spatial confinement properties. However, the exponentially decaying nature of evanescent waves renders it difficult to capture, extract and engineer the wealth of energy and information that they can carry[1,2].
In this work, we propose and experimentally demonstrate an innovative method to overcome this fundamental challenge by using a novel metasurface that consists of C-aperture resonators, so that we can efficiently convert evanescent waves to the far field emission. Meanwhile, the phase and polarization of the emission can be well controlled with the metasurface. Both electric and magnetic dipoles are utilized by changing the orientation of the C-aperture resonators[3] to achieve a complete control of the phase and polarization of optical waves. This approach is distinctly different from most previous metasurface works that only utilize electric dipole resonances. As a result, we can manipulate the near-field to far-field radiation with desired functionalities, such as the focusing, holography and polarization switching, without any additional, bulky optical components. In the experiment, we generate evanescent waves with total internal reflection scheme and the evanescent waves are modulated by the designer metasurface. The experimental results rigorously show focusing, holography and polarization switching as the theoretical designs, clearly confirming the new metasurface strategy.
Our findings offer a versatile platform to extract and explore the near-field energy carried by evanescent wave and manifest promising applications in integrated optics. It will also further inspire studies related to evanescent waves, such as near-field energy harvesting, electron induced emission, subsurface sensing, and so on.

Reference
1. J. B. Pendry. Phys. Rev. Lett. 85, 3966-3969 (2000).
2. Z. W. Liu, H. Lee, Y. Xiong, C. Sun & X. Zhang. Science 315, 1686-1686 (2007).
3. Z. J. Wang, K. Yao, M. Chen, H. S. Chen & Y. M. Liu. Phys. Rev. Lett. 117, 157401 (2016).

Optical platforms enabling the dynamic real-time control of the fundamental properties of light at visible and near-infrared frequencies are the essential components for the future optical devices. Combining materials with magnetic and plasmonic properties gives a possibility for a simultaneous enhancement and mutual control of their magneto-optical and chiro-optical properties [1-4]. Here we examine magnetoplasmonic nanoantenna consisting of noble metal-ferromagnet sub-nanometer multilayers to employ it as an element in magnetically-controlled chiroptical surfaces and in plasmonic color printing. We design a nanodisk antenna comprising 15 interchanging Co (0.5 nm)/Au (1.5 nm) layers and perform a careful tuning of the layers to earn the strong perpendicular magnetic anisotropy [5] and the low magnetic saturation field to ease the magnetic control. By this we reach the magnetization switching at 0.7 kOe with the strong magnetic signal despite the insignificant amount of Co (7.5 nm of pure Co (15 Co layers x 0.5 nm thickness) in the multilayer nanodisk of the total thickness 31.5 nm). We propose using the developed Co/Au magnetoplasmonic element to control with the markedly low magnetic fields the chiroptical transmittance [6] and to be implemented as a dynamic color pixel in the plasmonic color printing. For the latter we show numerically that such Co/Au nanoelement provides a color-selective reflection [7-9] that could be potentially modulated by the external low magnetic fields.
[1] J. C. Banthí, et al., High Magneto–Optical Activity and Low Optical Losses in Metal–Dielectric Au/Co/Au–SiO2 Magnetoplasmonic Nanodisks. Adv. Mater., 24, OP36 (2012)
[2] G. Armelles, et al., Interaction Effects between Magnetic and Chiral Building Blocks: A New Route for Tunable Magneto-chiral Plasmonic Structures. ACS Photonics, 2(9), 1272 (2015)
[3] V. Bonanni,et al., Designer Magnetoplasmonics with Nickel Nanoferromagnets. Nano Letters 11(12), 5333 (2011)
[4] K. Lodewijks,et al., Magnetoplasmonic Design Rules for Active Magneto-Optics. Nano Letters 14(12), 7207 (2014)
[5] F. J. A. den Broeder, et al., Perpendicular Magnetic Anisotropy of Co-Au Multilayers Induced by Interface Sharpening. Phys. Rev. Lett. 60, 2769 (1988)
[6] I. Zubritskaya, et.al., Magnetic Control of the Chiroptical Plasmonic. Surfaces Nano Letters 18(1), 302 (2018)
[7] C. Frydendahl, et al., Optical recofniguration and polarization control in semi-continuous gold films close to the percolation threshold. Nanoscale 9, 12014 (2017)
[8] A. S. Roberts, et al., Subwavelength Plasmonic Color Printing Protected for Ambient Use. Nano Letters 14(2), 783 (2014)
[9] X. Zhu e al., Digital resonant laser printing: Bridging nanophotonic science and consumer products. Nano Today 19, 7 (2018)

ε- and µ-near-zero (EMNZ) metamaterial has been demonstrated for unconventional tailoring and manipulation of the light-matter interaction. Therefore, it provides a platform for new optoelectronic devices having desirable properties and functionalities. We have studied several ITO films with different thicknesses and annealing processes to explore the possibility to develop a host EMNZ metamaterial for a new type of optoelectronic devices that is environmental insensitive. We have characterized these ITO films with ellispsometry, X-ray diffraction, TEM and SIMS. The results indicated that a thicker self-assembled annealed ITO film has a non-uniform permittivity (ε) which varies from a negative to a positive value from the substrate to the top surface. In order to make the ITO metamaterial useful, we studied the method for producing a thicker than 1.5 m ITO with ε-near-zero (ENZ) and slight negative condition. A series of 2 µm-thick ITO films were deposited on a 3 µm-thick SiO₂ on Si wafer that were annealed at different temperatures and times. These sample were further investigated by a cutting-edge ellispometry tool (Woollam M-2000 system). The optical constant depth profile at 1550 nm has been produced. The goal is to develop an ENZ metamaterial process that is compatible with a Si-based integrated optoelectronic platform.

Optical polarization is an important characteristic of electromagnetic waves that has a significant impact on a number of applications, including communications, 3D imaging, and quantum computation. However, conventional optical polarizing components, such as linear polarizers and waveplates, are either bulky or static. Metasurfaces, which are artificially designed planar nanophotonic structures, have attracted immense attention due to their ability to control the amplitude and phase of electromagnetic waves at a subwavelength scale. Metasurface devices hold promise for versatile polarization generation in a compact form factor by introducing different electromagnetic amplitudes as well as phase shifts between orthogonal electric field components. In this work, we demonstrate that the polarization state of reflected light can be dynamically controlled by an indium tin oxide (ITO)-based tunable metasurface. The proposed metasurface consists of an aluminum back reflector, a 20 nm thick gate dielectric layer and a 5 nm-thick ITO layer on which we fabricate an aluminum nano-antenna array. The period of the metasurface structure is 400 nm while the operating wavelength is about 1550 nm. When applying an electrical bias between the ITO layer and back reflector, the carrier concentration at the gate-dielectric/ITO interface is modulated, resulting in a change of the effective index of the ITO layer. The epsilon-near-zero (ENZ) mode, which is accessed under applied external DC bias, alters the interaction between the induced plasmonic modes (which correspond to the orthogonal polarization components), leading to modulation of the polarization state of the reflected light. By suitably biasing the metasurface structure, the linearly-polarized incident light can be converted to cross-polarized, circularly-polarized or elliptically-polarized light. This dynamic control of the amplitude, phase as well as the polarization state of the scattered beam provides prospects for various applications, such as dynamic wave plates, spatial light modulators, adaptive wavefront control, signal monitoring and detection.

We show that periodic nanostructures of high imaginary part of permittivity (HIPP) materials can support well- localized modes, and spectral position of the modes are mainly defined by the structure periodicity. Propagating surface waves are supported by the interface of materials with positive permittivities if at least one of them has a non-zero imaginary part, and these waves are called Zenneck modes. Recently an interference of Zenneck waves has been imaged on the surface of transition metal dichalcogenides (TMDCs) where the modes are mainly defined by the layer thickness and interfaces with air and substrate [1]. Here, we show that Zenneck modes can also be localized on subwavelength particles and facilitate direction light scattering, Kerker-effect, and reflection suppression. First, we demonstrate the effect with spherical particles and hypothetical permittivity ε = 1 + 18i to emphasize the crucial role of HIPP, and then we analyze several practical cases such as particles of TMDC material and disk shapes.
The single-particle resonance is relatively weak, but the periodic arrangement of the particle supports collective lattice resonances [2] and plays a crucial role in the observation of localized Zenneck waves. Resonance position can be controlled by the lattice period [3], and electric dipole and electric quadrupole moments of the particles are controlled independently with a possibility to overlap. As an example, we show that simultaneous excitation of the electric dipole and electric quadrupole lattice resonances allows achieving significant reflection suppression and observation of a generalized lattice Kerker effect. Full-wave numerical simulations with a finite-difference time-domain method agree well with analytical calculations based on coupled dipole-quadrupole equations. One can engineer nanostructures of TMDC, vanadium oxides, and many other commonly occurred materials with HIPP to enhance the performance of photonic devices based on such HIPP materials.

Acknowledgment: This material is based upon work supported by the Air Force Office of Scientific Research under Grant No. FA9550-16-1-0088.

[1] V.E. Babicheva, S. Gamage, L. Zhen, S.B. Cronin, V.S. Yakovlev, Y. Abate, "Near-field Surface Waves in Few-Layer MoS2," ACS Photonics 10.1021/acsphotonics.7b01563 (2018).
[2] V.E. Babicheva and A.B. Evlyukhin: Resonant lattice Kerker effect in metasurfaces with electric and magnetic optical responses. Laser and Photonics Reviews 11, 1700132 (2017).
[3] V.E. Babicheva and A.B. Evlyukhin: Metasurfaces with electric quadrupole and magnetic dipole resonant coupling, ACS Photonics 5, 2022 (2018).

For the past ten years, plasmonic surface lattice resonances (SLRs) have been a growing topic of interest for photonic and plasmonic devices. We report on the fabrication of SLR-supporting arrays of aluminum nanodisks on glass, and the coupling of these SLRs to organic dye excitons on arrays coated with dye or dye-doped polymer films. In particular, the interaction of the SLRs with the excitons of the pyrromethene laser dye P580 is examined via angle-resolved transmission, reflection, and photoluminescence spectroscopy. Through variation of the concentration of the dye, the plasmonic particle diameter, and the spacing of the particles in the periodic lattice, the wavelengths of the relevant resonant modes and the Rayleigh anomaly are varied to determine the strength of the interaction between these modes, as well as the wavelengths and dispersions of the resulting SLR-exciton hybrid modes. The effects of coupling between the SLRs of various arrays with dye excitons, including angle-dependent emission, are reported.

Surface plasmon resonances confine light into subwavelength dimensions which results in a corresponding increase in electric field intensities near plasmonic nanostructures. Phase-matching conditions are exploited to enable nonreciprocal optical propagation and enhanced magneto-optic responses in magnetoplasmonic systems [1]. Here we show that exploiting diffraction in conjunction with plasmon excitations adds further versatility and flexibility in the design of photonic systems. As a testbed we analysed transverse magneto-optic Kerr (TMOKE) responses in magnetoplasmonic gratings etched into gold/cobalt multilayers. The grating coupler was chosen as the simplest system where we can combine the three distinct phenomena: magneto-optics, diffraction and plasmonics. The presence of transverse magnetization modifies the propagation conditions of the SPPs along the grating and lifts the degeneracy between the forward- and backward propagating modes. Angular resolved measurements revealed narrow line-shape plasmon resonances that were accompanied by large magneto-optical intensity effects in the diffracted beams. We show that exploiting plasmon resonances in diffraction gratings allows unexpectedly large diffracted TMOKE responses that exceed 3% - up to one of order of magnitude larger than reported diffracted magneto-optical effects in systems where plasmon resonances are not present [2,3]. Our results pave the way towards using magneto-optical modulation of SPPs to build non-reciprocal diffraction components that could be used to control the direction of propagation of light beams.
[1] V. I. Belotelov et al., Nat.Nanotechnol. 6, 370 (2011).
[2] O. Geoffroy et al., Journal of Magnetism and Magnetic Materials 121, 516-519 (1993)
[3] J. L. Costa-Krämer et al., Nanotechnology 12, 239-244 (2003)

Nanoscale metamaterials exhibit extraordinary optical properties and are proposed for various technological applications. Here, a new class of novel nanoscale two-phase hybrid metamaterials is achieved by combining two major classes of traditional plasmonic materials, metals (e.g., Au) and transition metal nitrides (e.g., TaN, TiN, and ZrN) in an epitaxial thin film form via the vertically aligned nanocomposite platform. By properly controlling the nucleation of the two phases, the nanoscale artificial plasmonic lattices (APLs) consisting of highly ordered hexagonal close packed Au nanopillars in a TaN matrix are demonstrated. More specifically, uniform Au nanopillars with an average diameter of 3 nm are embedded in epitaxial TaN platform and thus form highly 3D ordered APL nanoscale metamaterials. Novel optical properties include highly anisotropic reflectance, obvious nonlinear optical properties indicating inversion symmetry breaking of the hybrid material, large permittivity tuning and negative permittivity response over a broad wavelength regime, and superior mechanical strength and ductility. The study demonstrates the novelty of the new hybrid plasmonic scheme with great potentials in versatile material selection, and, tunable APL spacing and pillar dimension, all important steps toward future designable hybrid plasmonic materials.

Infrared (IR) absorption spectroscopy is one of the most powerful analysis tools, as it extracts essential information of chemical bonds and molecular structures in a label-free fashion. However, the low sensitivity which is originated from the intrinsically low absorption cross sections severely limits their practical applications. Plasmonic metamaterials-based IR spectroscopy has emerged as an promising approach that improves the sensitivity by exploiting the localized enhanced electromagnetic field (i.e., hot-spots) in plasmonic materials. This effect, however, is only effective when target molecules are located at the enhanced electromagnetic fields. It is thus of significance to control the spatial overlapping of molecules and hot-spots, yet it is a long-standing challenge.
Here we propose a plasmonics-nanofluidics hydrid device that enables the controllable delivery of molecules into the enhanced field of the plasmon modes. The device consists of a nanofluidic channel with a depth of several tens of nanometers sandwiched between a metal film and a batch of periodic nano square-disks. The structure forms a quadrupole resonant mode that traps the plasmonic energy inside the gap between two metal layers. This feature is exploited for ultra-sensitive detection and quantitative measurement of biomolecules. Furthermore, taking the advantage of precisely controlling the confinement of plasmonic fields inside the nanofluidic gap, we have also succeded in characterizing the molecular structures of nanoconfined water.
The performance of our device in ultra-sensitive detection of molecules and protein in aqueous solution were demonstrated. The device was purposely designed to spectrally overlap with the vibrational modes of the target molecules. When the analyte solution is introduced into the nanofluidic channel, the reflectance spectrum of the device reveals the signals of molecular vibrational modes as distinct peaks in the broad reflectance dip of the plasmonic structure. Our method confirmed the enhancement of sensitivity up to two orders of magnitude, with respect to previous reports.
Our device was also applied to measure the infrared absorption characteristic and elucidate the molecular structures of water confined in a 10 nm gap. It reveals the presence of a strong H-bond network with respect to bulk water, and the scaling behavior of confined water in the several tens of nanometer size regime. This effect is also found not being driven by the interaction with the interfaces; yet the constrained geometry itself promotes the intermolecular interactions of water and results in the modification of the H-bond network.
In conclusion, a plasmonics–nanofluidics hybrid device was proposed and demonstrated by empolying top-down fabrication techniques. Our device has offered an ultrasensitive platform for detection of molecules and in-situ probing of molecules or chemical reactions beyond the nanoconfinement.

Nanoscale confinement of light with the aid of plasmonics has stimulated research towards a number of future technologies including nanophotoniccircuits, on-chip micro- and nanosensor arrays, super-resolution imaging and designer flat-optics. Within this field, active plasmonics, for which the optical properties can be tuned in real time by an external stimulus are of great interest. Here, magnetoplasmonics provides a promising avenue for real-time dynamic plasmonic devices with the use of an external magnetic field [1,2].

Here, we use a top-down approach to fabricate large area ordered arrays of magnetoplasmonic nanocone antennas. Using electron beam lithography a nanodiskhard-mask is patterned on the Au/TbCo film. By tuning the mask diameter, either complete or truncated nanocones are produced after the Ar⁺ ion milling process. The base diameters range between 100 to 200nm with the plasmonic body of Au and TbCo nanocone tip. The latter is ferromagnetic at room temperature exhibiting perpendicular magnetic anisotropy. We characterise the magneto-optical enhancement as a function of wavelength and incidence angle. An enhancement of the Faraday rotation and ellipticity is observed at the plasmon resonance.

Varying the incidence angle altersthe contributions of vertical and base plasmon modes of the nanocone antenna, which could enable the generation of circularly-polarized electromagnetic near-fields in the vicinity of the nanocone tip. Preliminary simulations indicate that the nanocone geometry can sustain the input circularly polarisation, with an opticalspin,generated at the tip. Note that amorphous TbCo is known to exhibit helicity-dependent all-optical magnetization switching [3]. The combination of the TbCo tip and the Au plasmonic antenna capable of enhancing the opticalspin could therefore pave the way for the plasmon-assisted optical magnetization switching at the nanoscale.

References:
1. Zubritskaya, I. et al. Nano Lett. 15, 3204–3211 (2015).
2. Melander, E. et al. Appl. Phys. Lett. 101, 063107 (2012).
3. Mangin, S. et al. Nature Materials 13, 286–292 (2014).

Novel photonic materials not only rely on the properties of individual components, but also depend on the integration approach and structure of the building blocks. Here we report a new type of material assembly with emergent photonic properties: mesoscale optical matter clusters self-assembled from silver nanoparticles under laser illumination. These nanoparticles are discrete in space yet behave as a rigid body due to strong mesoscale electrodynamic interactions. The clusters rotate opposite to the direction of the incident beam’s angular momentum, i.e., a negative optical torque behavior. Normal materials, e.g., the individual Ag nanoparticles, will only show positive optical torque. We will discuss the rich experimental phenomena observed in the optical matter system and reveal the mechanism for the emergence of negative optical torque.

The application of conjugated polymers for various applications is expanding due to the introduction of pronounced electro-active behavior by doping. Some of these polymers include polyacetylene, polyaniline, polypyrrole, polythiophene as well as their various derivatives. Doping can switch a polymer from an insulator or a semi-conductor to a metal-like conductor and, is crucial for various electronic and optoelectronic applications. In particular, doped conjugated polymers can behave like disordered metallic materials and can exhibit conductivities ranging from ~10 S/cm to ~10,000 S/cm. We hypothesize that the afore-mentioned conductivities could result in a plasma frequency similar to that of metals, but in the mid- or far-infrared spectral region. Conventionally, heavily-doped inorganic semiconductors and metals are used as infrared plasmonic materials, but they are energy-intensive to produce due to high process temperatures and vacuum-based deposition. Utilizing conjugated polymers that are solution and room-temperature processable as infrared plasmonic materials would reduce the process energy and associated costs. However, a detailed study is required to prove that highly-doped polymers are suitable infrared plasmonic materials.
In this study, we prepare conjugated polymer (polythiophene) thin films on an indium-tin oxide (ITO) coated glass substrate using spin coating followed by electrochemical doping using tetrabutylammonium perchlorate. A three-electrode setup is employed using the polymer-coated ITO-glass as the working electrode, a platinum counter electrode and a non-aqueous reference electrode. There was an immediate change in the color of the films from a bright pink (undoped film) to dark blue (doped film) upon electrochemical doping. The surface morphology of the undoped film was smoother when compared to the doped film which exhibited local micron-sized clusters and increased roughness due to dopant inclusion. The electrical properties are compared between the doped and undoped polythiophene films using four-point probe conductivity measurements. Preliminary measurements show a clear increase in the conductivity because of doping. However, contributions from the underlying ITO must be deconvoluted from the data before conductivity can be accurately quantified. Both transmission and reflection-mode infrared spectroscopy are used to measure changes in the infrared absorption of the doped and undoped polythiophene films and to determine the plasma frequency. A relation between conductivity (controlled by doping concentration) and plasma frequency can be established using the Drude model which allows us to identify the doping concentration required for a maximum plasmonic response. If optimum doping concentrations can be obtained, doped conjugated polymer materials could be a class of low-embodied-energy plasmonic material used for sub-wavelength waveguides, gratings and nano-particles with strong infrared optical responses.

We use single particle spectroscopic imaging and statistical analysis to assess the plasmonic enhancement of NIR-to-visible upconversion luminescence (UCL) from single β-NaYF₄:Yb³⁺:Tm³⁺ upconverting nanoparticles (UCNPs) supported on substrates consisting of random arrangements of Ag nanowires (NW) and Au nano-cavity arrays. We measure both apparent luminescence enhancement, which is power dependent due to the nonlinear kinetics of energy transfer upconversion (ETU), and the excitation enhancement, defined as the ratio of the excitation intensity needed to produce a given UCL emission, divided by the equivalent power needed to produce the same emission on the plasmonic substrate, which we showed recently is power-independent over 5 orders of magnitude for Er³⁺ doped UCNPs on Au nano-cavity arrays¹. By examining the effects at the single particle level, and accumulating a statistical sampling of single particle emitters, both on and off the plasmonic substrates studied, we eliminate the effects of particle fluctuations on the apparent UCL emission enhancement, and observe a significantly larger excitation enhancement on the Ag NW substrate than previously reported (up to 36X, compared to 4.6X for Er³⁺ doped UCNPs on Au Nano-cavity arrays). We compare these results to statistical analysis of ensemble measurements made of Er³⁺ doped UCNPs supported on random Ag NW substrates².

(1) Fisher, J.; Zhao, B.; Lin, C.; Berry, M. T.; May, P. S.; Smith, S. Spectroscopic Imaging and Power Dependence of NIR to Visible Upconversion Luminescence from NaYF4:Yb3+,Er3+ Nanoparticles on Nano-Cavity Arrays. J. Phys. Chem. C 2015, 119, 24976–24982.
(2) Hor, A.; Luu, Q.; May, P.; Berry, M.; Smith, S. Non-Linear Density Dependent Upconversion Luminescence Enhancement of β-NaYF4: Yb3 : Er3 Nanoparticles on Random Ag Nanowire Aggregates. MRS Adv. 2016, 1, 2677–2682.

Plasmonic-based sensors exploit sensitivity of plasmon resonance conditions to the dielectric constant of the local environment. However, the resonances are usually broad; in order to enhance the sensitivity, the presence of sharp features in angular or spectral behavior of the plasmon resonance is very desirable. In our work we explore two approaches, involving such features. All optical approach uses the extra-sharp peak observed in the reflection from gold nanostrips. The reflectivity from the gold nanostrips immersed in an ethanol-water solution demonstrates a very high sensitivity to small variations in ethanol concentrations at the angle of the sharp reflection peak. The second approach involves electrical detection which does not require a bulk optical setup, and is based on strong enhancement and polarity switching of the photo-induced voltages (plasmon drag effect) under plasmon resonance conditions observed in the strongly nanostructured systems. We show that the presence of an additional polymer layer deposited on the top of the structure significantly shifts the angular position of the signal. Further studies are in progress.

Development of surface plasmon polaritons (SPPs) based optoelectronic devices is restricted mainly because of a high metallic energy loss and a weak nonlinearity of SPPs. This issue can be potentially addressed by making them work in the strong-coupling regime where the coupling strength outcompetes the energy loss. One way to achieve strong coupling is to couple a nonlinear material such as quantum emitter with the metallic nanostructure. In this work, the nanosphere lithography technique is used to fabricate a large area two-dimensional film-coupled gold nanopyramid array which supports gap plasmon mode. By incorporating a layer of quantum emitters at the gap, strong coupling is demonstrated between quantum emitters and the gap plasmon mode. This study has strong implications in designing state-of-the-art optoelectronic devices.

The negatively charged nitrogen-vacancy (NV^-) centre in diamond is extremely promising as an optically addressable defect for quantum applications. Only the NV^- state can be optically polarised and read out, however NV centres normally undergo photoionised charge-state conversion between NV^- and a neutral state. A 532 nm laser pulse can be used to repump the centre into the desired state; but the repumping can also change the electrostatic environment of the NV centre, shifting the optical transition frequency by several linewidths. Improvements in spectral stability have been shown by repumping at 575 nm, however this still relies on laser pumping to maintain the NV^- state. The recent demonstration of the state-of-the-art in quantum entanglement in diamond, with entangling rates of up to 39 Hz [1] also illustrates the limitations posed by repumping to maintain the NV^- state. Of their total ~75.5 μs protocol, ~50 μs is used for checking that the NV remains in the NV^- state and is on-resonance, and if the neutral state is detected, an additional 300 μs is needed to rempump into the NV-, which can be repeated many times before the entanglement experiment can begin. A novel solution to minimise repumping involves doping single crystal diamond (SCD) material n-type, so excess electrons stabilise the NV^- charge state, without initialisation. A recent study [2] using the n-type dopant phosphorus (P) in CVD grown SCD demonstrated NV centres that remained stable in the NV^- charge state. The precise control of the P concentration is critical for achieving long spin coherence (T₂) times, with T₂ ≈ 0.8 ms already demonstrated for P-doped diamond [3]. Achieving controllable levels of P is an area of significant current interest [4-6] and so controlled parameter studies of the growth of P-doped diamond for quantum applications are needed.

We have grown a series of thick SCD samples with varying P/C in the plasma feedgas on (111) oriented Sumitomo substrates in a series of 24 h deposition experiments at 1000 °C. We have also grown a second series of 24 h deposition P-doped growth experiments with 0.5% oxygen in the plasma feedgas with varying concentrations of methane, for comparison of NV properties across a wide parameter space. FTIR and Hall effect results confirm the P-incorporation at the high growth temperature studied (1000 °C). We will present photoluminescence mapping and spectroscopy results, and assess the factors affecting the quality of the grown films for NV^- stability and performance, to give strategies for achieving the growth of high quality n-type single crystal diamond for quantum applications.

References
1. P.C. Humphreys, et al. Nature 558, 268–273 (2018)
2. Y. Doi, et al., Phys. Rev. B. 93, 081203 (2016)
3. H. Morishita et al. arXiv:1803.01161 (2018)
4. R. Ohtani, et al., Appl. Phys. Lett. 105, 232106 (2014)
5. H. Kato, et al., Appl. Phys. Lett. 109, 142102 (2016)
6. T. A. Grotjohn, et al., Diamond Relat. Mater. 44, 129 (2014)

In recent years, research at the interface of chemistry, material science, and quantum optics has surged and now opens new possibilities to study strong light-matter interactions at different limits [1,2].

Combining theoretical concepts from the fields of material science and quantum optics presents an opportunity to create a predictive theoretical and computational approach to describe cavity correlated electron-nuclear dynamics from first principles. Towards this overarching goal, we introduce a general time-dependent density-functional theory to study correlated electron, nuclear and photon interactions on the same quantized footing [3].

In this talk, we demonstrate how Rabi splitting under strong light-matter coupling emerges in the absorption spectra and analyze cavity-modulated molecular motion of CO₂ molecules in optical cavities. Further, we use this novel framework to study how the potential-energy surfaces (PES) of a CO bond stretching in an ensemble of Formaldehyde molecules is modified under collective strong-light matter coupling, demonstrating the novel abilities to alter and open new chemical reaction pathways as well as to create new hybrid states of light and matter in this regime.

Our work opens an important new avenue in introducing ab initio methods to the nascent field of collective strong light-matter interactions.

[1] J. Flick, N. Rivera, P. Narang (under review) 2018.
[2] F. Benz et al., Science, 354 6313, 726-729 (2016).
[3] J. Flick, P. Narang, arXiv:1804.06295 (2018).
[4] J. Flick, A. Winniki, P. Narang (in preparation) (2018).

White LEDs (WLEDs) compared to traditional illumination have higher efficiencies, longer lifetimes and are also environment-friendly. Commercial technology uses yellow-emitting cerium-doped yttrium aluminum garnet [(Y_1-xCe_x)₃Al₅O₁₂ or YAG:Ce)] phosphor which is excited by a InGaN-based blue LED. However, such phosphors suffer from the following disadvantages: (1) limited phosphor performance due to thermal degradation, (2) significant backscattering losses, and (3) poor absorption. Current commercial WLEDs have a luminous efficacy around 140 lm/W, with research prototypes showing higher values, but with a trade-off in the color rendering indices (CRIs). To work towards the US Department of Energy target of a luminaire of 225 lm/W by 2025, it is necessary to develop new designs for phosphors for WLEDs with high efficiency and better color rendering.
Here, we propose and study theoretically core-shell (CS) metal-semiconductor and core-shell-shell (CSS) semiconductor-metal-semiconductor nanowires (NWs) as phosphor components in white LEDs, using a Mie formalism for absorption and a Green’s function approach for emission. The plasmon resonance oscillations at the metal surface couple with the electric fields of the incident light enabling an enhanced absorbance. We have shown that CS NWs show an absorbance of 0.6-0.9, whereas CS Quantum dots show only between 0.2-0.4. Utilizing the plasmon resonance of the metals, the core multi-shell nanowires improve the optical density of states and hence boost the emission rate, and correspondingly the power of the emitted radiation. Both these factors combined lead to an increase in the External Quantum Efficiency (EQE) of the NW phosphor, which is used as the judging parameter for the NW phosphors. We have predicted that the EQE can be enhanced by 11 times for red phosphors, by 36 times for yellow phosphors and as high as four orders of magnitude for the green phosphors relative to the bare semiconductor nanowires, when carefully choosing the semiconductor and metal materials and dimensions. We have also predicted the best dimensions and material combinations for the CS/CSS NW phosphors. We also show that the EQE of these NW phosphors can be boosted further by as much as 4.5 times when using emerging plasmonic materials such as TiN and ZrN, compared to the EQE values when using traditional plasmonic metals (Au, Ag). These materials are relatively cheaper than the traditional plasmonic metals and their ease of fabrication and integration make them ideal candidates for the use of phosphors. CSS NWs further improve values of the EQE by 1.5 times relative to the CS nanowires for red phosphors and as high as 3 times for yellow phosphors, due to the coupling of another enhanced electric field from the semiconductor core to the outer shell, resulting in an increased emission. Experimental progress based on the simulation results will also be discussed.

The prospering family of two-dimensional (2D) transition metal carbides/nitrides, known as MXenes, have exhibited a variety of unique optical and optoelectronic properties, making them attractive for many potential sensing and photonic applications. Recently, nanosheets of the most studied MXene by far; i.e., Ti₃C₂T_x (T_x denotes surface functional groups such as =O or -F), were shown to exhibit intense surface plasmons (SPs) when excited by electron beams. However, their spatial variation over individual Ti₃C₂T_x nanosheets remains undiscovered.
In this work, we used scanning transmission electron microscopy (STEM) combined with ultra-high resolution electron energy loss spectroscopy (EELS) to investigate the spatial and energy distribution of SPs (both optically-active and -forbidden modes) in mono- and multi-layered Ti₃C₂T_x nanosheets. By means of spatially resolved STEM-EELS mapping we were also able to directly visualize the inherent inter-band transition in addition to a variety of SP modes (both transversal and longitudinal), and correlate them with the shape, size and thickness of nanosheets. The independent polarizability of Ti₃C₂T_x nanosheets is unambiguously demonstrated, and attributed to their unusual weak interlayer coupling. This characteristic makes engineering a new class of nanoscale systems possible, where each monolayer in the multi-layered structure of Ti₃C₂T_x has its own set of SPs with distinctive multipolar characters.
Furthermore, we probed the tunability of the SP energies by conducting in-situ heating STEM to monitor the change in the surface functionalization of Ti₃C₂T_x through annealing at temperatures up to 900 ^oC. At temperatures above 500 ^oC, fluorine (F) desorption multiplies the metal-like free electron density of Ti₃C₂T_x flakes, resulting in a monotonic blue-shift in the SP energy of all modes. Our results highlight the great potential of Ti₃C₂T_x for photonic applications, ranging from visible to MIR ranges, such as broadband photodetectors and plasmonic waveguides.

Plasmonics utilizes surface plasmons that are generated when light enters the interface between metal and dielectric materials, resulting in new optical properties not seen in nature. Using plasmonic materials such as metals and dielectrics, various studies have been intensively performed to control optical characteristics. Among them, a plasmonic color filter that selectively extracts light from the visible spectrum by inserting a nanostructure into a plasmonic material has been reported.[1] In plasmonic color filter research, improving the transmittance and color reproducibility are the main issues.[1,2] In addition, using a transparent conducting oxide, there was an attempt to produce a plasmonic color filter with good electrical conductivity as well as high transmittance. As a result, plasmonic color filter electrodes (PCEs) that function at the same time as conductive electrodes as well as a color filters have been developed as a new type of color filter.[3]
Most plasmonic color filters have been fabricated through patterning methods such as nano-imprinting or e-beam lithography. In terms of cost and processing time, such patterning methods have disadvantages for large areas; this has limited their application to display devices. However, in our previous research, PCEs were implemented with laser interference lithography, which is beneficial for large area patterning and allows for practical large-area applications such as displays and lightings.
In this study, we first proved the feasibility of fabricating PCEs embedded in organic light emitting diodes. Each PCE for red and green light filtering was applied to a light-emitting device, instead of using an ITO electrode, which is used in counterpart devices as an anode. Fabricated devices showed diode operation characteristics, which verifies that PCEs performed electrically in a stable fashion. Also, the broad electroluminescent spectra in the above devices, ranging from 500nm to 700nm, were selectively filtered into red and green light, demonstrating the optical performance of the PCEs. Consequently, these results confirm that PCEs function as color filters and electrodes simultaneously.
The optimized PCE structure has a height of within some hundreds of nm. This thin and metal-based structure is advantageous for malleable devices that are flexible, stretchable, and etc. In addition, since PCE is based on metal, it can overcome the drawbacks of conventional color filters, e.g. dyes and pigments, which have poor thermal and chemical stability. Therefore, the PCEs in this work, are not simply used to reduce device structure by integrating two functions (color filter and electrode). We expect this work will open new opportunities to develop and expand the use of novel plasmonic nanostructures.
[1] Q. Chen et al., Optics Express, 2010
[2] Y. S. Do et al., Advanced Optical Materials, 2013
[3] Y. G. Moon, Y. S. Do, M. H. Lee et al., Scientific Reports 2017

The quest for alternative plasmonic materials is vital for further progress in plasmonics-based photonics. Nanostructures, which can act simultaneously as optical detectors, stimulators, or photocatalysts are crucial for many applications in the fields of chemo- and biosensing, electro- and photocatalysis, electrochemistry, and biofuel generation. Plasmonic materials with a huge surface-to-volume ratio as well as a high density of active surface sites and electromagnetic hot spots, are desirable. In many cases, however, a large surface-to-volume ratio is counterproductive due to surface damping of the plasmonic resonances.
We will discuss the plasmonic properties of sponge-like, in other words, fully porous, nanoparticles, so-called nanosponges.¹ In such sponges, both the gold and the air phase are fully percolated in three dimensions, providing a huge surface area but still sufficiently sharp plasmonic resonances. We correlate, on a single nanoparticle basis, their optical scattering spectra (using dark field microscopy) with their individual morphology (using electron microscopy).² The scattering spectra of such gold nanosponges depend only weakly on their size and outer shape, but they are decisively influenced by their unique inner percolation pattern, in good qualitative agreement with numerical simulations.
In addition to scattering, we investigate the photoluminescence (PL) from gold nanoparticles due to electron-hole recombination^3-4 in the specific case of gold nanosponges. We find that the polarization anisotropy is much larger in the scattering spectra than in the PL spectra for individual gold nanosponges. We interpret this finding that the excitation of local plasmons by a plane wave from the far field (scattering) and the local creation of plasmons via electron hole recombination with subsequent radiation to the far field (PL) are not reciprocal. We interpret this finding by the causality principle and the existing of a “plasmonic horizon” of less than 60 nm,⁵ given by the limited lifetime of plasmons (less than 10 femtoseconds) and by the finite speed of information.

1. Wang, D.; Schaaf, P., Nanoporous gold nanoparticles. J. Mater. Chem. 2012, 22, 5344-5348.
2. Vidal, C.; Wang, D.; Schaaf, P.; Hrelescu, C.; Klar, T. A., Optical Plasmons of Individual Gold Nanosponges. ACS Photonics 2015, 2, 1436-1442.
3. Sivun, D.; Vidal, C.; Munkhbat, B.; Arnold, N.; Klar, T. A.; Hrelescu, C., Anticorrelation of photoluminescence from gold nanoparticle dimers with hot-spot intensity. Nano Lett. 2016, 16, 7203.
4. Cai, Y. Y.; Liu, J. G.; Tauzin, L. J.; Huang, D.; Sung, E.; Zhang, H.; Joplin, A.; Chang, W. S.; Nordlander, P.; Link, S., Photoluminescence of Gold Nanorods: Purcell Effect Enhanced Emission from Hot Carriers. ACS Nano 2018, 12, 967-985.
5. Vidal, C.; Sivun, D.; Ziegler, J.; Wang, D.; Schaaf, P.; Hrelescu, C.; Klar, T. A., Plasmonic Horizon in Gold Nanosponges. Nano Lett. 2018, 18, 1269-1273.

Hybrid nanostructures containing plasmonically active metallic nanoparticles exhibit many effects interesting from both fundamental and application perspectives. For instance, silver nanowires (NW) with submicron diameters and lengths of about tens of microns can act as energy propagators, by employing surface plasmons polaritons (SPP). These quasiparticles can not only propagate along a metal-dielectric interface for distances extending tens of microns, but also can remotely activate luminescence of emitters located in the closest vicinity of the NW.
The ability to remote activation of emission is demonstrated using both Stokes and anit-Stokes emitters. For this purpose we constructed a two-objective confocal fluorescence microscope, where the top objective is used for excitation of SPPs in a NW, while the detection of fluorescence is facilitated with the second one. The experimental setup allows for probing both stationary and time-resolved fluorescence of the photosynthetic proteins (Stokes emitters) and up-converting nanocrystals (anti-Stokes emitters).
The results indicate that the efficiency of SPPs propagation and luminescence activation depends strongly on the length of the nanowire as well as polarization of the laser beam. In addition, we show that the substrate affects the efficiency of the remote activation, which is less effective on graphene. Systematic analysis of many nanowires characterized with different lengths allowed to study the influence of the graphene substrate on the damping efficiency, which originates from polariton-graphene interactions. The efficiency of this process is also found to depend on the laser polarization and is more efficient for laser polarized along the nanowire.

S. Mackowski. et al., Nano Letters 8, 558-564 (2008).
A. Prymaczek, M. Cwierzona, J. Grzelak, D. Kowalska, M. Nyk, S. Mackowski, D. Piatkowski, Nanoscale, 2018, in print
M. Szalkowski, K. Sulowska, J. Grzelak, J. Niedziolka–Jonsson, E. Rozniecka, D. Kowalska, S. Mackowski, Sensors 18, 290/1-10 (2018)
D. Piatkowski, N. Hartmann, T. Macabelli, M. Nyk, S. Mackowski, A. Hartschuh, Nanoscale 7, 1479–1484 (2015)

Research was partially financed by the National Science Centre (Poland) within the projects no 2016/21/B/ST3/02276, 2017/27/B/ST3/02457, 2017/26/E/ST3/00209, and the project 3/DOT/2016 funded by the City of Gdynia, Poland

The growing interest in Terahertz (THz) radiation in a wide range of application fields is due to recent technological advances that have made emitters and detectors available in this spectral range. The THz radiation is particularly suitable for the study of systems, ranging from textiles to biological tissues, whose structural complexity makes unavoidable understanding how scattering affects its propagation [1].
On the other hand the propagation of electromagnetic (em) waves in random media is itself a topic of considerable interest in many research fields, since it can lead to a plethora of phenomena. With respect to standard techniques in the visible range, the detection technology available in the THz spectral range provides the advantage to easily access the amplitude and the phase of the em field.
We report the measurements of the electric field transmitted by media with a three-dimensional disorder, obtained by means of THz time-domain spectroscopy.
The samples are made by dispersions of 1 mm diameter Silica spheres in a Paraffin matrix with refractive index n_Si=1.95 and n_p=1.5 respectively, prepared at filling fraction, ν, ranging from 0.05 to 0.5. These dispersions are then solidified in cylindrical molds with 15mm diameters and variable thickness L.
We collected the temporal signal E(t) transmitted by the sample over 800 ps at different filling fractions. By taking the Fourier transform of these signals, we can extract the spectral amplitude, E(ω), and phase, φ(ω), in a frequency range between 0.2 and 2 THz. We analyzed these quantities by following the paradigm [2] of light diffusion in order to explore the various transport regimes of THz radiation.
At low filling fractions, the spectral amplitude E(ω) shows signatures of Mie scattering that is gradually washed out by the increasing de-coherence effect introduced by the increasing number of scatterers. In order to provide a dynamic observable able to quantitatively characterize the photon transport regime, we estimate the group delay τ_g = d[φ(ω)]/dω. This quantity provides the product between the inverse of group velocity and the propagation length L inside the sample. We then compare τ_g with the phase delay τ_p obtained as the ratio between the speed of light and the mean refractive index of the samples, <n>=νn_Si+(1-ν)n_p. The discrepancy between τ_g and τ_p increases for ν >0.1 denoting a transition from a ballistic to a diffusive transport regime. At ν >0.25, τ_g reaches a plateau, signature of the onset of photon trapping mechanism.

[1] Terahertz spectroscopy and imaging—Modern techniques and applications, P. U. Jepsen et al., Laser Photonics Rev. 5, 124 (2011).
[2] Ultrashort pulse propagation and the Anderson localization, Gentilini, Opt.Lett. 34, 130 (2009).

Enhancement of optical fluorescence has enabled breakthroughs in areas such as fluorescence-driven DNA sequencing, rapid disease detection, and other numerous biological studies.^1,2 While several plasmonic nanostructures eliciting strong fluorescence enhancement have shown promise for these applications, they have been restricted to detection of small dye molecules due to their shallow and relatively flat 2D electromagnetic (EM) enhancement profile.^3,4 However, bioassays in general utilize small dye molecules conjugated to larger proteins, and small dye molecules alone are rarely used in research and clinical diagnostics. Therefore, successful application of nanoplasmonics-enhanced bioassays requires that the devices exhibit excellent EM enhancement with a 3D volume that could accommodate molecular probes of all sizes.

Here, we have demonstrated a 3D tapered nanofocusing plasmonic nanocavity for fluorescence enhancement using probes of all sizes and targeted single-molecule detection. Our device focuses propagating surface plasmon polaritons (SPP) into a 3D tapered metal-insulator-metal (MIM) cavity that provides extreme volumetric enhancement for visualization of single molecule fluorescence. We fabricated our device on a silicon substrate with gold and silica layers to form a 3D-tapered Au-SiO₂-Au plasmonic cavity. The plasmonic nanocavity achieves strong EM-field confinement⁵ while the channel-like geometry of the nanocavity and attached waveguide provides test-fluid access and host surface functionalization for capturing molecules. A large, spatially uniform plasmonic hotspot plane at the tip of the 3D-tapered MIM gap allowed uniform visualization and molecular-specific capture of diversely-sized molecules diffusing in solution, including single-stranded DNA, streptavidin and anti-biotin antibodies with calculated fluorescence enhancement close to 1500. Appropriately optimizing the tip geometry also allowed individually capturing and detecting single antibody molecules at the tip.

References:
1 Eid, J. et al. Real-time DNA sequencing from single polymerase molecules. Science 323, 133-138 (2009).
2 Yokoe, H. & Meyer, T. Spatial dynamics of GFP-tagged proteins investigated by local fluorescence enhancement. Nat. Biotechnol. 14, 1252-1256 (1996).
3 Kinkhabwala, A. et al. Large single-molecule fluorescence enhancements produced by a bowtie nanoantenna. Nat. Photonics 3, 654-657 (2009).
4 Punj, D. et al. A plasmonic/antenna-in-box/'platform for enhanced single-molecule analysis at micromolar concentrations. Nat. Nanotechnol. 8, 512-516 (2013).
5 Choo, H. et al. Nanofocusing in a metal-insulator-metal gap plasmon waveguide with a three-dimensional linear taper. Nat. Photonics 6, 838-844 (2012).

Polaritonic materials that support epsilon-near-zero (ENZ) modes offer the opportunity to design light-matter interactions at the nanoscale through extreme sub-wavelength light confinement, producing phenomena like resonant perfect absorption. However, the utility of ENZ modes in nanophotonic applications has been limited by a flat spectral dispersion, which leads to small group velocities and extremely short propagation lengths. In this talk, I will describe a strategy to overcome this constraint by hybridizing ENZ and surface plasmon polariton (SPP) modes in doped cadmium oxide epitaxial bilayers. This results in strongly coupled hybrid modes that are characterized by an anti-crossing in the polariton dispersion and a large spectral splitting on the order of 1/3 of the mode frequency. These hybrid modes simultaneously achieve modal propagation and ENZ-mode-like interior field confinement, adding propagation character to ENZ-mode properties. It is additionally possible to tune the resonant frequencies, dispersion, and coupling of these polaritonic-hybrid-epsilon-near-zero (PH-ENZ) modes by tailoring the modal oscillator strength and the ENZ-SPP spectral overlap. PH-ENZ modes ultimately leverage the most desirable characteristics of both ENZ and SPP modes, allowing us to overcome the canonical plasmonic tradeoff between confinement and propagation length.

The ability to confine light now routinely to of order 10 nm³ volumes enables new functionalities for optoelectronic switching at ultralow energies. We present a range of devices in which an active polymer is sandwiched into nm-scale plasmonic gaps that can deliver fast and reversible actuation. Using the thermo-responsive polymer PNIPAM combined with directed nano-assembly using Au or Ag nanoparticles, allows high densities of nano constructs to be created, that change colour either in response to light or heat. High throughput synthesis allows the scale-up of these actuating nano-transducers (ANTs), opening up video-rate large area display technologies. We show measurements on single ANTs which confirm that nN forces are developed, and discuss their application for optically-driven microfluidic valves. Close-packed films of ANTs act to pump water in and out of permeable membranes using light, and produce a switchable metamaterial. The integration of these devices with DNA origami machines is demonstrated for the first time. We also show that lithium can be optically-pumped in and out of perovskite materials, making light-rechargeable batteries.

References:
Dynamic- and Light-Switchable Self-Assembled Plasmonic Metafilms, Adv.Opt.Mat. 1800208 (2018); DOI: 10.1002/adom.201800208
Photo-Rechargeable Organo-Halide Perovskite Batteries, Nano Letters (2018); DOI 10.1021/acs.nanolett.7b05153
Thermo-responsive Actuation of a DNA Origami Flexor, Adv.Func.Mat. 1706410 (2018); DOI 10.1002/adfm.201706410
The Crucial Role of Charge in Thermoresponsive-Polymer-... Au Nanoparticles, Adv.Opt.Mat. 1701270 (2018); DOI 10.1002/adom.201701270
Actuating Single Nano-oscillators with Light, Adv.Opt.Mat. 1701281 (2018); DOI 10.1002/adom.201701281
Light-Directed Tuning of Plasmon ..Polymerization Using Hot Electrons, ACS Phot. 4, 1453 (2017); DOI: 10.1021/acsphotonics.7b00206
Light-induced actuating nanotransducers, PNAS 113, 5503 (2016); DOI 10.1073/pnas.1524209113

Key challenges limiting the adoption of plasmonic nanostructures for practical devices include the nanoscale fabrication, control over light-matter interactions at subwavelength scale, as well as thermal and structural durabilities. In this work, we demonstrate a unique hybrid plasmonic thin film, with metallic (e.g. Au, Ag) nanopillars being embedded in a robust transition-metal-nitride matrix as epitaxial, self-assembled metamaterial. The metallic nanopillars are well-ordered, confined within 10 nm diameters, and tailorable in terms of density and geometry. Our optical measurements coupled with numerical simulations demonstrate that such novel hybrid metal-nitride structures show strong optical tunability and anisotropy, enhanced Raman (SERS) and photoluminescence (PL) signals, which can be utilized for highly-tailorable optical metamaterial designs for sensing and high temperature applications.

Vanadium dioxide (VO₂), with its dramatic change in optical properties across its insulator-metal transition (IMT), is a promising material for a variety of applications, including optical limiting and isolation, thermometry, and switching. The key piece of information necessary for the design of optical components based on VO₂ is its temperature- and wavelength-dependent refractive index across the phase transition. Although there have been various studies characterizing the optical properties of VO₂ in different wavelength ranges, a complete, broadband dataset that spans the phase-transition region is not available. It is particularly difficult to find reliable datasets in the mid- and far-infrared spectral ranges.
In our talk, we will describe our optical characterization of thin-film VO₂ across its thermally driven phase transition, for free-space wavelengths from 300 nm to 30 μm. First, we used spectroscopic ellipsometry to extract the complex refractive index of VO₂ in its insulating (30 °C) and metallic (100 °C) phase. Then, we applied an effective-medium theory to calculate the refractive index at the intermediate temperatures across the phase transition. The results are verified by measuring temperature-dependent normal-incidence infrared reflectance using a different instrument (FTIR spectrometer) and comparing to calculations using the standard transfer-matrix formalism.
Because the properties of VO₂ are known to differ based on synthesis procedures and substrate choice, we repeat this process for films grown on several substrates (sapphire, Si, GaAs) using magnetron sputtering and sol-gel synthesis. Our results indicate that (1) there are differences in the optical properties of VO₂ synthesized under different conditions, but (2) the differences are relatively minor for various optics application that use the large switching ability of VO₂. As an example application of this dataset, we will briefly describe our design of infrared optical limiters and diodes based on VO₂ thin films.

Tailoring light-matter interaction plays an imperative role in the field of plasmonics. This relies on precise yet scalable fabrication of structures few nanometers in dimensions. The limitations of conventional top-down fabrication techniques in terms of resolution and precision can challenge fabrication of such devices as dimensions approach the few-nanometer regime. This becomes ever more important if structures require mechanical tunablility. In such architectures, the presence of dominating surface adhesive forces can lead to structural collapse and device failure. Such mechanical tunability if implemented successfully however can lead to the emergence of dynamically tunable plasmonic structures. In this work, we present an integrative fabrication approach in which directed self-assembly of chemically synthesized nanostructured building blocks are used to achieve functional plasmonic units. Here, selective surface functionalization of nanomaterials is combined with physical templating and external stimuli including light and electric field to promote the deterministic fabrication of devices with nanometer precision and uniformity. Through incorporation of mechanically reconfigurable components, we are able to further achieve dynamic tuanblity of the structures. As an example device, we will discuss precisely defined nanometer-thin plasmonic junctions that can be actively tuned in resonance through induced mechanical reconfiguration.

Dynamically tunable infrared filters open pathways to multifunctional optical components needed for hyperspectral applications. Capitalizing upon this technology requires directing the filtered light to a detector. Overwhelmingly, however, tunable infrared filters operate in a reflective geometry that necessitates a complex optical path. Tunable transmission filters, on the other hand, can be placed immediately in front of the detector or can even be monolithically integrated with the sensing element to allow for independent pixel-by-pixel tuning. For these reasons, we have developed wafer-scale tunable infrared filters operating in transmission based on graphene plasmonics that simultaneously offer > 75 cm^-1tuning while maintaining >40% transmission and requiring biases less than 10V. This technology is amenable to direct monolithic integration with LWIR detector technologies for which we have developed a proof of concept demonstration.

Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia LLC, a wholly owned subsidiary of Honeywell International Inc. for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.

In the last several years, metasurfaces have demonstrated promise to control constitutive properties of light via interaction with nanoscale elements. To date, the functional performance of metasurfaces has typically been fixed at the time of fabrication. However, active control of metasurface properties would enable the realization of novel low-profile optical components, which can be used in numerous technologically relevant applications. These applications include but are not limited to dynamic holograms, focusing lenses with reconfigurable focal lengths, and beam steering, a key requirement for future chip-based light detection and ranging (LIDAR) systems.
In the present work, we report a multifunctional electrically-tunable metasurface, which may act both as a beam steering device and a focusing lens with reconfigurable focal length, depending on the voltage application configuration. The proposed multifunctional metasurface is actively controlled by incorporating a material with voltage-tunable optical properties, into the metasurface. In our work, we use indium tin oxide (ITO) as a voltage-tunable material. First, we experimentally demonstrate an electrical control of the phase of the metasurface unit element at telecommunication wavelengths. By carefully choosing the properties of the ITO and gate dielectric layers, we show that the phase shift of the metasurface unit element can be actively changed from 0° to 300°. Our design enables independent electrical control of each metasurface element via individual application of the DC voltage. Next, using this property, we show that the same metasurface can exhibit multiple functionalities, acting both as a beam steering device and a reconfigurable lens.

Photonic integrated circuits (PICs) provide a platform to control a large number of electromagnetic modes with high phase stability for applications including beam formation, quantum photonic technologies, and optical neural networks. A central challenge in scaling such applications is the ability to program or trim large numbers of phase shifters in a non-volatile fashion to reduce power consumption and enable cryogenic compatibility. For example, to program an arbitrary linear-optics unitary transformation on N input/output modes, it is necessary to set O(N²) phase shifters, a number which can be in the hundreds ([1], [2], [3], [4]). Here, we investigate two schemes for non-volatile phase shifters to solve these scaling challenges in silicon-on-oxide (SOI) PIC platforms. These rely on (1) phase-change chalcogenide glasses and (2) shape-memory ceramics.

1. For phase modulators based on chalcogenides [5], we consider germanium-antimony-selenium (GSSe) deposited over an exposed waveguide section in the SOI PIC platform. We consider the use of built-in silicon heaters to control the phase of the GSSe through tailored heating-cooling curves. As shown previously, there is a strong index difference between the amorphous (n=2.99) and crystalline (n=3.60+0.0398i) states of GSSe [6]. Through eigenmode expansion simulations, we find an effective index shift on the order of 10^-2 for the fundamental mode of a silicon ridge waveguide, a corresponding length of 8.5 μm required for a π phase shift, and a loss of < 0.3 dB per phase shifter.

2. We also introduce a new concept for phase modulation based on the photoelastic effect in silicon, controlled via strain through nonvolatile actuators. In particular, we propose the use of shape-memory ceramics [7], such as ceria-doped zirconia, to strain SOI waveguides through transformations between the austenite (tetragonal) and martensite (monoclinic) states. We explore this non-volatile phase shifter concept through finite-element simulations based on experimentally measured material properties, and show that the shape-memory effect in polycrystalline zirconia can produce an effective index shift on the order of 10^-3 for the fundamental mode of a silicon ridge waveguide. A corresponding length of 40.4 μm is required for a π phase shift.

References:

[1] Harris, N.C. et al. Nature Photonics 11.7 (2017): 447
[2] Annoni, A. et al. Light: Science & Applications 6.12 (2017): e17110.
[3] Shen, Y. et al. Nature Photonics 11.7 (2017): 441.
[4] Wang, J. et al. Science (2018): eaar7053.
[5] Raoux, S. et al. Chemical Reviews 110.1 (2009): 240-267.
[6] Zhang, Y. et al. CLEO: Applications and Technology (2017), JTh5C. 4
[7] Lai, A. et al. Science 341.6153 (2013): 1505-1508.

Metasurfaces utilize strong interactions between light and two-dimensional nanostructured interfaces to control light at will, and enable us to break away from the reliance on light propagation by transferring an optical interface into functional devices. In this talk, I will describe my lab’s work on flat lenses and holograms based on metasurfaces. I will show you how we can control phase dispersion, i.e., phase as a function of wavelength, to create metalens triplets that correct both chromatic and monochromatic aberrations. I will describe how we can realize independent and complete control of optical phase and amplitude for multiple colors of light and demonstrate colorful holographic objects with realistic surface qualities.

Data storage and processing is always a heated research area as we have moved into the era of big data and artificial intelligence. High efficiency, high speed, and stable data storage technology is greatly needed for today’s applications. However, traditional storage systems rely on devices which require binary data storage techniques due to noise and drift limitations. These devices are increasingly difficult to scale down to satisfy the demand of big data.

To address this, several kinds of alternative electronic storage cells have been developed using new materials or different physical mechanisms. Memristors and phase change RAM are promising potential multilevel storage cells which have gained much attention for unique advantages, such as in-memory computing, but are still limited by resistance drift over time and the reliability of programming a given level. What’s worse, typical issues associated with electrical circuits, such as current leakage and high crosstalk, still exist with these devices. Photonic memory, on the other hand, is particularly promising since it addresses these issues while enabling multilevel storage with drift-free readout.

Here, we experimentally demonstrate a single multilevel phase change photonic storage cell with up to 34 non-volatile levels with high stability. Using a Ge₂Sb₂Te₅ thin film to tune the transmission of light through a waveguide, we develop a single-pulse method to encode arbitrary storage levels regardless of the previous state of the material. With this technique, we show an order of magnitude reduction in both time and energy consumption which is a significant improvement over previous demonstrations of photonic phase-change memory. Furthermore, we develop three different methods to program multiple levels in the memory cell and investigate influence of different pulse parameters on the process of amorphization/recrystallization. This research is a significant step forward in the development of integrated photonic storage devices, demonstrates the possibility and advantages of all-optical storage network, and provides a potential platform for novel computing architecture like neuromorphic computing.

Recently we reported laser induced amorphization of silicon nanoparticle and demonstrated its optical response shift. In this presentation, we demonstrate that nanosecond pulsed laser can readily crystallize and amorphize different size of silicon nanoresonator at Mie resonance. Due to the switch of the optical dielectric constant, the resonance peak of nanoresonator can be shifted significantly. Raman mapping and SEM validated the phase transformation can be carried out reversibly. Melt-induced phase transformation of nanostructures usually suffers from the deformation and dewetting process. The first demonstration of reversible phase transformation is enabled by an abnormal geometry pinning effect, where we found the silicon thin disk melts but not dewets. We further demonstrated single “bit” writing and erasing with a sub-diffraction feature size. Combined with motion stage, arbitrary patterns can be written, including structural coloring and optical elements.

In this talk, we discuss dynamic plasmonic displays and holograms based on catalytic magnesium (Mg) metasurfaces in the visible range. Through the unique hydrogenation and dehydrogenation between Mg and magnesium hydride (MgH₂), different information components on the plasmonic metasurfaces become fully addressable in space and can be individually switched on/off. This results in dynamic plasmonic displays and holograms with designated multiple states, giving rise to high-level information control with unprecedented dynamic performance. Our work outlines the inevitable transformation from metasurfaces to metadevices, opening the door to a futuristic research horizon. Such dynamic plasmonic devices will allow for a wealth of applications for high-resolution displays, advanced security labels, high-density data storage and information processing.

Cerium and Bismuth doped yttrium iron garnet (YIG) films have excellent magneto-optical (MO) figure of merit (ratio of Faraday rotation to optical absorption) in the near-IR and have been incorporated into on-chip optical isolators. Recently, rare earth garnets have also been developed for MO applications that have the advantage of not requiring garnet seed-layers to crystallize on Si. Here we examine the growth on Si substrates and the optical and magnetic properties of polycrystalline thin films of terbium iron garnet (Tb₃Fe₅O₁₂, TbIG) substituted with Ce and Bi using pulsed laser deposition (PLD). The growth conditions that produced polycrystalline single-phase TbIG films were at temperatures of 720-750^oC and at oxygen pressures of 2-10 mTorr, with a rapid thermal anneal at 900^oC/3 min, yielding for example an in-plane coercivity of 47.7 kA/m and out-of-plane saturation field of 360 kA/m. The magnetic moment was 40 kA/m, lower than that of YIG due to the proximity to the compensation temperature of TbIG. Structural characterization revealed the growth of single phase polycrystalline TbIG films on Si without any secondary phases. Faraday rotation of TbIG/Si films measured at a wavelength of 1550 nm was ~600^ocm^-1 and the spectral dependence of the Faraday rotation, optical absorption and refractive index was characterized. The Faraday rotation can be increased significantly by Ce or Bi substitutions in the garnet. The sidewall growth of doped TbIG films on SOI waveguides was investigated for fabrication of TE mode isolators. Moreover, performance of these garnets in lithographically patterned TE and TM mode on-chip optical isolators based on ring resonators and Mach-Zehnder interferometers have also been studied.

KEYWORDS: magneto-optical garnets, terbium iron garnet, optical isolators, magneto-optics, sidewall growth.

In this communication we use plasmonically active silver nanowires (AgNWs) as building blocks of an ultrasensitive sensing platform for single photoactive proteins. The nanowires were synthesized by wet chemistry and have diameters and lengths around 100 nm and 50 μm, respectively. For achieving required functionality, we attach biotin molecules to their surface. Next they were deposited on a glass substrate, forming thus a sensor chip. The sensing potential of the nanowires was tested using Peridinin-Chlorophyll-Protein (PCP) equipped with a streptavidin linker. It is important to realize that PCP is roughly 4 nm in size, which is much less than diameters of the AgNWs.
Upon depositing AgNWs on a glass surface, we first determine their positions using transmission mode of a wide-field microscope. Next, a 2 μl droplet of protein solution was deposited on the substrate with simultaneous acquisition of fluorescence images of the same sample area. The concentration of the protien was changed from milimolar down to picomolar in order to determine the limit of detection. Using this approach, we are able to monitor protein attachment to AgNWs in real-time.
For highly concentrated sample the attachment is so efficient that within couple of seconds the nanowires are flaring, forming thus a distinct contrast to the background. Careful analysis shows that this contrast originates not only from the presumably higher concentration of the PCP complexes on AgNWs, but also from strong plasmonic enhancement of the PCP emission. Both this effects are crucial for detecting the emission of single PCP complexes when the concentration of the sample deposited on the substrate is reduced by 6 orders of magnitude. In fact, we are able to observe single protein attachment to AgNWs in a real-time mode.
We analyzed the influence of several key factors that determine the detection efficiency of our platform. These include: surface coverage by AgNWs, acquisition parameters, and additional functionalization of the substrate itself. The approach presented here is rather universal and can be applied for detecting variety of analyte molecules with essentially ultimate sensitivity.

[1] Marcin Szalkowski, Karolina Sulowska, Justyna Grzelak, Joanna Niedziolka-Jonsson, Ewa Rozniecka, Dorota Kowalska, Sebastian Mackowski, Sensors 18, 290 (2018)
[2] Sebastian Mackowski, J. Phys. Condens. Matter 22, 193102 (2010)

Research was partially financed by the National Science Centre (Poland) within the OPUS grant no 2016/21/B/ST3/02276 and the project 3/DOT/2016 funded by the City of Gdynia, Poland

Extraction of multidimensional information carried by optical beams has a wide range of applications including photography, optical communication, and augmented reality. To date, photodetection systems based on homogenous materials have limited abilities to measure more complex information beyond light intensity distribution, since these systems typically exhibit limited sensitivity to the polarization, wavelength, or phase of the incident light. Moreover, the bulky substrate-based detectors absorb all the light so that it cannot be integrated into an in-situ monitoring system. Here, we demonstrate a carefully engineered 110nm-thick Si nanowire (NW) array metasurface that functions as a transparent meta-photodetector able to in-situ extract the color, polarization as well as intensity of the incident light beam without the help of color filter arrays or polarizers. We show that a blue shift of the fundamental Mie mode can be engineered through radiative coupling between NWs, leading to a degenerate optical resonance (Kerker effect). As a result, photodetection with polarization- and wavelength sensitivity is combined with a polarization-insensitive anti-reflection at designed wavelengths in a single-layer structure.
To demonstrate this experimentally, we fabricate deep sub-wavelength arrays of nanowires in c-Si on a sapphire substrate using electron-beam lithography. The NWs are 110 nm high, and the widths are 30 nm, 55 nm, and 110 nm for efficient photodetection and suppression of reflection of blue, green, and red light, respectively. The measured reflection at resonant wavelengths is as low as 2%, with a transmission /reflection ratio larger than 40. Spectrally-resolved photocurrent measurements show a clear single peak at the three design wavelengths, behaving effectively the same as RGB color filters in front of a bulk Si substrate. The measured photocurrent ratio peaks are 2.4 at 470 nm (B/G), 2.1 at 540 nm (G/R), and 4.9 at 680 nm (R/B) with a bandwidth of ~80 nm, enabling color detection.
Finally, to improve the spatial homogeneity of photocurrent generated in the NWs, we fabricate a Si/ITO inter-digitated photodetector with ITO contacts sitting between Si NWs. The device optically functions as a NW array but electrically behaves like a homogeneous film because photo-excited charge carriers are extracted transversely across the NWs. The spatially-resolved photocurrent measurement shows only 10% signal fluctuation over 80% device area, revealing a reliable detection process for real applications.
Altogether, these results demonstrate the first single-layer and single-material detection system which can detect color, polarization as well as intensity information in the whole visible regime. A 60% overall average transmittance is observed experimentally, enabling in-situ beam monitoring applications. These transparent meta-photodetectors pave a new way for the next-generation transparent on-chip optoelectronic devices.

Tamm plasmon polariton (TPP) is proposed to demonstrate the ultra-sharp resonance wavelength across from UV to IR. A DBR-side TPP structure can overcome the limitations of the intrinsic property of the metal, which can support a stronger and narrower TPP resonance. The lithography-free, low cost, and refractory feature of the DBR-side TPP structures pave more possibilities for applications such as sensing and selective thermal emitters.

Current flow in metal films can be induced by the photon momentum carried by an obliquely-incident electromagnetic wave, a phenomenon known as photon drag. The prevailing intuition for the sign of this current assume that the absorbed light transfers momentum to the free electrons of the metal, either directly or indirectly via plasmon excitation, ultimately generating electron flow in the direction of the in-plane incident photon momentum. However, the direction of this photon-drag (PD) current, has been reported to puzzlingly vary with polarization state, surface morphology and excitation of surface plasmons [1,2]. In particular, measurements to date have typically been carried out with the illuminated surface facing ambient air, yielding, for smooth films, PD currents of opposite sign for illumination with s- and p-polarization, respectively, where only the sign in the p-polarized case is consistent with the standard hypothesis of free-electrons pushed in the direction of the incident momentum. Here, we demonstrate that for smooth metal films (including Au, Cu, and Ni-doped Ag) with illuminated surface facing vacuum, the PD current displays the same sign and nearly the same magnitude for both s- and p-polarized incident light, consistent with the expectation that light with different polarization states carries the same momentum. However, the shared sign of the observed current is always opposite to that implied by the intuitive model above, requiring reimagination of the microscopic processes of the light-metal interaction. We propose a new model for the PD phenomenon in which the bound electrons of the metal – far more numerous than the free electrons - are the primary recipients of the quantum momentum kicks from the incident light, leading to a net polarization of the metal to which the free electrons respond to yield a net PD current of the observed counterintuitive sign. The sign flip of the p-polarized signal upon subsequent exposure of the sample to air correlates with adsorption of ambient H₂0 molecules, and is proposed to result from generation of an extra polarization field induced by the intrinsic dipole moment of the molecules upon Lorentz-force induced rotation. [1] A. S. Vengurlekar and T. Ishihara, Appl. Phys. Lett. 87, 091118 (2005). [2] N. Noginova, V. Rono, F. J. Bezares, and J. D. Caldwell, New J. Phys. 15, 113061 (2013).

Giant enhancement of photoinduced electric currents in thin metal films under surface plasmon resonance conditions presents interest for various applications in plasmonic electronics and optoelectronics. In order to better understand the mechanism of the effect and explore possibilities to control and enhance it with nanoscale geometry, we study the photoinduced electric signals in strongly nanostructured systems, such as square-wave profile-modulated silver and gold films. Longitudinal and transverse photoinduced voltages are recorded as a function of incidence angle for various orientations of the grooves and are observed at both p and s polarizations. The angular dependence of the signal is found to have a shape reminiscent of Fano resonance, with a significant enhancement in magnitude in the range of plasmon resonance. A sharp switching of polarity is observed at p-polarization only, corresponding to efficient electron drag in the direction of plasmon propagation at smaller angles and against it at higher angles. Such a behavior is tentatively attributed to a coupling between the propagating surface plasmon polariton and localized surface plasmons excited at the sharp corners of the square-wave grating. Theoretically we estimate the effective forces acting on electrons and compare them with the experiment. We also demonstrate sensitivity of the polarity switching angle to the local dielectric environment, which makes the effect of interest for plasmonic based sensors with electric detection.

Interstellar space exploration requires the development of novel propulsion systems. Among the potential candidates, beamed energy propulsion of ultralight, nanometer-thin lightsails is regarded as one of the key candidates for unmanned exploration of neighboring star systems. A high-power phased-array laser capable of producing ~10 GW/m² illumination intensity will produce enough radiation pressure and thrust to accelerate a gram-scale spacecraft to relativistic speeds (up to ~0.2c). Among the many challenges facing such a propulsion system, dynamic stability of the sail, i.e., the ability to stay atop a laser beam during the entire acceleration phase, is regarded as one of the major obstacles to realizing laser-propelled spacecraft technology. In this work, we provide a comprehensive study of dynamical stability and consider several shapes that may be good candidates for stable beam riding.
First we consider a generic scenario of a laser beam interaction with various rigid, perfectly reflecting geometric shapes. We show that certain mutual configurations of beam intensity profile and sail shape may enable dynamically stable solutions. We analyze stability maps and basins of attractions for these regimes. We further find designs with the use of real materials, such as Si and SiO2, that provide suitable optical properties in the infrared frequency range and which satisfy the stability criteria.
Then, we consider the mechanical stability of the sail by relaxing the assumption of sail rigidity. We present first-order finite-element simulations of the thermal and mechanical behavior of free-flying sails undergoing optical acceleration. Hence, for the sail shapes and beam profiles identified in our stability analysis, we study numerically the whether the sail materials are capable of maintaining their shape under intense laser acceleration, and consider methods of shape reinforcement such as structural framing and spin-stabilizing. We analyze conditions and thresholds for materials stiffness that are required for stable sail beam riding.

The concept of light-driven cosmic sails is almost a century old, dating back to pioneering ideas that the pressure of sunlight could be used as a means of spacecraft propulsion. This category of light sails—known as solar sails—has been the focus of much research, culminating in the launch of the IKAROS solar sail prototype by the Japanese Aerospace Exploration Agency (JAXA) in 2010. In contrast to solar sails driven by broadband, relatively weak, solar radiation, the concept of a laser-propelled light sail envisions illumination by a focused, high-power laser phased array. More recently, the Breakthrough Starshot Initiative was founded around the idea of using a high power Earth-based laser source to accelerate an ultralight spacecraft to relativistic velocities needed to reach to the nearest star Proxima Centauri (4.2 light years away) and its potentially habitable exoplanet, Proxima b, in a matter of decades.

Propelling a light sail to ultra-high (potentially relativistic) speeds imposes substantial constraints on the size, mass, and shape of the spacecraft, as well as the optical, thermal, and mechanical properties of the constitutive materials. Here, we discuss how nanophotonic design of candidate dielectric and semiconducting materials (including high-refractive index materials such as crystalline-Si and MoS₂) at the scale comparable to the wavelength of the propulsion laser enables both a strong optical response but also thermal stability for a very low mass density (<1 g/m²) sail under high power laser irradiation. We present nanophotonic designs based on a range of motifs—including heterostructure thin-films, subwavelength metasurfaces, as well as one- and two-dimensional photonic crystal structures—that can achieve efficient photon momentum transfer, stability, and thermal management via emissivity-engineering for radiative cooling. We identify key tradeoffs between lightsail stability and reflectivity, needed for effective propulsion, and discuss further research directions in the emerging field of laser-driven light sails.

Since the pioneering introduction of holography [1], optical holography has been used for data storage, signal processing, sensing, security, etc. However, applications of real-time holography have been limited by the slow write and erase times which can be up to a minute for a typical holographic material. The implementation is further complicated by the needs for widely different wavelengths for writing (UV light) and reading (visible or near IR) the holograms. Furthermore, holographic materials are bulk crystals and thus not suitable for integrated optics.

We experimentally demonstrate that an epsilon-near-zero [2,3] material can be used to overcome these long-standing barriers. Specifically, we implement an efficient and broadband real-time holographic system using a highly nonlinear epsilon-near-zero material [4 - 7] which is four orders of magnitude thinner and which exhibits up to twelve orders of magnitude larger refresh rate than those of a typical holographic material. Our findings have significant implications in ultrafast signal processing and multimode communication.

References:
1. D. Gabor, Nature, 161, 777 (1948).
2. A. Alú, et al., Phys. Rev. B, 75, 155410 (2007).
3. B. Edwards, et al., Phys. Rev. Lett., 100, 033903 (2008).
4. M. Z. Alam and I. De Leon, R. W. Boyd, Science, 352, 795--797 (2016).
5. A. Kildishev, A. Boltasseva, V. M. Shalaev, Science, 339, 1232009 (2013).
6. V. E. Babicheva, A. Boltasseva, A. Lavrinenko, Nanophotonics, 4, 165--185 (2015).
7. L. Caspani et al., Phys. Rev. Lett., 116, 233901 (2016).

Conventional amorphous multi-component metal-oxide semiconductors with high-mobility, large-area processing feasibility, and environmental stability have UV-level wide optical bandgap (> 3 eV), the photoionization of oxygen vacancy (Vo) defects under energetic blue-light spectrum introduces restrictive spectral photosensitivity. However, Vo-defect-induced photocurrent generation under visible-light stimulus has several disadvantages for visible-light detection using oxide photo-TRs: 1) insensitive performance, 2) narrow spectral/unselective color detection, and 3) slow dynamic photoresponse speed and severe persistent photocurrent (PPC) behavior. We designed all-solution-processed multi-stacked metal oxide/chalcogenide semiconductor-based visible-light phototransistors via thermal-activation defect-healing process, multi-stacked functional materials, and chemically-stable stacking. Here, multi-stacked structure consist of high-mobility a-ZTO (interface), high-efficiency visible-light coarsened-crystalline CdS (cc-CdS) absorber, and defective surface passivation a-ZTO (surface) for high-efficiency photo-carrier generation, spontaneous separation, and defect-free transport. Here, thermal-activation defect-healing process were performed for defect suppression of hetero-interfaces (ZTO/CdS and CdS/ZTO) with rapid phase change and crystalline CdS bulk film. Next, artificial engineering of gate-terminal (V_GS condition) facilitate to tune and optimize photo-detection performance such as photo-sensitivity and dynamic photo-response (photo-rise/dark-recovery) speed.

Realistic material employment in the emerging field of metaphotonics, the convergence of plasmonics, metamaterials, and nonlinear optics, requires extensive control of material optical properties, a large nonlinear optical response, and robust mechanical stability. The transition-metal nitride material titanium nitride (TiN) has shown promise to fit these requirements at visible wavelengths, however, it suffers from a larger than ideal imaginary permittivity and limited tunablity. In this work, we develop a novel platform based on titanium silicon oxynitride (TiSiON) thin film optical ceramics of varying stoichiometry grown by reactive DC and RF co-magnetron sputtering and discuss its linear and nonlinear optical properties. Spectroscopic ellipsometry measurements reveal a decrease in imaginary permittivity, a tunable metallic behavior that can be extended to longer wavelengths, and evolution towards the value of real permittivity of a single material condition equal to zero at multiple wavelengths (i.e., multiple ENZ behavior). Consequently, we identify a novel class of materials with a double epsilon-near zero (DENZ) character. It was found that that vacuum post-deposition annealing allows for further tuning of these materials’ structural and optical properties. We show that these thermal annealing treatments can largely modulate charge carrier concentration by measuring their optical bandgaps as a function of annealing temperature. In addition, XRD and TEM measurements were performed to correlate the observed material structure and optical properties. DENZ materials have the potential to be especially relevant for advancing the field of optical modulation, and this development of Si compatible materials DENZ materials along with materials with potential for a tunable nonlinear response and decreased optical losses in the visible spectrum, will extend the engineering feasibility of implementing integrated metaphotonic devices. Nonlinear optical characterization based on Z-scan measurements will also be presented.

Being able to tame the speed and confinement of light is of immense important in a range of current photonic applications, such as on-chip all-optical circuitry, photonic neuromorphic computing, higher harmonic signal generation etc. Waveguides that support slow-light propagation have therefore been the subject of intense investigation efforts in the last decade. Different designs of mesoscale-patterned features had been realized primarily inspired by theoretical calculations relying on time-independent modal analyses.

We discuss here why such analyses are not appropriate for real systems, leading to erroneous predictions for the actual light propagation speed. We will show why a theoretical analysis based on the time-evolution of the wave is needed for the proper design of slow-light mesophotonic waveguides that can be high performing in the practical realm. In particular, we will present a counterexample demonstrating that when two modes are compared it is actually the mode with the higher group-velocity, as predicted from time-independent analysis, that ends-up effectively slowing-down light more. Our results contradict the widespread notion that the group-index is generally a good measure of the light’s speed effective slow-down factor in practical waveguide systems.

Our results suggest that in order to realize robust platforms for slow-light, waveguiding systems should be designed to exhibit simultaneously a large group index and a large modal index bandwidth. These results are important to inspire new slow-light waveguide designs that can break current performance limitations, enabling its applicability in optical signal manipulation devices.

[1] S. Foteinopoulou and J. P. Vigneron, Extended slow-light field enhancement in positive-index/negative-index heterostructures, Phys. Rev. B. 88, 195114 (2013).
https://journals.aps.org/prb/abstract/10.1103/PhysRevB.88.195144

[2] (Invited) S. Foteinopoulou, Frequency-Domain versus Time-Domain analysis of slow-light mesophotonic waveguides: theoretical insights for practically realizable devices, Conference paper, ACES Conference, Denver 2018.
https://ieeexplore.ieee.org/abstract/document/8364280/?reload=true

One-dimensional hybrid Distributed Bragg Reflector (DBR) with high reflectivity is constructed using Tris (8-hydroxy) quinoline aluminum (Alq₃) molecules and Titanium dioxide (TiO₂) nanoparticles via spin coating process. Light emission Alq₃ thin film is dominated by excitons. This material has been widely used as a superior emitter for organic light emitting diodes. Titanium dioxide (TiO₂) is an inorganic semiconductor with a high band gap. Photoluminescence (PL) of thin films of Alq₃ showed a broad PL peak at 532 nm. In DBR structures, PL quenching is observed but there is no shift in the PL peak of the Alq₃. The PL quenching is tentatively attributed to energy transfer via sensitization to wide band gap TiO₂ layers. The observation can be explained using a excitonic model. Fabrication process and optical properties of the structure are presented.

We proposed a method for precision control of 808 nm ultra short pulses in Er-doped fluoroindate crystals. The infrared laser of 808nm wavelength is widely used in medical applications such as laser therapy for pain management, inflammation reduction and orthodontal procedures. Even though its initial use was mainly for wound healing and pain relief, the medical applications of low-level laser therapy have broadened to include diseases such as stroke, myocardial infarction, and degenerative or traumatic brain disorders. Such medical applications require these laser pulses to be precisely controlled. This particular wavelength is also highly applicable as the infrared light source for military night vision targeting tools and weapons. Moreover, it is a commonly used pump source for producing other popular wavelengths such as diode pumped 532nm green laser. Frequency up-conversion in rare earth doped crystals have been studied extensively. Previously, authors have reported the model based controller design, in which the controller varies and controls the pump rate in real time through the pump power resulting in the enhanced emission of 808nm wavelength in Er-doped fluoroindate crystals under 1.48 μm pump. In model-based design, the performance of the resultant controller depends on the accuracy of the mathematical model used to represent the device in the design process. Therefore, this method is sensitive to modeling errors. In this paper a more robust control scheme using model-free approach is presented. In this recently developed model free approach the controller design is independent of the mathematical model and hence any modeling error have no effect on the device performance. Both theoretical analysis of the control methodology and simulation results will be presented.

Highly strained ultrathin InAs layer in GaAs has attracted much interest because of its use for study of fundamental physics as well as its application in electronics and optoelectronics devices. Recently submonolayer (SML) InAs deposition has been proposed as an alternative method to the extensively used Stranski-Krastanow (SK) mode of quantum dot (QD) growth. Moreover, both theoretical and experimental works have added interested for a InAs single quantum well (QW) for efficient excitonic lasing applications. Here, we discuss results from a set of samples containing a single ultrathin InAs layer with varying thickness from 0.5 to 1.4 ML in a GaAs matrix grown by molecular beam epitaxy on GaAs (001) substrate at low temperature and investigated by low temperature excitation power dependent photoluminescence (PL). We will discuss results, including observed asymmetric PL spectra having a low-energy-tail at low and moderate excitation power for non-integral ML samples. We will also discuss the observed linear change in emission energy with InAs thickness a PL line shape from InAs/GaAs heterostructures that is excitation power and InAs thickness dependent. The discussion will be based on the interplay of uncorrelated electron hole pairs, free excitons and localized excitons with excitation power and its effect on the optical properties of the InAs layer.

Semiconductor quantum dots (QDs) have become particularly important for quantum information science. Due to their atomic-like discrete energy spectrum they can behave as quantum light sources. Such sources operating at room temperature can be invaluable and can be enabled using III-nitride materials due to their large exciton binding energy. Here we demonstrate a quantum size controlled photo-electrochemical etch (QSC-PEC) approach^1-2 toward the integration and deterministic placement of quantum dots (QDs) within prepatterned nanostructures. An array of III-nitride nanowires containing a single InGaN quantum well is first fabricated using electron-beam lithography (EBL) patterning followed by inductively coupled plasma reactive-ion etching (ICP-RIE). Next, QD is formed within the prepatterned nanowires using a bandgap-selective, wet-etching technique. Here the nanowires are immersed in acidic etch solution with laser illumination of specific frequency above the energy gap of the SQW. The illumination enables the etching of SQW within the nanowire from the sides until quantum size effects causes increase in energy gap to ultimately stop the etch process, resulting in a quantum dot within the wire. The size of the QD can be tuned by the illumination frequency. Low-temperature microphotoluminescence (μ-PL) measurements of individual nanowires reveal sharp spectral signatures, indicative of QD formation. Further, internal quantum efficiency (IQE) improves an order of magnitude following QSC-PEC etching. Finally, second-order cross-correlation (g(2)(0)) measurements of individual QDs exhibit antibunching behavior indicating nonclassical behavior. Our results illustrate an exciting approach toward the top-down integration of nonclassical light sources within nanophotonic platforms.

Sandia National Laboratories is a multimission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525. The views expressed in the article do not necessarily represent the views of the U.S. Department of Energy or the United States Government.

1. Fischer, A. J.; Anderson, P. D.; Koleske, D. D.; Subramania, G., Deterministic Placement of Quantum-Size Controlled Quantum Dots for Seamless Top-Down Integration. ACS Photonics 2017, 4 (9), 2165-2170.
2. Xiao, X.; Fischer, A. J.; Wang, G. T.; Lu, P.; Koleske, D. D.; Coltrin, M. E.; Wright, J. B.; Liu, S.; Brener, I.; Subramania, G. S.; Tsao, J. Y., Quantum-Size-Controlled Photoelectrochemical Fabrication of Epitaxial InGaN Quantum Dots. Nano Lett. 2014, 14 (10), 5616-5620.

Photonic pigments, which is color materials using photonic materials, show fadeless colors caused from resonances, scattering and interferences of visible lights. Angle-independent photonic pigments especially are important for application of reflective displays and color materials. In our previous work, colloidal amorphous arrays, where monodispersed colloidal particles form an aggregate with short-range order, were found out as angle-independent structural colored materials under the conditions where a sample is illuminated from all directions. Although they usually appear whitish due to the contribution of multiple scattering of visible lights, we succeeded in taking on brilliant colors of the arrays by adding black particles such as carbon black diminishing the multiple scattering lights. However, the colors from the arrays exhibit angle-dependence under a directional light. We have to seek alternative photonic materials to improve the angle-independence.
Recent studies have revealed that in the reflection spectra observed from colloidal aggregates, the peaks due to different mechanism appear on the lower wavelength side than the scattering peak caused by the aggregates. The positions of the peaks don’t change at all even when changing the irradiation direction of light or the observation direction. These peaks are thought to be due to the phenomena called “Mie resonances”. Mie resonances are known as phenomena that light waves comparable to particle diameter are enhanced by circumnavigating of the light waves in dielectric spherical materials such as silica particles. In this research, we prepared photonic pigments displaying angle-independent colors by Mie resonances.
Silica particles from 200 nm to 500 nm in diameter mixed with carbon black particles to absorb multiple scattering lights are pressed into colloidal aggregates as pellets. In order to evaluate the angle dependence of the color observed from the prepared pellets, diffuse scattering was measured by injecting white light onto the flat surface of the pellets and changing the angle of the detector. The peaks derived from Mie resonances occurs in the visible region, and the pellets become photonic pigments without angle dependence when a colloidal particle having a diameter of 350 nm or more is used. Fine tuning of scattering lights is achieved by just changing the particle diameter, because the scattering lights of Mie resonances shift depending on the diameter. The color saturation of the photonic pigments due to Mie resonances can be controlled by adding amount of carbon black particles.
In summary, we succeeded to prepare angle-independent photonic pigments by Mie resonances even under a directional light.
The results are useful for making energy-efficient color materials, optical devices and reflective displays having low viewing angle dependence. We are now investigating the contribution of the aggregation structure of the particles to the color development of this aggregates.

Research highlights
- Application of large-scale flexible infrared emitter for IR camouflage
- Suggestion of flexible infrared emitter without flexible dielectric materials considering mechanical stress
- Verification of flexible infrared emitter in the various conditions to apply for IR camouflage
Abstract
During several decades, the enhancement of survivability of ally has been one of issues on the modern battlefield. In particular, the infrared regime is important for the increase of survivability because the infrared emission from target occurs continuously from target, which makes the late recognition of target compared to the microwave based on radar using reflection signal. To control the infrared signal, we can control the temperature and emissivity on the surface based on the radiative heat transfer equation. In terms of manipulating emissivity, metamaterials have also superior potentials to apply for the military applications because it can realize the exotic properties contrary to natural resources. Especially, the selective emitter, which is one of metamaterials, has ability to manipulate the emissivity for modifying the emissive power from target. In addition, the IR detector has the limited range of detection wavelength because the atmospheric transparency window exists the range of 3~5 and 8~14 um due to the existence of molecules to cause high infrared absorption. For this reason, via infrared selective emitter, we can match the infrared emission from target with the consideration of atmospheric transparency window. Furthermore, to apply the arbitrary surface on aircraft, military clothes and others, the selective emitter should have the material flexibility.
Here, we present the large-scale flexible infrared selective emitter for IR camouflage for ally’s survivability. To achieve the material flexibility of selective emitter, we use metal-dielectric-metal (MDM) structures. We adjust the flexible substrate of polyimide. And, contrary to the past research, the discrete dielectric material considering mechanical stress is applied for the selective emitter. In addition, we change the dielectric material to silicone dioxide and nitride which are common materials to use MEMS fabrication process. The emissivity is measured by FT-IR (Bruker) using Kirchhoff’s law. The results showed that we can achieve the selective emitter with single peak to deceive the emissive power from target. In addition, the presented emitter is high reliability against temperature and peel&stick test. The maximum curvature of presented emitter is down to 250 um, and the high machinability applies for the various surfaces and sizes. Furthermore, we attach the replicated aircrafts and confirm the performance of IR camouflage in the supersonic flowfields (Ma = 3) with the most severe condition of the aircrafts. This study will be helpful for the enlargement of applications using metamaterials.

Thin film interference is one of the well-known optical phenomena that can be found in many places around us, and numerous optical coatings that utilizes thin film interference are already available in the market at the moment. In general, these optical coatings largely rely on Fabry-Perot interference that occurs between one or more films of dielectric or metallic materials. On the contrary, optical coating that consists of highly absorbing film on a substrate with finite optical conductivity has been investigated recently as a new type of optical coating. In the previous related studies, reflective optical coating at visible frequencies have been successfully achieved by coating Germanium (Ge) film at subwavelength thicknesses on gold (Au) substrate using conventional lithographic techniques.
We found that iron oxide film (Fe2O3) on platinum (Pt) substrate, having similar refractive indices trends as Ge and Au, exhibit similar interference effect as the Ge film on Au substrate. In this study, Fe2O3 thin film is grown directly on Pt substrate by laser-induced photothermal reaction and applied as an absorbing dielectric for the optical coating. In brief, Pt substrate is immersed in aqueous Fe2O3 precursor solution and 532 nm continuous wave (CW) laser beam is focused on the substrate at 5 to 10 mW power. It is observed that the reflected spectrum from the substrate varies in full visible regime according to the resultant thickness of Fe2O3 film, which is easily controlled by changing the irradiation time. While on the other hand, the film thickness, and hence the reflected color, can be also altered by the scanning instead of changing the irradiation time. By choosing the proper scanning speed in the range of 10 μm/s to 30 μm/s, continuous line of Fe2O3, which exhibits the designated color in the visible frequencies, is created along the scanning path at microscale width. By utilizing the proposed laser process, thin Fe2O3 absorbing media is easily and selectively created on Pt substrate without any photolithographic techniques. It is also worth mentioning that Fe2O3 films at numerous thicknesses can be created on the same substrate with a single step, while it requires multi-step complex photolithography process with careful alignments to achieve the identical result with the conventional techniques.
Another virtue of the proposed laser process comes from the in-situ measurement of the thin film thickness in real time. The incident laser is absorbed by Pt substrate to induce the photothermal growth of Fe2O3, whereas the reflected laser contains the information about the Fe2O3 thickness. Therefore, the height of the Fe2O3 film can be predicted according to the thin film interference theory by collecting the reflected laser beam intensity signal. The precision of the thickness measurement is further tested by comparing the height measured from AFM data with the predictive value from the reflected intensity signal.

The ground states of strongly correlated oxides are very sensitive to disorder which can alter the orbital occupancies. In this work, we will discuss how oxygen vacancies and hydrogen interstitial defects reversibly can be injected into vanadium dioxide and rare-earth nickelates, by which orders of magnitude change in electrical resistivity and reconfiguration of optical bandgap becomes feasible. Electrochemical incorporation of point defects allows phase modulation beyond limitations of screening length and electric field control independent of thermal constraints. The defect induced phase change in these correlated materials can be attributed to the charge filling which alters the electron-electron interaction. The phase change can be spatially localized to sub-wavelength scales by use of electrodes fabricated by e-beam lithography. The large change in optical properties of complex oxides upon phase change enables a promising platform for tunable photonics, plasmonics and metamaterials.

Solid-state microchip lasers attract attention because of their compact size (~cm), high stability, and high efficiency. Giant pulse power and high repetition rate can be realized with active modulators and used in prolific applications. However, these modulators are large in size although Q-switches should be small to obtain shorter and higher output pulse. To overcome these issues, we have suggested the use of magnetooptical (MO) materials as Q-switches. Since the rare-earth iron garnet (RIG) film has the similar crystal structure and thermal expansion coefficient as the ones of Nd:YAG, we expected that the combination of RIG and Nd:YAG is suitable for realizing MO microchip lasers, and reported a MO Q-switch laser using Nd:YAG with the output peak power of 1.1 kW^[1]. However, the cavity length was 10 mm at the previous report although the physical length limit is 5 mm. Because of the large size of the component holders of the laser optics, there were air gaps between the components and they limited the cavity length. In order to achieve a microchip MO Q-switch laser, the air gaps must be eliminated. In this report, the design and implementation of BM on the RIG film to remove the air gap between the RIG film and output coupler are discussed, and the Q-switching is demonstrated.

We used a matrix approach method to design the BM onto a single crystalline RIG [(Tb, Bi)₃(Gd, Fe)₅O₁₂] film, and SiO₂ and Ta₂O₅ were used as a low and high refractive index materials, respectively. The optical constants of the materials were numerically estimated from their transmittance spectra using a simulation software (SCOUT) based on Fresnel interference. Refractive indexes of the RIG, SiO₂, and Ta₂O₅ were 1.70, 1.43, and 2.02, and extinction coefficients of the RIG, SiO₂, and Ta₂O₅ were 1.57×10^-4, 2.79×10^-3, and 1.48×10^-3, respectively. The BM was designed to have a partial reflection of 50%. The BM was coated onto the RIG film via radio frequency ion beam sputtering. The atmosphere was filled with O₂, and the ion beam power and voltage were 114 W and 1200 V, respectively. The fabricated structure of the BM was RIG/SiO₂/Ta₂O₅/SiO₂/Ta₂O₅, and the thicknesses of the films were 190 μm, 102 nm, 141 nm, 167 nm, and 129 nm, respectively. Two three-turn coils with diameters of 5.3 mm sandwiched the RIG film. A single crystalline 1 at.% Nd-doped YAG which has a high reflection BM at the wavelength of 1064 nm on its input surface was used as a lasing material. The cavity length was 7 mm, which is 3-mm smaller than the one of our previous report^[1]. The pumping power was 27.4 W and the repetition rate was 1 kHz at the wavelength of 808 nm, and the pulsed current with 2.3 μs width was applied to the coil for Q-switching. The output pulse had the pulse width of 25 ns, and the obtained pulse energy was 40 mJ corresponding to the peak power of 1.6 kW, which is 1.4 times larger than the previous report^[1].

[1] R. Morimoto, et al., Sci. Rep. 7, 15398 (2017).

Colloidal quantum dots (QDs) are one of the favorable nanomaterials having outstanding optical properties such as narrow emission bandwidth with high quantum yield. These properties can be useful for optical or optoelectronic device applications like lasers, solar cells or flat panel displays. Recently, there are several reports for QDs combining with metal structures, expecting enhance photo-luminescence (PL) by surface plasmon resonance (SPR) [1, 2]. There is another report mentioning that the thickness is important factor for the PL from QDs spin-coated films [3]. In this study, we controlled the QDs layer thickness precisely by use of layer-by-later deposition on glass and gold substrates, and compared their emission properties.
A self-assembled monolayer composed of colloidal CdSe/ZnS QDs was fabricated by Langmuir-Schaefer (LS) method. The CdSe/ZnS crystal size is 5 nm in diameter and the thickness of organic ligand layer is 1 nm. The sheet was transferred to hydrophobized glass and gold substrates. To avoid a quenching by a Förster Resonance Energy Transfer (FRET), a 10 nm of SiO₂ layer was deposited on gold substrate prior to the fabrication of multilayered QDs. The PL images and spectra of the multilayered QDs were taken under an epifluorescence microscope.
The PL intensity of the multilayered QDs sheet on glass substrate increased monotonically against the number of QDs layer. However, it showed a different feature on gold substrate. The PL intensity was maximized at 7 layers (the layer thickness: 50 nm), where the PL enhancement factor was 9 against that on glass substrate. Neither the mirror effect nor SPR effect can explain this layer-number-dependent PL intensity on gold substrate. If SPR is the main reason of PL enhancement, the maximum should appear at 10 to 20 nm layer thickness on gold in consideration of SPR and FRET [2]. Hence, the light confinement effect in the “optical resonator” composed of multilayered QDs sheet needs to be considered. We assume that a similar phenomenon to our previous study with multilayered silver nanoparticle sheets [4] was induced in the multilayered QDs sheet as well (although electromagnetically induced transparency (EIT) was not found in this case). The detailed data analysis with Finite-Difference Time-Domain (FDTD) simulation provides an overall picture of the layer-number-dependent PL profile, which includes three independent contributions of mirror effect, SPR enhancement, and light confinement.

[1] K. Matsuda, et al,Y., Appl. Phys. Lett., 92 (2008) 211911-1.
[2] E. Usukura, et al, Appl. Phys. Lett., 104, (2014) 121906.
[3] T. Shin, et al, Sci. Rep., 6, (2016) 26204.
[4] K. Okamoto, et al, Sci. Rep., 6, (2016) 36165.

Amorphous Si (a-Si) is used for fabrication of commercial low-cost flat panel image detectors for radiographic applications such as computed tomography (CT) imaging. a-Si photodiodes are known to exhibit a rapid decrease in quantum efficiency near 750nm. While crystalline Si does not suffer from such an early decline, the large-area and low-cost constraints of medical imagers make it challenging and costly to use crystalline Si for such devices. In this work, we report on the development of a sensitive layer for upconversion from 785 nm to green region of the spectrum, which nearly matches the peak quantum efficiency of a-Si detectors. Various host materials have been extensively studied in literature with rare earth ions such as Er³⁺(emission: green+red), Tm³⁺(emission: blue), Ho³⁺(emission: red+green) along with Yb³⁺ as a sensitizer for upconversion to the visible regime at high incident optical power (~100 mW) for colloidal solutions. We carried out a thermal decomposition synthesis of NaYF₄:Yb(18%):Er(2%):Gd(15%) at moderate temperature (~320°C), resulting in a nearly pure hexagonal phase material. This is confirmed by powder X-ray diffraction (PXRD) of the unannealed sample with a lattice constant (~5.17 Å). High-resolution transmission electron microscopy (HRTEM) measurements reveal the formation of nearly spherical nanoparticles. The observed plane ([100]) inferred from lattice fringes in TEM data with a visibly estimated interplanar distance (4.4±1.6 Å) is in reasonable agreement with standard data (~5.17 Å) for comparable NaYF₄-based materials. Excitation (785 nm) of the deposited thin films of Gd-doped unannealed material at relatively low incident power (~0.4 mW) exhibits a PL response in green (539 nm) and red (665 nm) region of the spectrum. Gd-based upconversion material based thin films are thus a feasible photonic material for potential effective extension of high quantum efficiency range in a-Si for flat panel image detectors.

Quantum dot light-emitting diodes(QD-LEDs) that allow solution processes and are capable of light emission with high color purity have drawn much attention. We can control their wavelength and the full width at half maximum (FWHM) of the emission spectrum by using the ability of size control; therefore, QD-LEDs are expected to be applied for displays with a wide color gamut. Recently, Cd-based QD-LEDs have been reported to show high color purity and external quantum efficiency (EQE). However, because of the toxicity and restrictions on the use of these Cd-based materials, their substitution with Cd-based materials such as InP is highly desired. In this study, we developed QD-LEDs using high-performance Cd-free QDs composed of core/shell spheres of ZnInGaP/ZnS. The Zn added to the InP core improved the crystal growth and the Ga helped control the lattice matching between the core and shell, increasing the stability in air. This is because the interfacial state of the core/shell has a huge impact on the deterioration mechanism. The photoluminescence (PL) emission peak, FWHM and quantum yield (QY) of this material are 521nm, 60.9nm and 73.8%, respectively. We fabricated a QD-LED with an inverted structure using these Cd-free ZnInGaP/ZnS QDs then examined the relationship between the hole transport materials and EQE. Some of these materials exhibited low EQEs, partly because electrons and holes were not efficiently able to recombine in the light-emitting layer. In other words, electrons were not fully blocked by the hole transport layer and penetrated through it. Therefore, a material with a high Lowest Unoccupied Molecular Orbital (LUMO) level to block and confine more electrons in the light-emitting layer was desired. We found that TCTA (4,4’,4”-Tri(9-carbazoyl)triphenylamine) was effective for blocking electrons. The device fabricated with TCTA for the hole transport layer showed an EQE of about 3.4%. This value is comparatively high for common Cd-free QD-LEDs, and a better charge balance was achieved and the recombination probability between electrons and holes was increased compared with those of previously reported Cd-free QD-LEDs. We found that the combination of ZnInGaP/ZnS and TCTA was effective for realizing high-efficiency QD-LEDs.

There is an ever growing interest in organic electro-optic (EO) polymers as the active material in a range of applications from broadband terahertz emitters and detectors [1] to ultrafast electro-optic modulators [2]. Large electro-optic response [3] and ease of tethering on varying device platforms make EO polymers desired over conventional EO crystals (e.g. LiNbO3) [4]. Push-pull chromophores with large first hyperpolarizability (β) are the key constituent of EO polymeric materials. The translation of their second order molecular nonlinearity into a macroscopic nonlinear optic effect (Pockels effect) depends on their alignment in an acentric orientation. This is achieved by a so called poling process, in which the chromophores dipoles are oriented with a strong external electrical field at elevated temperature [2-5]. One of the challenges of this technology is the long-term stability of the poled arrangement associated with a high EO response [4,5]. Poling efficiency and stability relies crucially on the host polymer and the degree of chromophore mobility after poling. Strategies widely being employed to stabilize the poling induced acentric order include, use of high glass transition temperature host polymers, cross-linking the chromophores with the host after poling, covalently bonding chromophores to the polymeric chain, adding site-isolation groups to the chromophores [4-6]. Presently, there is an intense research interest to achieve a high EO activity together with long-term thermal and photochemical stability.

In this work we focus on the study of host-guest polymeric materials and their EO performance with the purpose of understanding the mechanisms leading to a stable EO activity. We used known polyene-type push-pull chromophores with adequate molecular nonlinearity [4,5]. Miscible amorphous polymeric host materials with different glass transition temperatures were selected. Thin films of host-guest EO materials were poled using contact electric field poling and their EO coefficients were measured using Teng-Man modulation ellipsometry[7]. To understand the mechanism behind change in persistence of poled state with different host, thermally stimulated discharge studies were conducted and parameters governing the relaxation of the chromophores were obtained. The role of the host polymer on the poling efficiency and stability will be discussed. In-device figure of merit of selected materials will be presented for silicon-organic-hybrid (SOH) EO modulators [2,3].


[1] McLaughlin et al., Appl. Phys. Lett. 151107 (2008) 92
[2] Wolf et al., Scientific Reports, 2598 (2018) 8
[3] Kieninger et al., Optica, 6 (2018) 6
[4] Dalton et al., Chem. Rev. 25-55 (2010) 110
[5] Yesodha et al. Prog. Polym. Sci. 45-74 (2004) 29
[6] Elder et al., Chem. Mater. 6457-6471 (2017) 29
[7] C. C. Teng and H. T. Man, Appl. Phys. Lett., 1734 (1990) 56

The planar waveguide modes TE_n or TM_n show n nodes, i.e. positions where the mode intensity equals zero. Within symmetric waveguides one node of all odd modes (odd n) is situated in the center plane of the waveguide. Remarkably, this accounts for the entire spectral range where the modes exist, which paves the way for tailoring large bandwidth light matter interactions. For instance, light scattering inside a waveguide strongly depends on the filling factor of the scattering film. Placed in the node plane the scattering is strongly suppressed. On the contrary, only slight detuning of the waveguide could efficiently switch that scattering, which paves the way for new display concepts. In planar solar concentrators a diffracting layer placed in the node plane will hardly interact with the excited waves. As a consequence, by exploiting the node plane the same concentration factor and acceptance angle are reached at a much lower thickness of the concentrator.

Remarkable properties of gaseous UV photon detectors make them attractive for relativistic particle identification in high energy physics, sky observation in astronomy and position emission topography in medical physics. Photocathode used to convert UV photons into electrons play a key role in deciding detection efficiency of these photon detectors. Cesium Iodide (CsI) is one of the most efficient UV photon convertor among other alkali halides because CsI has the highest quantum efficiency in the UV wavelength range and is relatively stable in air. However, a major drawback of the photocathode material is ageing process which limits their life time and hence performance of these photon detectors. UV photon detectors used in high energy physics, suffer from a high radiation background. High radiation fluences of electrons or protons are continuously hitting on the detector photocathode surface. Such high flux would significantly affect the morphological, structural and photo-emission properties of the photocathode film.

In this work, we demonstrate the impact of electron bombardment on 50 nm thermally evaporated CsI film. It has been studied by SEM, XRD and HRTEM. A severe modification in surface morphology is observed under high energy electron beam exposure. This surface modification involves the creation of void areas which coalesce upon further exposure. Effect of exposure time and energy of incident electron beam on film continuity will be presented here.

Band-engineered Ge has become a promising candidate for monolithically integrated light sources on Si. The direct bandgap of Ge at 0.8 eV corresponds to the most technically important wavelength for telecommunications (1550nm), making Ge an ideal candidate for large scale photonic integration on Si chips for photodetection/optical sensing [1], modulation, and lasing [2]. Even though Ge is an indirect bandgap semiconductor, the energy difference between its direct and indirect bandgaps is only 136 meV [1], which can be compensated by tensile strain and n-type doping. However, growing high quality n+ Ge for efficient light emitters remain a significant challenge. Existing delta-doping approach in chemical vapor deposition (CVD) tends to adversely affect the crystallinity of Ge [2], while high-temperature defect-reduction annealing undesirably drives out the dopants. Along with reducing materials defects it is important to minimize Si-Ge interdiffusion [3,4] for optoelectronic applications since it drives the materials towards an indirect gap semiconductor, and is one of the main causes of low efficiency of devices such as lasers. Therefore, it is highly desirable to achieve high-quality n+ Ge on Si without high-temperature processing. In this paper, we report strong room-temperature photoluminescence (PL) from low dislocation density n+ Ge-on-Si grown by metal-organic chemical vapor deposition (MOCVD) without the need of post-growth high temperature annealing for defect reduction. The as-grown MOCVD samples with n=1x10¹⁹ cm^-3 show a low threading dislocation densities of ~10⁶ cm^-2 [3], two orders lower compared to 10⁸ cm^-2 from as-grown delta-doping samples. Correspondingly, compared to delta-doped samples with a similar doping level, the MOCVD n+ Ge PL intensity is 5-10x higher at room temperature. In fact, their PL intensity is similar to delta-doped samples with 4x higher n-type doping level. Furthermore, the MOCVD samples also shows almost no out-diffusion of phosphorous or arsenic dopants upon annealing, in contrast to the delta doping approach. Finally, after annealing the dislocation density only decreases by a factor of 2-3 for the MOCVD samples, compared to ~100x for delta doped samples. However, the PL intensity of the high-temperature annealed MOCVD sample decreases and the PL peak blueshifts mainly due to Ge-Si interdiffusion. These results indicate that it is promising to significantly improve the material quality and light emitting properties of n+ Ge-on-Si via MOCVD growth without high-temperature defect-reduction annealing for better monolithic light emitters on Si.

[1] J. Michel, J. Liu, and L. C. Kimerling, Nature Photonics 4, 527-534 (2010).
[2] J. Liu, Photonics 1, 162-197 (2014)
[3] Guangnan Zhou et al. Opt. Mater. Express 8, 1117-1131 (2018)
[4] X. Li et al. Semicond. Sci. Technolo. 31, 065015 (2016)

Lead sulfide (PbS) based colloidal quantum dots (QDs) are very attractive materials for the realization of novel optoelectronic devices, combining low cost synthesis and processing, deposition over large area and on any substrate with a tunable band gap that enables selective detection in wavelengths ranging from the visible light to the short-wave-infrared (SWIR). Great effort has been made in the last years to incorporate PbS QDs in thin-film photovoltaic devices (TFPV), leading to improved efficiencies through better ligand passivation schemes of the QD surface. Even though these devices report high external quantum efficiencies (EQE) they lack the required characteristics that would make them suitable for imaging applications, namely a low dark current and high speed. In this work we demonstrate how a combined electronic and optical design PbS QDs can enable high performance infrared sensitive photodiodes.

In the electronic domain, we study devices of QD layers sandwiched between two transparent, thickness tunable transport layers as an n-p junction photodiode. We demonstrate how the proper selection of hole (HTL) and electron (ETL) transport layers through band alignment engineering can decrease the dark current of the device substantially. We argue that a major contribution to dark current in these devices derives from dangling bonds at the surface of the QDs. Time-resolved photoluminescence (TRPL) is used to characterize the recombination processes in the QD films under different ligand termination and to study the relation between the trap states and the dark current of the devices. Taking these data as a feedback we fabricated optimized devices with dark currents below 1 uA/cm² at 1450 nm and below 10 nA/cm² at 950 nm.

In the optical domain, we optimize our devices to achieve high EQE at the wavelength of our choice by properly manipulating the optical cavity inside the layers. Using variable angle ellipsometry and reflection/transmission measurements we build an optical model for PbS QD based thin-films. We show that different ligands result in different packing of the QDs, having a crucial effect on the IR light absorption coefficient. Furthermore, we tune the transport layer thickness to optimize the intrinsic cavity effect of the device structure, boosting the absorption of our stack from 40% to above 80% at the wavelength of interest even with very thin QD layers between 100 – 200 nm. Through this path we demonstrate photodiodes with low dark current and high EQE of 30% and 50% at the wavelengths of 1450 nm and 950 nm, achieving specific detectivity of 10¹² Jones.

Quantum Dots (QD) for optoelectronic devices have received much attention over the past decade due to their large-area solution processability as well as size-dependent optical properties. Although there has already been remarkable breakthroughs in visible-light emitting QDs for LED applications that has led to commercialization, efficient and stable IR (infrared) emitting QDs that have high potential applications in photodetectors and lasers have yet to be fully realized. The major bottleneck for commercialization is the low band gap material’s low chemical stability in ambient environments as well as difficulties in precisely tuning and maintaining the target optical characteristics.

Our strategy utilizes a well-controlled yet simple cation exchange process to create Core/Graded Shell-Shell QDs with precisely tuned spatial composition to accurately control the optical properties in the IR region as well as enhance stability.

In our research, we first synthesize CdSe/Cd_1-xZn_xSe_1-yS_y/ZnS QDs with precisely-tuned spatial composition by utilizing the different chemical reactivity of the four component atoms. Subsequently, we successfully synthesize PbSe/PbSe_1-xS_x/PbS QDs and AgSe/AgSe_1-xS_x/AgS QDs with tailored dimensions via a simple yet robust cation exchange process that effectively replaces the Cd and Zn cations with cations of interest (Pb or Ag). It is worth noting that the cation exchange process occurs in less than five minutes and continued reaction time leads to a gradual and controllable increase in either the PbS or AgS shell layer thickness resulting in precision control over optical properties via band gap alignment tuning of the core and shell materials.

Our results show that we can precisely control the absorption properties of our IR QDs from 1000 nm to 3000 nm. Colloidal stability of the IR QDs in solution are also excellent for several months. Moreover, absorption and photoluminescence properties are well-maintained for more than 30 days without noticeable shifts.

Resonant Bragg Structure (RBS) based on GaAs/AlGaAs multiple quantum wells (MQWs) was designed aiming to coincide the Bragg resonance with the exciton energy of the second quantum state in the GaAs quantum well to achieve double-resonance condition. The RBS samples with 60 periods of GaAs/AlGaAs quantum wells/barriers were grown on semi-insulating GaAs substrates by molecular beam epitaxy. We employed optical reflectance (OR) and electro-reflectance (ER) spectroscopies to study resonant optical properties of the RBS samples.
At low temperature optical reflectance measurements, we observed enhanced Bragg reflection intensity when exciton energy coincides with the Bragg energy peak. In the electro-reflectance spectra, we also observed an enhanced and broad ER features related to excitons at the excited states under the double-resonance conditions manifesting a strong light-matter interaction. Bragg peak can significantly be tuned by changing the angle of incidence of the light. Exciton energies can be tuned by changing the temperature and external electric field. The exciton energies are very sensitive to the thickness of the quantum wells. There was noticeable change in exciton energy in the samples in the different regions from the same wafer. We performed the numerical calculations to estimate the possible exciton states. By tuning these variables, we performed OR and ER measurements to study the resonant optical behaviors at the excited energy states.
By tuning the Bragg peak for double resonance in the RBS samples of different thicknesses, we observed the electro-reflectance features related to the transitions of x(e2-hh2), x(e2-hh1), x(e2-hh3) and x(e1-hh3) excitons from the electro-optical experiments. The excitonic transitions x(e2-hh1), x(e2-hh3) and x(e1-hh3) which are prohibited at zero electric field, were allowed due to the increased overlap of the electron and hole wave functions caused by the electric field; built-in electric field or a DC bias applied. Details about our findings on the resonant optical properties of GaAs/AlGaAs MQW based RBS along with its implications will be presented.

Herein we show that nanophosphor integration in an optical cavity allows unprecedented control over both the chromaticity and the directionality of the emitted light, without modifying the chemical composition of the emitters or compromising their efficiency. GdVO₄:Dy³⁺/Eu³⁺ nanoparticles of controlled size and shape are synthesized using a solvothermal method with which highly transparent nanophosphor thin films are prepared. Key to the achievement herein reported is the careful analysis of the structural and optical properties of thin nanophosphor layers with the processing temperature in order to achieve efficient photoluminescence while preserving the transparency of the films. An optical cavity was produced by embedding one layer of the nanophosphors in the middle of a one-dimensional photonic crystal (1DPC) made by ultraviolet (UV) transparent ZrO₂ and SiO₂ layers. Strict control over the structural parameters yield a photonic cavity mode that couples to the target emission peak of the nanophosphors, which results in an enhancement or suppression of the spontaneous emission of Dy³⁺ or Eu³⁺ ions compared with their corresponding reference. As a result the chromaticity and the directionality of nanophosphors can be tuned with high precision due to the interplay between photonic resonances and the natural emission of the rare earth cations.^[1,2] Our approach opens a route towards the development of nanoscale photonics based solid-state lighting and displays.
References
[1] D. Geng et al., Photonic Tuning of the Emission Color of Nanophosphor Films Processed at High Temperature, Adv. Optical Mater., 2017, 5, 1700099 –cover story-.
[2] D. Geng et al., Photonic structuring improves the colour purity of rare-earth nanophosphors, Mater. Horiz., 2018, DOI: 10.1039/C8MH00123E –cover story-.

The III-V growth on Al₂O₃(0001) can create the opportunity to realize monolithic integrated combining high-performance III-V semiconductor light sources, modulators and detectors, low loss waveguides and passive devices, and CMOS and RF silicon circuits on a sapphire platform. The potential integration of microwave photonics (MWP) functionality on a photonic chip can dramatically increase speed, bandwidth, processing capability and dynamic range. Here we study the initial nucleation mechanism and interface between III-As grown on an Al₂O₃ (0001) substrate, very dissimilar materials (lattice parameters, crystal structure, thermal expansion). Previous studies for arsenides on sapphire have focused on thick layered growth as opposed to investigating the initial nucleation. In our research, molecular beam epitaxy (MBE) is used to grow ternary (InGaAs) and binary (GaAs, AlAs, and InAs) arsenide materials on well define step and terrace surface of the substrate. At the initial stage of the growth, we observed the 3D growth mode for the InGaAs, GaAs, and InAs from 1ML to 50 nm. For the 50 nm GaAs the faceted crystal structure is observed and from rocking curve measurements of X-ray diffraction (XRD) only one out-of-plane orientation [111] has been detected. Rocking curve of 50 nm GaAs shows small linewidth (242 arcsec) indicating the high quality of the grown crystals. Asymmetric (113) phi-scan shows a weak correlation with the sapphire substrate. The In incorporation is very low for the very small amount of InGaAs deposition on the sapphire substrate and the amount of In incorporation increased with increasing the thickness of InGaAs. Twin formation is a major challenge for obtaining a single crystalline material and these twin crystals are rotated by 60° to the original phase. We have suppressed the twin formation and improved the GaAs crystal quality by introducing a thin layer of AlAs between GaAs and sapphire as well as the in-plane orientation relationship between GaAs and sapphire has improved. Further in-situ annealing has been found to decrease the twin volume and reduced to less than 2% of the total grown material.

The metallic, robust and high-melting point qualities of titanium nitride (TiN) make it an interesting and viable material for plasmonics and electrodes. Previously, we analyzed how substrate temperature of (0001)-oriented sapphire during TiN deposition affected the microstructure and optical properties of the film. We found that optical plasmonic properties correlated with the microstructural quality of the TiN films. In this work, we provide a further investigation into the strain mechanisms and stoichiometry of TiN films as a product of substrate temperature, as these mechanisms affect microstructure and therefore optical properties. Six (111)-oriented TiN films were grown with controllably unbalanced reactive magnetron sputtering, with substrate temperatures varying from 280°C to 760°C. The TiN films were analyzed with X-ray photoelectron spectroscopy, which revealed no compositional difference between samples. Additionally, Rutherford backscattering spectrometry on the 550°C film showed that it was stoichiometric. These results indicate that strain is the most prominent component contributing to changes in TiN microstructure, as oppose to stoichiometry changes. To further investigate any present strain, the films were analyzed with high resolution X-ray diffraction (XRD) by completing 2theta-omega scans around the (111) TiN orientation (symmetric) as well as the (113) orientation (asymmetric). Rocking curve scans were also completed around the (111) TiN peak. With the assumption that any strain in the sample would be rhombohedral, the 2theta values from each orientation were compared, and the best estimate for the strained lattice constant and bond angles were extracted. Lattice constants increased with increasing substrate temperature until 490°C. At this substrate temperature, the lattice constants plateaued around 4.245Å, and the deviation from a 90° bond angle remained at a minimum. This trend correlated with plasmonic qualities of the films, with the more relaxed films grown above 490°C yielding greater plasmonic quality. Additionally, two smaller peaks were found symmetrically placed around the film peak in the XRD rocking curve scans for the TiN grown with 550°C, suggesting some planes in the film are uniformly tilted. The film grown with 550°C had the least amount of background counts, indicating the least amount of point defects in this film. We previously found that TiN grown with 550°C resulted in optimized optical properties for plasmonic materials; this contributes to the previous conclusion that microstructure correlates with optical properties in TiN. The results of this work emphasize the importance of specific deposition conditions—in this case, substrate temperature—to minimize strain and defects for achieving optimal plasmonic properties for TiN thin films.

Focal plane arrays (PFAs) based on type II superlattice (T2SL) operating in mid-wave infrared (MWIR- 3-5 mm) and long-wave infrared (LWIR 8-12 mm) are of great importance for many advanced surveillance and imaging systems for a variety of civil and military applications. These detectors have better accuracy and sensitivity with low false alarms even in complicated backgrounds and are promising alternatives to the technically developed HgCdTe in MWIR and LWIR detection. When the focal plane contains hundreds or thousands of semiconductor elements a single spot of incident energy can stimulate more than one element. The unwanted electrical and optical crosstalk is the most important factor that hinder the performance. It has been found that III-V semiconductor (InAs, InSb, and InSb) strained layer superlatice (SLS) are very promising in terms of reducing the crosstalk, because of the intriguing broken gap alignment and interesting fundamental physics (Bose-Einstein condensation of excitons and the quantum spin Hall effect.

In the present work, the band structure of (InAs)_n/(InSb)_m type-II superlattices is calculated within the density functional theory (DFT) framework using Perdew-Burke-Ernzerhof (PBE) approximation for the exchange-correlation functional using the Qunatum Esspresso (QE) package. For the calculation of spectroscopic properties of the material (absorption spectra), time dependent density functional theory (TDDFPT) package in the QE distribution is used. We investigate the bandstructure of (InAs)_n/(InSb)_m T2SLs with different layer thickness. Understanding and controlling the bandstructure of InAs/InSb T2SL will facilitate to acheive any desired bandgap for infrared detection and the negative bandgap will open a new avenue for the new physics phenomena.

Despite multiple attempts to achieve optical refrigeration in very high (99.5%) external quantum efficiency (EQE) GaAs, no cooling has been observed to date. In this study, we investigate optical refrigeration in GaAs by numerically solving the transient drift-diffusion equation coupled to Poisson’s equation. Obtained charge carrier distributions, together with the heat diffusion equation, allow us to observe the spatial and temporal evolution of cooling/heating within GaAs. Our results indicate that maximum cooling occurs at a laser intensity different from that which maximizes its EQE. A 6-fold difference in cooling power exists. We ultimately find that samples suspended in vacuum using a 100 µm SiO₂ fiber cool to 83 K. These results emphasize the critical importance of choosing an appropriate laser excitation intensity to achieve optical refrigeration along with minimizing the conductive heat load on the refrigerator. Beyond this, results of the study are applicable towards analyzing the optical response of other optoelectronic systems where accurate charge and/or heat diffusion modeling is critical.

Metallic oxide thin films are used for applications ranging from anti-reflective coatings to microelectronics. Sol-gels are useful for creating these films due to the ability to create high quality films without expensive and complex deposition equipment.

In this work, sol-gel blends are created using a combination of previously developed high index (n~2.4) TiO₂-based sol-gel made directly from the metal chloride and a low index (n~1.5) SiO₂ based sol-gel synthesized by the standard alkoxide route. Blends are prepared with different ratios of these sol-gels and spun onto glass or ITO-on-glass substrates. The thicknesses, refractive indices, and dielectric constants of the resulting films are measured using profilometry, prism coupling, and an LCR meter, respectively.

Results show that including more SiO₂ based sol-gel in the initial mixture enables thicker films ranging from 1-10 μm, while resulting in lower refractive indices and lower dielectric constants. Refractive indices measured were 1.5 and 1.75, and dielectric constants up to 8 were observed at 100 kHz. This is consistent with the expected results due to SiO₂ having a lower refractive index and dielectric constant over the range of wavelengths and frequencies explored. The ability to fine tune the properties is explored, with the trade-off between processability and dielectric properties being the primary focus.

In the future, these sol-gel blends can be used within photonic circuitry, or within either stand-alone or integrated capacitors devices.

In this study, we have investigated the influence of the geometrical confinement and impurity position on the optical properties (linear and third order nonlinear) associated with the 1s-1p intersubband transition of a single dopant in a SiO2/Si cylindrical core/shell nanotube. Our calculations are performed in the framework of the effective mass and parabolic band approximations and the energies are obtained by using a variational method. Our results show that linear and nonlinear parts of the absorption coefficient and the refractive index associated to the intersubband 1s-1p transition undergoes important changes. There are different interesting results to point out such as the shift of the absorption coefficients and refractive index to high values of photon energy. Another significant result is that the donor position considerably affects theoretical properties and their corresponding magntitudes.

Detection techniques using waves are used in many areas such as ultrasound wave, radar, and infrared wave detection. However, these detection technologies also have a side effect that the ally's operational activities that emit radiation by body temperature are exposed to the enemy. Therefore, it is required to develop anti-detection techniques for protecting our assets. To diminish the detection possibility, many researches about anti-detection for radar and ultrasound wave have been progressed. In case of the Infrared wave, there is difficulty to realize camouflage, because Infrared emission passively depends on the surface temperature of the body. For Infrared dectection, Infrared detectors mostly detect wavelengths outside the band of 5 ~ 8 microns, because the absorption by atmospheric air are the largest in the 5 ~ 8 microns band. Therefore, if the emission characteristics can be manipulated that energy is emitted through in 5 ~ 8 micron band to a maximum and out a range of 5 ~ 8 micron to a minimum, the infrared camouflage can be realized.
Recently, metamaterials have been cultivated as means to control emission characteristics. By modifying the structure’s size of selective emitters, we can control specific wavelengths to emit. In this study, we design the broadband infrared emitter which emits in 5 ~ 8 microns band through the multiple-size structure for reducing the thermal instability caused by the narrow bandwidth which can cause the out of control emission.
Firstly, to determine the diameter of cylinders, a numerical method was used by using Finite element method. Commercial code (COMSOL 5.2a) was used to simulate the physical phenomenon of MDM structure. For the validation, a single-diameter structure was fabricated by micro-nano fabrication process and compared with numerical results. After the emission characteristics of single-diameter structure are analyzed, we could derive two diameters of the cylinder for having the MDM structure emit in the band of 5 ~ 8 microns. In addition, we simulated the derived structure. As a result, the multiple-diameter structure shows the dual peaks within the in the band of 5 ~ 8 microns. Additionally, we compared the total radiated energy between the selective emitter and broadband emitters for evaluating the radiation performance. Total radiated energy is increased by 33% for comparing the single-diameter structure and multiple-diameter structure and is 120% and 130% more for two layers of MDM and three layers of MDM.
In conclusion, through on this study, we derived a structure that emits broadband in 5 to 8 bands to camouflage from infrared detectors. In order to realize the broadband, the absorption peaks could be widened to the left and right through the multi-diameter structure and stacked method and this study will help to understand the effect of the metamaterial structural characteristics on the radiation performance and to understand the application of metamaterials to real life.

The application of nano-photonic structures for organic solar cells (OSCs) has been quite popular and successful, leading to increased optical absorption and better spectral overlap to solar irradiances. With the introduction of plasmonic cavity structures [1-3], textured light trapping structures [4-5], or multi-plasmonic effects [6-8], significant improvements in the power conversion efficiency (PCE) have also been reported, now exceeding 11%.
Nonetheless, with the limitations of given material properties of OSCs such as low optical absorption and narrow absorption band, together with the high-Q narrow band plasmonic effects, the PCE of single-junction OSCs has been stagnant over the past few years, at the barrier of 12% [9-10].
In this work, we demonstrate an ultra-thin inverted OSC structure which provides near-perfect optical absorption over the entire absorption band of commonly used organic active material. By introducing a smooth spatial corrugation to the vertical plasmonic cavity which encloses the active layer, we successfully derive a strong multi-peak in-plane plasmonic propagation modes, in addition to the significant broadening of the Fabry-Perot cavity modes in the UTMF-electrode OSC. As a result, we achieve highly uniform (> 85%) and ultra-low Q (330~775 nm) broadband absorption in the ultra-thin active layer, which directly translates to the record-high PCE approaching ~ 13%. With the newly introduced concept of long-period spatial corrugation for the UTMF based OSC, we expect further applications of the same concept for solar cells of different materials and structures, targeting for even broader responses and higher PCE. The significance of spectral engineering in the PCE of ultrathin OSC will be discussed, together with the detailed study on electrical carrier transport dynamics in the device.

[1] J. Huang, and et al. Adv. Energy Mater. 2015, 5, 15, 1500406.
[2] J. Ham and et al. Adv. Mater. 2015, 27, 27, 4027-4033.
[3] K. S. Chen, and et al. Adv. Mater. 2014, 26, 20, 3349-3354.
[4] J. D. Chen, and et al. Adv. Mater. 2015, 27, 6, 1035-1041.
[5] C. Cho, and et al. Adv. Opt. Mater. 2015, 3, 12, 1697-1702.
[6] K. Yao, and et al. Adv. Funct. Mater. 2015, 25, 4, 567-574.
[7] L. Lu, and et al. Nano Lett. 2012, 13, 1, 59-64.
[8] X. Li, and et al. Adv. Mater. 2012, 24, 22, 3046-3052.
[9] Y. G. Bi, and et al. Org. Electron. 2015, 27, 167-172.
[10] J. D. Chen, and et al. Adv. Energy Mater. 2014, 4, 9, 13

Surface-enhanced Raman scattering (SERS) is a convenient and highly sensitive method for even single molecular level detection. SERS has been extensive use in the field of chemical, material, environmental, and medical sciences. SERS sensitivity is an important role in sensor field because high sensitivity means recognizing target molecular as early as possible. In order to improving SERS sensitivity, advanced and complex metal nano structures have been used to produce a huge electric field in hot spot. Although the metal nano structures have shown high performance, the high cost and poor spatial uniformity are critical drawbacks. In this work, we propose an inexpensive, reliable, and ultrasensitive SERS platform based on M13 bacteriophages (M13 phage) and a simple metal nano structure. The employed simple metal nano structure is a sandwich structure between single silver nanowire and gold film. When the nanowire is well dispersed on the film, the individual hot-spot can be well specified through optical microscope. Therefore, we can believe that each different position of SERS hot-spot shows same properties. Diameter of the silver nanowire is around 300 nm and length is around 30 um. Absorption and localized surface plasmon resonance spectra were measured to understand basic plasmonic properties of the structure. Three-dimensional finite difference time domain simulations (3D FDTD) was also calculated. The most important reason for using the structure is that the shape of its hot-spot is long as like M13 bacteriophage. Because the structure and M13 phage are similar in shape, they can be aligned well. M13 bacteriophage is a promising functional material due to its interchangeable and functionalized peptides on its surface. Genetic manipulated M13 phage can have selective adhesive strength with a target material. When the M13 phage placed in the hot-spot of the structure, the SERS platform can have selectivity for the target material. To verify this, SERS measurement were performed using the structure with and without the M13 phage. Target molecular, 20 microliters of paraquat (PQ), was dropped on the structure and dried. the droplet size is around 0.5 cm x 0.5 cm. Without the M13 phage, the structure shows the limit of detection over picomole range for PQ. When we introduced the M13 phage that was genetically engineered for selective biding of PQ, the structure shows the limit of detection around femtomole range for PQ. M13 phage-based SERS platform exhibited a dramatic improvement of sensitivity compare to the structure without the M13 phage.

Yttria-Stabilized Zirconia (YSZ) is known to be a thermal and chemical stable functional oxide with a refractive index of about 2.12, which allows good light confinement of the optical mode. Moreover, it has a large energy bandgap avoiding two photon absorption (TPA) in near and mid-IR and its transparency covers the wavelength range from visible to mid-IR. Additionally, Kerr effect has been recently demonstrated. While these optical properties are very appealing for various applications including on-chip optical communications and sensing, YSZ has remained almost unexplored in photonics [1,2]. In this regard, we recently demonstrated YSZ waveguides with propagation losses as low as 2 dB/cm at a wavelength of 1380 nm [3]. Based on the promising results obtained in such passive photonic structures, we have recently looked into the development of active systems based on YSZ thin films. For that, we have introduced optically active rare-earth (RE) dopants into the matrix to investigate the correlation between the luminescence properties and the microstructure of thin films grown under different conditions.
In this work, we report on the optical and structural properties of Er-doped YSZ thin films showing strong emission at 1.53 and 1.536 µm under a continuous-wave pump laser excitation at about 980 nm. Yttrium-to-erbium substitution is performed in the Yttria-Stabilized Zirconia (YSZ) crystal. The observed outstanding luminescence will be analyzed and discussed. Remarkably, the Er-doped YSZ system provides a nearly perfect allocation of Er ions in the host matrix caused by a similar atomic radius between Y and Er, enabling an efficient optical activation of such dopants.
These results pave the way towards the implementation of new rare-earth-doped functional oxides into hybrid photonic platforms in a customized and versatile manner, adding new functionalities including light amplifiers that may be instrumental for nanophotonics applications.

[1]. Liu, X., Nature Photonics, 4(8), 557 (2010)
[2]. Ma, P., Optics express, 21(24), 29927-29937 (2013)
[3]. Marcaud, G. Phys. Rev. Materials 2, 035202 (2018)

Recent advancement in nano-engineered structures based on deep subwavelength-scale confined electromagnetic waves (polaritons) enable tight spatial confinement of light, which is greatly benefiting nanophotonic and optoelectronic applications. Hyperbolic media, where the permittivity is opposite in sign along orthogonal axes, support highly directional propagation of volume-confined, hyperbolic polaritons (HPs) which have been demonstrated for use in super-resolution imaging via the hyperlens concept. Hexagonal boron nitride (hBN), a natural hyperbolic material, has been demonstrated to support deeply subdiffractional, low-loss HPs in both planar slabs and nanoscale resonators within the mid- to long wave IR. Recently it was demonstrated that through the implementation of monoisotopic (i.e. material with just a single boron isotope) hBN these losses could be reduced even further. Here we exploit these ultralow losses and natural hyperbolic response to realize unprecedented spatial resolution in hyperlensing with long-wavelength IR light. We provide a direct comparison of the imaging power of hyperlens designs using flat slabs of naturally abundant and monoisotopic hBN via scattering-type near field optical microscopy (s-SNOM). Our experimental (s-SNOM) and simulated results show the ability to resolve features as small as 50 nm with 6-7.1 µm free-space wavelength light, providing at least l/125 spatial resolution. We complement this with electromagnetic field simulations of the hyperlens response to demonstrate and quantify the improvements from the monoisotopic over the naturally abundant materials.

Photovoltaic devices can provide a mobile source of electrical power for a variety of applications in both space and terrestrial environments. Many of these mobile power applications can directly benefit from enhancements in the efficiency of the photovoltaic devices. Space-based PV systems can also utilize technologies that improve radiation hardness, operating temperature range, efficiency, and specific power. To improve the radiation tolerance of multijunction III-V solar cells, we describe a device design which integrates reflector structures and quantum well absorbers into existing space power cell technology.

Thinning the GaAs-based middle subcell of a triple junction solar cell provides a means of improving radiation tolerance and thus extending end-of-life performance. To maintain the current generating capability of a thinned GaAs subcell, we are developing advanced structures that employ InGaAs quantum wells to extend infrared absorption and leverage optical cavity effects to increase the path length of near band edge photons. Device structures incorporating these elements we call a resonant cavity enhanced quantum well multijunction solar cell (RCE-QW MJSC). Simulations suggest enhanced absorption via resonant optical cavity effects can enable the GaAs middle subcell to be thinned to less than 1 um without sacrificing short circuit current density.

Prior work has demonstrated that InGaAs wells can be added to a GaAs cell while still maintaining a high open circuit voltage (V_oc ~ 1.05 V). Carrier collection efficiency in the wells can then be significantly increased with integrated photon management techniques. In particular, simulations indicate that adding a bottom reflector and a partial top reflector to generate resonant cavity effects can enable thin InGaAs quantum well subcells to match the current output of a conventional thick GaAs subcell. Prototype test structures incorporating both quantum wells and integrated top and bottom optical reflectors have been fabricated and tested. Fabry-Perot oscillations are observed in the external quantum efficiency and add to the collection efficiency in the well region and just above the GaAs band edge. These preliminary results validate the use of resonant cavity structures for increasing carrier collection and provide concrete insights into how to best optimize RCE-QW MJSC structures.

We present photonic structures having different heights and color intensities through a flow coating method on a single substrate. Flow coating is one of the solution coatings that use coffee-ring effect. Coffee-ring effect is a non-volatile solute remaining on the surface after evaporation. This pattern originates from the capillary flow that is induced by the different evaporate rate between the edge and center of the droplet. Flow-coater confines the position of droplet with blade on specific position and deposits the solute along the shore. There are various methods of depositing thin films on solid substrates and are of substantial importance in numerous field such as microelectronics, optics and sensors. Flow coating has several important features that are different from other similar methods to fabricate photonic structures. First, lithography or nanocasting need complex steps to achieve photonic structure but flow coating do not need a develop process. Second, deep coating need a long time until liquid evaporate and form a thick film. However flow coating overcomes such problems and has more advantages. Especially, flow coating can control the overall thickness easily, it changes spontaneously along the speed of blade. Particles deposit on the substrate forming close packed and well-ordered structure. Photonic structure can be made in that way. And photonic structure is three dimensional periodic structures that influence on incident light lay. When the periodicity of the photonic crystal is about half the light wave, photonic surface diffracts certain range of wave length. It has been found in nature, like avian feathers and wings of butterfly. They exhibit vivid color that reflect depends on their one structure. Two different nano size (195±5 nm and 300±5 nm) of silica Particles are prepared through the stober method. And different particles reflect different wave length showing blue or red colors when is being film structure. Ethanol is good medium to disperse silica and for flow coating. Solution evaporate fast inducing strong capillary follow, it make film quickly. The speed of deposition determines the thickness of film form 0.37±0.1 μm to 2.93±0.3 μm and shows different height of reflection peaks. Therefore, two different colored with different intensity can be made in a one substrate with one step of patterning process.

Studying light-matter interactions at the molecular level is critical to accelerate our understanding of life sciences. Nanoscale infrared spectroscopy, combining Atomic Force Microscopy (AFM) with IR spectroscopy, has made it possible to explore the vibrational modes excited in the sample. Using AFM subwavelength spatial resolution in IR spectroscopy can now be reached. Previous work shows that it is possible to design plasmonic substrate to locally enhance the electromagnetic field used to excite the molecules for higher sensitivity.
Here we use a cavity coupled plasmonic substrate to polarize and enhance the field used to excite the molecular vibration, detected by AFM. Our study focuses in studying the effect of chirality of biomolecules on the nanoscale resolved measurements captured with the AFM. We show that it is possible to generate a stronger confined electromagnetic field in the range of 1500-1800 cm-¹ by exploiting the cavity-coupled achiral plasmonic structure. We compare the response of the sample to both linearly and circular polarized incoming IR pulsed laser. For circular polarization, we acquire signals resulting from left handed and right - handed circular polarizations before subtracting them to reach information on the chirality. Molecules laying in the confined field at the plasmonic structures show stronger circular dichroism signal. The results suggest that using the “hot spots” of the cavity coupled plasmonic structures are significant and offer great potential for characterizing the chirality of single molecule. By using this approach, we expect to help distinguish chirality of biomolecules at nanoscale.

Most vivid colours in nature originate from the interference of light with nanostructured materials. In many cases, the nanostructure is not perfect and introducing disorder results in additional optical effects that might change the reflection properties. Our current research is broadly focused on two questions: (i) how are optical properties influenced by disorder? and (ii) can we replicate natural photonic structures with a controlled degree of disorder? For this, we systematically investigate the colour change of distributed Bragg reflectors in relation to its key parameters, i.e. the total numbers of layers, the refractive index contrast and their thickness.
Here, we have manufactured distributed Bragg reflectors (DBRs) using different techniques. Using spin coating, we deposit several layers of alternating porous materials where each layer has a controlled thickness and refractive index. We show that a reflector constructed of three repeating series of six layers gives optical properties that are a result of the disorder. In this case, the disorder is not fully random, but small deviations in the structure have a large influence on the colour and still produce a narrowband response. Alternatively, we show a different proof-of-concept solution using lithographically produced multi-layered patterns that readily assemble into a variety of Bragg reflectors with a different number of layers. We discuss the influence of disorder and the use of these materials in novel optical applications.

Nanostructured plasmonic systems support strong resonances and have led to new photonic applications. Nowadays, researchers compose metals and dielectrics to build even more sophisticated hybrid devices possessing optical properties exceeding the limits of purely dielectric or plasmonic systems. Here, we focus on a novel symmetric hybrid waveguide system. According to a semi-analytic Fourier Modal Expansion simulation, outstanding optical properties originate from hybridization between plasmonic and dielectric waveguide modes. Due to the symmetry, transmission resonances with remarkably high Q-factors as well as electromagnetically induced transmission (EIT) like phenomena with high refractive index sensitivity are predicted. These predictions were verified experimentally. The hybrid waveguides are fabricated by spin coating, UV-nanoimprint lithography, physical vapor deposition and symmetry-enabling lamination. In comparison to a non-symmetric device, the transmission at resonance is enhanced by a factor of 80 with a Q-factor of 400 through a 100 nm thick silver layer. These results show that both the advantages of plasmon modes and dielectric modes can be combined with our approach, and promise to improve upcoming generations of sensor and filter applications.

Since the first report of room-temperature photoluminescence from nanostructured porous silicon (PSi),¹ scientists have fantasized about PSi-based lasers² enabling the realization of integrated silicon photonic circuits.³
Although some trials on the fabrication of Si nanocrystal-based lasers have been attempted after the discovery of Si nanocrystal optical gain, a PSi-based laser has not been reported yet.⁴ In the past few years, leveraging a cheap and robust material preparation technique coupled with a high flexibility and high quality in optical structure fabrication, PSi has gained renewed interest in integrated optics and photonics, spanning from gradient refractive index (GRIN) optical elements,⁵ capable to finely control light propagation, and resonant microcavities.⁶

Here we report, for the first time, on low-threshold lasing from fully-transparent nanostructured porous silicon (PSi) monolithic microcavities (MCs) infiltrated with a polyfluorene derivative, namely poly(9,9-di-n-octylfluorenyl-2,7-diyl) (PFO).<span style="font-size:11px">7</span>
The hybrid PSi/organic laser is fabricated by electrochemical etching of crystalline silicon, with a square-wave current density profile that produces alternating nanostructured porous silicon (PSi) layers with different porosity and thickness values tuned to achieve a resonant microcavity (MC) operating in the visible region. The as-fabricated PSiMC was oxidized to avoid absorption of silicon, and then peeled off the silicon substrate using a PDMS slab to enable transmittance operation. Eventually, infiltration of PSiMC with PFO was carrid out via drop-casting.
The hybrid laser supports single-mode blue lasing at the resonance wavelength of 466 nm, with line width of ~1.3 nm and lasing threshold as low as 5 nJ (i.e. fluence of 15 μJ/cm²), which is among the lowest values of state-of-the-art PFO-based lasers.

[1] Cullis, A. G.; Canham, L. T., "Visible Light Emission Due to Quantum Size Effects in Highly Porous Crystalline Silicon" Nature 353, 335 (1991).
[2] Canham, L., "Gaining Light From Silicon" Nature 408, 411 (2000).
[3] Liang, D.; Bowers, "Recent Progress in Lasers on Silicon " J. E., Nat Photon 4, 511 (2010).
[4] Priolo, F.; Gregorkiewicz, T.; Galli, M.; Krauss, T. F., "Silicon Nanostructures for Photonics and Photovoltaics"Nature nanotech 9, 19 (2014).
[5] Ocier, C. R.; Krueger, N. A.; Zhou, W. J.; Braun, P. V., "Tunable Visibly Transparent Optics Derived from Porous Silicon "ACS Photon 4, 909 (2017).
[6] Ning, H.; Krueger, N. A.; Sheng, X.; Keurn, H.; Zhang, C.; Choquette, K. D.; Li, X. L.; Kim, S.; Rogers, J. A.; Braun, P. V., "Transfer-Printing of Tunable Porous Silicon Microcavities with Embedded Emitter" ACS Photon 1, 1144 (2014).
[7] V. Robbiano, G. M. Paterno, A. A. La Mattina, S. G. Motti, G. Lanzani, F. Scotognella, G. Barillaro, Room-Temperature Low-Threshold Lasing From Monolithically Integrated Nanostructured Porous Silicon Hybrid Microcavities, ACS Nano 12, 4536−4544 (2018).

Smart surfaces with dynamically tunable micro/nanoscale patterns have drawn a great deal of attention because their unique optical, electronic, biological, wetting, and adhesive properties are promising for a wide range of applications. Wrinkle structures, as one kind of mechanical instabilities, have been widely exploited recently to create hierarchical patterned surfaces because of their fast and simple fabrication process, feasibility to generate large-scale patterns, tunability and even complete reversibility in response to various external stimuli. To realize reversibly responsive surface patterns, various smart materials have been applied to wrinkling systems to realize dynamical and on-demand tuning of surface morphology and properties. However, these existing systems have at least one of several challenges such as high sensitivity to external stimuli, multiple and tunable responsiveness, mild and environmental friendly stimuli condition, or “functional” dynamic patterns.

Reconstituted silk fibroin, derived from the native Bombyx mori silkworm fibers, has been intensively investigated because of its outstanding combination of biocompatibility, biodegradability, all aqueous processing, optical and electronic properties, mechanical flexibility, and the resulting multiple applications in biomedical applications, bio-optics/photonics, electronics, and optoelectronics. Most importantly, silk fibroin undergoes conformational transition when triggered by external stimuli, such as water vapor, methanol or deep UV light. This polymorphic transition of silk fibroin enables the control of the molecular chain movement on the nanoscale, offering the possibility to controllably tune the pattern morphology formed due to mechanical instability.

We report the facile fabrication of reversible, multi-responsive wrinkle patterns using silk protein as the responsive component. We show that the wrinkle structures can be dynamically tuned and/or erased easily by water vapor, methanol vapor or UV irradiation. We demonstrate that the wrinkle evolution rate is dominated by the starting conformation of silk protein, and that the occurrence of conformational transitions accelerated the reversibility of wrinkle patterns. The wrinkle evolution behavior is confirmed by investigating the molecular mechanism governing the conformational transition of silk fibroin. Finally, we demonstrate that the tunable wrinkling systems can be used for information storage, encryption, collection and extraction, as well as light diffusion related smart window with switchable transparency and thermal regulation.

High haze, high transparency substrates can increase the power conversion and extraction efficiency of solar cells and light emitting diodes (LEDs), respectively. In this paper, we demonstrate a new octadecyl trichlorosilane (OTS)/silica nanostructured substrate that displays high transmission (91.5 ± 0.5% at 550 nm wavelength) and near unity haze (98.1 ± 0.5% at the same wavelength) with 143° scattering angle. The OTS/silica nanostructures are fabricated through a scalable and facile maskless reactive ion etching (MRIE) process followed by OTS coating. The OTS coating enhances the transmission of the structures by merging silica nanostructures together by capillary forces and effectively grading the index of refraction. The OTS/silica nanostructures display the highest combination of both transmission and haze in the literature as defined by Pareto optimality. The OTS/silica nanostructured glass exhibits lotus leaf-like wetting with a 159.7 ± 0.6° water contact angle (WCA) and 4.9 ± 0.6° contact angle hysteresis. We demonstrate the structures have self-cleaning functionality where about 100% of transparency can be easily recovered after graphite soiled substrates are rinsed with water. This self-cleaning functionality is maintained after 200 cycles of soiling and cleaning. The OTS/silica nanostructured glass may be an important substrate in optoelectronic applications where a combination of high transmission, high haze, and self-cleaning function are important

The nanoscale metal dot array has been used in many plasmonic devices and metamaterials. There are numerous methods to fabricate the nanoscale metal dot array including complex lithography and etching, random dispersion or coating of chemically-engineered metal particles. Here, we use the dewetting of gold (or platinum) thin film on a nano-patterned surface to create a periodically arranged metal dot arrays. By using nanoimprint techniques, nanohole array patterns are transferred to an alumina thin film and dewetting of gold (or platinum) thin film is followed. The dewetted metal pucks are periodically arranged on the top of the surface and inside the nanohole at the same time. These dual layers of periodic metal dot array can exhibit useful optical properties such as a broadband high absorption. Furthermore, by using FDTD simulation, we calculate the optical properties of the periodic metal dot arrays with different-sized metal dots and show the possible applications with this structure. Experimentally, the dewetting behaviors are presented with different conditions and surface patterns on which the dewetting occurs. The whole process here we use (nanoimprint and dewetting) can create dual layers of periodic metal dot array without complex lithography on various-shaped substrates with great scalability.

Infrared emitting materials are of great importance for LED, Laser, night-vision, and chemical sensor technologies. Superlattice (SL) structures of compound semiconductors such as from AlAs and GaAs semiconductors have been studied and used extensively for IR emitting Quantum Cascade Lasers. Until now MBE and MOCVD are the primary fabrication methods for SL fabrication but they are expensive and limited in their scale up. The hybrid organometal halide perovskite (HaP) materials, such as MAPbI₃ and MAPbBr₃, are a new class of semiconductors which are efficient, cheaper to fabricate, and potential candidates to replace expensive semiconductors which are currently used in solar and LED applications. Here we show a novel and efficient way to fabricate HaP SL's that emit in the Near-IR (NIR) region. We demonstrate that a cascade of layers of MAPbI₃ and MAPbBr₃ can shift an intense emission peak at 770 nm for 1 pair of layers to 900 nm for 3 pairs. The new technique and resultant SL's of MAPbI₃ and MAPbBr₃ are a promising candidate for extending the spectral range of HaP emission from NIR to the Mid-IR, with potential to deliver low-cost, efficient, tunable IR emitting LEDs and lasers, and new classes of spectroscopic chemical detectors.

The isolation of stable atomically thin two-dimensional (2D) materials on arbitrary substrates has led to a revolution in solid state physics and semiconductor device research over the past decade.¹ A variety of other 2D materials (including semiconductors) with varying properties have been isolated raising the prospects for devices assembled by van der Waals forces.² A fundamental challenge in using 2D materials for opto-electronic devices is enhancing their interaction with light, ultimately responsible for higher performance and efficiency in the devices. In particular, for photovoltaics; inorganic materials (e.g., Si, GaAs and GaInP) can concurrently maximize absorption and carrier collection. But thin film absorbers have lacked the above ability often due to due to surface and interface recombination effects. In contrast, Van der Waals semiconductors have naturally passivated surfaces with electronically active edges that allows retention of high electronic quality down-to the atomically thin limit.
In this seminar, I will show our recent work on photovoltaic devices from transition metal dichalcogenides of molybdenum and tungsten such as MoS₂, WSe₂ etc. We have recently demonstrated near-unity absorption in the visible part of the electromagnetic spectrum in < 15 nm films of these semiconductors by placing them on reflecting metal substrates such as gold and silver. We have further shown that these highly absorbing, ultrathin films can be further used for fabrication of simple Schottky junction photovoltaic devices with microfabricated metallic top contacts.³ While, this work helps solve the light-absorption problem, the external quantum efficiency EQE was < 10% for our Schottky junction devices. Very recently, we have extended this early work to fabricate p-n heterojunctions from p-WSe₂/n-MoS₂ and use graphene as a transparent top contact to amplify our current collection efficiency and push the EQE up to 50%,⁴ approaching that of many emerging photovoltaic technologies with active layers in the 100s of nm range. This represents a significant development as both light-absorption and charge collection have been addressed in these devices. Finally, I will then present ongoing work on addressing the key remaining challenge for application of 2D materials and their heterostructures in high efficiency photovoltaics which entails engineering of interfaces and open-circuit voltage. I will conclude by giving a broad perspective⁵ on 2D materials for mainstream as well as specialty applications as well as highlight some other novel applications of combining nanophotonic concepts with two-dimensional semiconductors.
1. Jariwala, D. et al. ACS Nano 2014, 8, 1102–1120.
2. Jariwala, D. et al. Nat. Mater. 2017, 16, 170-181.
3. Jariwala, D. et al. Nano Lett. 2016, 16, 5482-5487.
4. Wong, J.; Jariwala, D. et al. ACS Nano 2017, 11, 7230–7240.
5. Jariwala, D.et al. ACS Photonics 2017, 4, 2962-2970

In this work, we show how the near-field radiative heat transfer (NFHT) can be tuned using Al plasmonics, which spans to broader frequency range, as compared to noble metals, and it oxidizes. The oxide layer (Al₂O₃) changes the plasmonic response and even inhibits it. The formation of the oxide layer is the basis for our work in the control of NFHT.
The NFHT is mediated by the interaction of surface plasmons, in the case of metals and surface-phonons polaritons in polaritonic materials. The hybridization of these two modes changes the heat flux between surfaces [2,3]. Using Rytov theory of fluctuating electrodynamics, we calculate the heat transfer between two Al surfaces at a different temperature, separated by a gap. As the surface oxidizes as a function of time, the surface-plasmon response is inhibited, and the polaritonic modes of the Al₂O₃ layer begin to play an essential role in the heat transfer, increasing both the spectral heat transfer and the total heat transfer significantly. As the oxidation increases, plasmon modes of the Al and phonon modes of the oxide hybridize favoring the radiative heat transfer.

[1] M. W. Knight, N. S. King, L. Lie, et al. Aluminum for Plasmonics, ACS NANO, Vol. 8, 834 (2014).
[2] J. Perez Rodriguez, G. Pirruccio and R. Esquivel-Sirvent, Fano interference for tailoring near-field radiative heat transfer, Phys. Rev. Mat.(R) vol. 1, 062201 (2017).
[3] J. Perez Rodriguez, G. Pirruccio and R. Esquivel-Sirvent, Spectral thermal gaps due to plasmon-phonon mode interaction in bilayer systems arXiv:1807.10253 (2018).

Silicon photonics has grown to be a mature platform that is compatible with complementary metal-oxide–semiconductor (CMOS) technology and is one of the most potential solutions offering intrinsic advantages in terms of higher bandwidth and lower loss which microelectronics is limited to. However, due to the intrinsic indirect band gap of silicon, silicon photonics has strong limitations on the areas of light generation, modulation and detection. The emerging integration of two dimensional (2D) layered materials onto the silicon photonics platform provide viable solutions to the above-mentioned concerns owing to their unique electronic and optical properties, mechanically flexibility, low fabrication and integration complexity, robustness, and high compatibility with CMOS technology. The tunable electronic and optical characteristics, stability in aqueous environments, large surface area and the intercalatable layered structure of 2D materials can be potentially beneficial to expand the functionality of silicon photonics platform in the telecommunication prospect chemical and biological sensing field. Traditionally, silicon photonics platform heavily relies on the variation of ambient refractive index. However, many chemical and biological events do not lead to a measurable change of ambient refractive index by silicon photonic sensors. Meanwhile, complex surface functionalization processes with low repeatability is essential to the sensor selectivity.

Here, the concept of the 2D materials-silicon photonic chemical sensor is demonstrated using a representative tunable plasmonic 2D candidate. As the operation wavelength of the silicon device is either in the 1300 and 1550 nm telecommunication wavelength, the emerging plasmonics of 2D degenerately hydrogen doped molybdenum oxide (H_xMoO₃, 0 < x ≤ 2) can be a suitable candidate for coupling onto the silicon photonics platform as its plasmonic absorption wavelength can be precisely tuned across the edge of UV-Vis and the near infrared (NIR) region (>1000 nm). More importantly in such a degenerately doped plasmonic system, the H⁺ dopants and concurrently injected free electrons are easily exchanged from the 2D MoO₃ host structure in the presence of redox chemical and biological events, leading to the alteration of its plasmonic properties which provides the fundamentals for label-free chemical sensing with superior sensitivity.

The achievement of ultrathin films that strongly interact with light, absorbing photons over a wide spectral range is of central importance for applications such as sensing, energy harvesting, or biology among others. Strong broadband absorption is challenging in view of the intrinsic limitation in the absorption coefficient of every material, which is especially deleterious for infrared wavelengths. Many wave optics based designs are being currently investigated to surpass this limitation, from photonic crystals to plasmonics and microresonators. All these architectures provide new and exciting means of confining light in sub-wavelength thin films. Nevertheless, the absorption enhancements exhibited are typically restricted to a specific frequency range or mostly take place in the metal part. In sum, a photonic architecture capable of increasing the absorption of a semiconductor throughout its entire absorption coefficient has hitherto, remained elusive.

In our work, we demonstrate^[1] a germanium photonic architecture deposited on a metal film acting as a metasurface. Our metasurface sustains the simultaneous excitation of Fabry-Perot resonances, Brewster modes and plasmonic-photonic modes that result in an omnidirectional enhanced absorption in a Ge ultra-thin film of 70 nm from 400 nm until 1500 nm. This represents an unprecedented advance in broadband light harvesting, well beyond previous reports^[2].

The key aspects of our findings are:

The absorption in the metasurface exceeds over 100% that of a flat a-Ge film on gold over an impressive bandwidth of 1100 nm. Moreover, the high refractive index of the Ge renders the absorption profile of the metasurface independent to the angle of incidence of the incident light.

We provide the key design guidelines to tune the absorption profile of the metasurfaces and the physical origin beneath each resonant mode sustained by the architecture. With these findings, we show metasurfaces exhibiting strong broadband absorption (appealing to PV community) or NIR absorption peaks reaching 100% at the telecommunication windows (interesting to the photodetection field).


Remarkably, we fabricated the 16-mm² Ge metasurfaces via nanoimprinting lithography, the most promising method for mass-produced nanostructures^[3]. This inexpensive and large area technique adds feasibility to the exciting photonic properties described in the manuscript.

We expect our work to set a new benchmark in the field of light trapping in ultra-thin films and it paves the way for the demonstration of this technology into practical large area applications related to sensing, energy harvesting and photocatalytic devices.


[1] P. Molet, J. L. Garcia-Pomar, C. Matricardi, M. Garriga, M. I. Alonso, A. Mihi, Advanced Materials 2018, 30, 1705876.
[2] M. a. Kats, R. Blanchard, P. Genevet, F. Capasso, Nat. Mater. 2012, 12, 20.
[3] Y. Xia, J. A. Rogers, K. E. Paul, G. M. Whitesides, Chem. Rev 1999, 99, 1823.

Plasmonic sensors are known as an efficient tool for real time monitoring biomolecular interactions. The detection mechanism is based on the field perturbation at the plasmonic sensing interface induced by the binding of molecules. This molecular binding process could be recorded through reflected light signal when the surface plasmon is excited by the incident light. In this talk, we will present the use of hybrid 2D perovskite-based metasurface nanostructures as a plasmonic sensing substrate. The thickness of the sensing substrate is tuned in an atomic scale and optimized to obtain an enhanced sensing effect. More specifically, a sharp phase signal change and phase-related Goos-Hänchen signal shift was achieved that results from the strong resonance. The improved sensitivities of 2D Perovskite nanostructures were investigated. It is worth noting that the atomic layer design led to the sensing substrate optimized with a tuning scale less than 1 nm. By precisely design the metasurface substrates, more than 4 orders of magnitude improvement of the sensitivity (900,000 um/RIU) were obtained in comparison to the one with pure gold sensing substrate (400 um/RIU). This 2D perovskite-based metasurfaces would pave a way for the development of ultrasensitive and compact biosensors for in-situ applications.

Two-dimensional (2D) materials continue to be investigated in optical, photonic, and plasmonic applications due to their tunable properties, solution-processable control of lateral size and thickness, and exceptional performances compared to their bulk counterparts.¹ The largest family of 2D materials, known as transition metal carbides and/or nitrides (MXenes), has a general formula of M_n+1X_nT_x, where M represents a transition metal (Ti, Mo, Nb, V, Cr, etc.), X is either carbon and/or nitrogen, and T_x represents surface terminations.² Early optics work pioneered on titanium carbide (Ti₃C₂T_x) MXene showed high conductivity at high transparency (6500 S/cm, >97% transparency per-nanometer thickness in the visible range) and an optical absorbance peak in the ultraviolet as well as the visible to near-infrared range (750-800 nm), making it useful in transparent conductors, SERS substrates, photothermal therapy, as a metamaterial, etc.^3-5 With the material family containing ~30 different synthesized compositions so far (and millions possible), optimization and light-matter characterization of other MXene compositions remains important.

Here, we will discuss optical absorbance characterization from the ultraviolet to the infrared for a variety of MXenes beyond Ti₃C₂T_x, including MXenes composed of Nb, V, and Mo metals. For example, thinner M₂XT_x MXene structures, such as Ti₂CT_x, exhibit an optical absorbance peak shift to the visible range (~550 nm), demonstrating the optic/electronic differences from similar-but-thicker structures, such as Ti₃C₂T_x.⁶ This work provides insight into the optical properties of MXenes as well as spectroscopic information which can be applied to designing next-generation plasmonic and photonic devices, such as photodetectors, electro-chromic devices, random or femtosecond lasers, photothermal therapy agents, transparent conductors, photonic diodes, metamaterials, and more.

References
1. Xia, F. N., et al., Nature Photonics 2014, 8 (12), 899-907.
2. Anasori, B. et al., Nature Reviews Materials 2017, 2(2) 16098.
3. Dillon, A. D., et al., Adv. Funct. Mater. 2016, 26 (23), 4162-4168.
4. Sarycheva, A., et al., The Journal of Physical Chemistry C 2017, 121, 19983-19988.
5. Chaudhuri, K., et al., ACS Photonics 2018, 5, 1115-1122.
6. Alhabeb, M., et al., Chemistry of Materials 2017, 29, 7633-7644.

Split-ring resonator (SRR) based metamaterials have been extensively studied due to their enhanced ability to confine light of wavelength several orders of magnitude larger than the dimension of the resonator. The strong confinement of incident light has been explored for the development of a diverse range of sensors including biological and chemical sensors that can assess the properties of the molecules in the vicinity of the split. The polarization dependent switching of resonant modes and resonant frequency of two-dimensional (2D) SRR structures presents a major hurdle in the widespread application of metamaterial sensors where the orientation of the SRR is hard to control. Furthermore, the 2D planar SRR structures do not undergo any resonance when the incident electric field is polarized perpendicular to the plane of the resonator. However, if the 2D metamaterials are self-assembled to form three-dimensional (3D) structures, novel optical properties can be achieved that cannot be realized in 2D planar SRRs. When six symmetric 2D X-shaped resonator segments fabricated on top of square surfaces are self-assembled, the resulting 3D cubic structure forms an 8-pointed star-shaped octagram SRR (OSRR). The octagram resonator consists of splits only at the corners of the cube, thus, achieving 3D SRR splits that are equally moderated by all three vectors (electric field, magnetic field, and wave) for every possible orientation of the OSRR. The 3D splits in OSRR achieve a perfectly isotropic (angle-invariant) transmission response. The strong OSRR coupling and angle-invariant amplitude offer a two-fold advantage i.e. a 25 times higher shift in resonant frequency (25 times higher sensitivity) than 2D SRRs and amplitude-based detection at low concentrations that cannot cause a measurable shift in resonant frequency. However, even with the enhanced sensitivity the constant need for replenishing antibodies used for binding the targeted molecules to the surface of the OSRR presents a major challenge and can also limit the detection capabilities for unknown target molecules. If the surfaces of the cube forming the OSRR are composed of porous nanomaterials (graphene oxide), the properties of the 3D metamaterials can be further tuned for desired sensory application. The strong affinity of GO functional groups towards all chemical and biological molecules and sieving properties of the porous GO layers provide an ideal surface for adhesion of targeted molecules for non-labeled sensing mechanisms. By varying the number of GO layers forming the 3D cubes, properties of the OSRR are tuned for control over sieving and molecular adsorption as well as extending the sensitivity of octagram sensors due to enhanced adhesion of targeted molecules.

Engineered metamaterials and photonic crystals with non-trivial topological properties represent an emerging class of photonic structures exhibiting inherent robustness to continuous deformations. While topological photonic insulators have been the subject of growing interest and research efforts in the past few years – especially for the possibility of realizing back-scattering-immune unidirectional wave-guiding systems – relatively less attention has been devoted to scattering and radiating topological photonic structures in the presence of radiation loss or material loss/gain, a research direction that may lead to novel functionalities and applications. In this talk, we will review our recent efforts on different fronts in this exciting area. (i) We will discuss our theoretical and experimental demonstration of the topological nature of embedded eigenstates, or bound states in the continuum, in suitably engineered optical metasurfaces [H. Doeleman, F. Monticone, W. den Hollander, A. Alù, and A. F. Koenderink, Nature Photonics, in press (2018)], confirming a theoretical prediction made in [B. Zhen, et al., Phys. Rev. Lett. 113, 257401 (2014)]. (ii) We will present our recent work on topological wave-guiding structures with radiation loss, which support topologically-protected one-way leaky modes that may act as a bridge between free-space radiation and unidirectional guided waves propagating on the surface of complex bodies [S. A. Hassani Gangaraj and F. Monticone, J. Phys. Condens. Matter (2018)]. These findings may lead to relevant advances in the design of leaky-wave (nano)antennas. (iii) We will discuss the possibility of engineering the coupling between topological modes in Hermitian and non-Hermitian waveguides, and the presence of exceptional points accompanied by anomalous propagation effects, such as one-way, defect-immune, loss-immune “Jordan modes” [S. A. Hassani Gangaraj and F. Monticone, arXiv:1803.06419 and arXiv:1805.03767]. (iv) Finally, if time permits, we will also present our recent findings on giant optical forces and torques on localized emitters near photonic topological materials.

Topological photonic engineering leverages the inherent nonlocality of Bloch states in periodic media to enable scatter-free and unidirectional light transport. A DC magnetic field can lift Dirac point degeneracies in a 2D photonic crystal, opening a topologically nontrivial band gap which supports unidirectional edge modes. However, due to the weak and lossy nature of optical Faraday rotation, such magnetic-field sensitive configurations are too bulky for practical implementation and are challenging to reconfigure on fast time- and small length-scales. To date, compact, reconfigurable topological protection at optical frequencies remains an outstanding challenge.

Here, we propose an entirely new route to achieve unidirectional edge modes and topological protection based on photon-phonon interactions. Our approach is inspired by Floquet driving schemes of electronic topological insulators. We break time-reversal symmetry using photon-spin degrees of freedom to excite lattice phonon modes with the same chiral symmetry. Notably, we utilize Stimulated Raman Scattering with circularly polarized light to generate nontrivial topology in a photonic structure. If the pump illumination is circularly polarized, the off-diagonal elements of the Raman- induced effective susceptibility become complex conjugates at the Stokes-scattered frequency, meaning Raman gain inherits a magneto-optic like form without the need for a magnetic field. Unlike magneto-optical materials, stimulated Raman scattering leads to spin dependent dissipation/amplification, bringing a new degree of freedom to topological photonic experiments. Therefore, resonant nanophotonic structures incorporating Raman active materials open the possibility for deeply nanoscale topological protection at optical frequencies.

As a proof of concept, we design a hexagonal photonic crystal consisting of cylinders of Silicon in the near infrared. We use Silicon due to its high Raman susceptibility. First, we calculate the bandstructure of the photonic crystal; a period of 675 nm and cylinder radius of 200 nm exhibits a Dirac cone at the K point at 1500 nm. By locally pumping the photonic crystal with circularly polarized light such that the Dirac point is near the Stokes or anti-Stokes line, a topologically nontrivial band gap is opened. By varying the pump wavelength and helicity in neighboring crystals, we gain intimate control over the formation and evolution of topological defect states. We theoretically demonstrate typical one-way edge state phenomena, as well as unidirectional edge state amplification. We also show how a variety of reconfigurable optical devices such as waveguides, circulators, and resonators can be readily generated by this system.

The investigation of surface plasmon polaritons is a highly investigated field of research due to their high potential to be applicable tosensors1, information technologies2, high-resolutionimaging3, etc. These coherent delocalized electron oscillations are common at metal-dielectric interfaces. However, they also exist in highly doped semiconductors, conducting oxide system or graphene, i.e., in many other systems with high carrier mobility. This poses the question whether such resonances can also be observed at the insulator interfaces.

Bismuth Telluride family can be count as a role topological insulator materials system which has been heavily investigated. It was already shown in 2013 Bi2Se3 one of the member of the family supports Dirac plasmons with energy in the range of 0.5 – 1 eV4. The Dirac state is not the only reason for the existence of plasmon resonance in Bi2Se3. The highly anisotropic tetradymites crystal structure also has highly anisotropic dielectric properties5 allowing plasmon excitations.

In this study we would like to show electron energy loss spectroscopy (EELS), energy-filtered transmission electron microscopy (EFTEM) and a finite-difference frequency-domain (FDTD) study for investigating Bi2Se/Te3 and try to find an explanation for plasmon polaritons.

References:
1. Wang, Z et al., Analytical Chemistry, 86 (2013), 1430-1436.
2. Kosmeier, S. et al., Scientific Reports. 3 (2013), 1808.
3. Cai, W.; Gao, T.; Hong, H.; Sun, J., Science and Applications 2008, 17-32.
4. Di Pietro et al., Nat Nano 8 (2013),556-560.
5. Esslinger, M. et al., ACS Photonics 1 (2014), 1285-1289.
6. Talebi, N, et a., ACS Nano 10, (2016), 6988-6994

Topological photonic structures in analogy to their electronic counterparts can provide new functionalities in nanophotonics through topological protection. Topologically protected photonic modes can propagate unidirectionally without scattering and can have an extreme photonic density of states (PDOS). These unique properties can directly impact many photonic systems used in quantum information processing applications such as single photon transport. Enabling such properties at optical frequencies and on chip-scale will be very important for practical applications of such phenomena. Photonic system composed of appropriately designed two-dimensional photonic crystals (PhC) preserving time-reversal(TR) symmetry can exhibit pseudo-spin based topological behavior. A membrane-type PhC composed of modified honeycomb lattice ( HC) can show topologically protected unidirectional photonic pseudospin transport with excellent bandwidth (~ 50nm) at optical wavelengths ( ~ 1550nm) , important for chipscale nanophotonics¹. We will discuss design, fabrication and optical response of such topological photonic structures in semiconductor based systems like silicon-on-insulator (SOI)².

Sandia National Laboratories is a multimission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525. The views expressed in the article do not necessarily represent the views of the U.S. Department of Energy or the United States Government.

1. Anderson, P. D.; Subramania, G., Unidirectional edge states in topological honeycomb-lattice membrane photonic crystals. Optics Express 2017, 25 (19), 23293-23301.
2. G. Subramania, P. Duke Anderson, S. Foteinopoulou, J. Dominguez, A. James, " Optical Frequency Topological Photonic Structures" , In Preparation.

Using infrared microspectroscopy, the localized surface plasmon resonances (LSPRs) of metallic nanoarc antennas on dielectric substrates have been systematically investigated. The reflection/transmission spectra are rich with information regarding the fundamental and higher order LSPR modes, the dipole component polarization, and mode coupling. The relationship between the LSPR wavelength and the geometric parameters of single nanostructures was established. Furthermore, the relative strength of the different order LSPRs was shown to depend on the central angle of the nanoarc. These rules are used in optimizing the nonlinear optical response of nanoarcs, specifically enhancing the efficiency of Second Harmonic Generation. Mode coupling was studied in a pseudo-chiral assembly of four 90°-nanoarcs in a propeller-like configuration. At the center of the nano-propeller, the arcs were separated by narrow gaps, down to 15nm. The strength of coupling of the plasmons in the adjacent nanoarcs, inversely proportional to the gap size, was manifested in the infrared absorption and circular dichroism (CD) spectra. The CD response may be significantly stronger in 3-dimensional propeller-like configurations that have a polarizability component normal to the surface. This information is utilized in the design of thin-film plasmonic metamaterials with engineered chiroptical responses.

Two-dimensional materials exhibit a fascinating range of electronic and photonic properties vital for nanophotonics, quantum optics and emerging quantum information technologies. Merging concepts from the fields of ab initio materials science and nanophotonics, there is now an opportunity to engineer new photonic materials whose optical, transport, and scattering properties are tailored to attain thermodynamic and quantum limits. In this context, I will present a new class of engineered 2D metals, Argentene and Cuphene, that exceed properties of state-of-the-art optical materials. Achieving high carrier density and mobility in a 2D material like could be transformative for atomic-scale photonics for optical elements such as monolayer waveguides, sensors, and emission control layers. However, realizing the potential of 2D metals (truly monolayer metals) requires an understanding of single-crystalline atomic layers of metals and suitable combinations of substrates, with computational screening of thermodynamic stability. In addition to 3D crystalline substrates, I will present the feasibility of metal monolayers on existing 2D materials in order to facilitate their incorporation into van der Waals stacks. This is an example of a carrier lifetime-driven approach to quantum materials where we expect time-domain properties of a monolayer to be distinct from few-layer and bulk. Further, I will discuss the interplay between optical, topological and transport properties of Weyl semimetals that are promising new nanophotonic platforms.

Single-photon sources are elementary building blocks for photonic quantum technologies where the photons are used as “flying” qubits [1]. One of the central challenges towards practical applications of single-photon sources is the ability to efficiently extract light from the emitters. Efficient extraction of light from a single emitter in free space is limited due to its isotropic nature of emission. So, a useful single-photon source must emit into a well-defined direction because in practice one can collect light only in a finite solid angle. Here, we propose to harness the exceptional light molding capabilities of photonic metasurfaces [2] to engineer the emission from quantum emitters and achieve highly directional emission. Photonic metasurfaces mold optical wavefronts at subwavelength spatial resolution via phase gradients imparted through the interaction of light with the metasurface elements. They have revolutionized optical designs by enabling the realization of virtually flat optics. Recently, it has been proposed that a judiciously designed metasurface can be harnessed at single-photon level [3].

In this work, we designed a phase gradient metasurface which efficiently collects spontaneous emission from a quantum emitter, located in the far-field (d~5λ), and redirect it back to the source. By controlling the phase imprinted by the metasurface on the incident light, we control the directionality, radiation pattern, and emission rate. In the experiment, our quantum emitters are dibenzoterrylene (DBT) molecules embedded in a thin film of anthracene. In the past decade, such organic molecules embedded in an optimal host have emerged as an excellent source of indistinguishable and Fourier-limited single photons with high fidelity and high rate of emission [4]. Remarkably, the spectral region covered by optimal guest-host molecule combination lies in the range of silicon-based photodetectors, whose detection efficiency peaks around 800nm. Furthermore, the emission in the near infrared wavelength range would lower the ohmic loss in noble-metal based nanoantennas. Our simulations show that at 785nm which corresponds to the zero phonon line of DBT in anthracene, the reflection efficiency is greater than 85% with gold nanoantennas. We show that the emission from these molecules are highly directional with deviation from upward direction 2Δθ~20°. Furthermore, we show that we can not only control the emission pattern but also decelerate (or accelerate) the decay rate of the emitters. Moving forward, scaling up to multiple quantum emitters and integrating them with tunable metasurfaces [5] may open new opportunities in quantum information sciences.

References:
[1] B. Lounis and M. Orrit, Rep. Prog. Phys. 68, 1129 (2005).
[2] N. Yu and F. Capasso, Nat. Mater. 13, 139 (2014).
[3] P. K. Jha, et al., Phys. Rev. Lett. 115, 025501 (2015).
[4] A. A. L. Nicolet, et al., ChemPhysChem 8, 1929(2007).
[5] G. K. Shirmanesh, et al., Nano Lett. 18, 2957 (2018).

The past decades have brought increasing interest in using materials to control light at the nanoscale for diverse applications such as light sources, quantum optics, spectroscopy, spintronics, and sensing. A key paradigm in controlling electromagnetic energy at the nanoscale are materials that support strongly confined surface polaritons. The most well-known example of surface polaritons are surface plasmon polaritons (SPPs) on conductors, which have been actively studied for several decades. The enabling principle of the strong sub-diffractional light confinement by these materials is that they can sustain a negative electric permittivity. In this work, we propose a new twist on the aforementioned paradigm: negative magnetic permeability modes in antiferromagnets to enable nanoscale control light-matter interactions and spin dynamics. These modes, known as surface magnon polaritons (SMPs) have been observed in antiferromagnetic materials such as MnF₂ [1] and FeF₂ [2] at THz frequencies. Here, we discuss for the first time enhancement of spin relaxation in emitters using these highly confined magnetic polaritons.

SMPs on antiferromagnetic films offer several advantages over dielectric-based polaritons for engineering strong interactions with spin and other magnetic transitions. We use spontaneous emission of a dipole emitter into SMPs on MnF₂ as an example system for understanding the strength of interactions. We show that a ω=1.6 THz magnetic dipole may transition and emit SMPs into a 100 nm MnF₂ film at rates more than 10¹¹ times higher than the free space rate of 10^-6/s, for a total rate of 10⁵/s. These extreme Purcell factors are possible because SMPs on antiferromagnetic systems can reach wavelengths on the order of 10-100 nm, in some cases more than 10⁵ times smaller than for the same frequency in free space. This is particularly advantageous for speeding up magnetic transitions which are highly inefficient in free space. Another advantage of an antiferromagnetic platform is that SMP mode properties, and by extension dynamics of magnon-matter interaction, can be manipulated by applying an external magnetic field. We explore SMPs as an electromagnetically dual analog to SPPs, and find that Purcell factors for magnetic transitions on magnonic media scale identically to those of electric transitions on plasmonic media [3], offering a more complete picture of interactions between polaritons and matter.

Until now, methods of magnetic transitions enhancement have been insufficient to allow spin relaxation and electric dipole transitions to compete on the same playing field. Our results present a new route towards bridging this gap.

[1] R. E. Camley and D. L. Mills, Phys. Rev. B 26, 1280 (1982)
[2] R. W. Sanders et. al., Phys. Rev. B 23, 1190 (1981)
[3] Rivera et. al., Science 353, 6296 (2016)

Reciprocity is a fundamental principle in optics, stating that wave transmission is the same in opposite direction. Breaking this symmetry is necessary for the design of isolators and circulators, which are used to protect sources from reflections and separate signals propagating in opposite directions. Furthermore, it has been recently shown that nonreciprocity is tightly connected to the realization of photonic topological insulators, which allow propagation of signals around sharp corners or other discontinuities without scattering. Conventionally, nonreciprocity is achieved through the magneto-optical effect, but this approach is challenging to integrate and generally leads to large devices. For this reason, there has recently been interest in looking for other ways to break reciprocity. One option is to use nonlinearities, in particular Kerr-type nonlinearities, with spatial asymmetries. Kerr nonlinearities allow changing transmission of a structure versus the input intensity, while spatial asymmetries lead to different field intensities inside a structure when exciting from opposite sides. As a result, in nonlinear asymmetric devices, transmission will generally follow different dependence versus the input power from opposite sides, leading to nonreciprocal response.
During the previous years, many devices of this type have been proposed based on heuristic principles, leading to sub-optimal responses. In this talk, we provide a systematic study of nonlinear isolators, identifying ultimate bounds for their characteristics and showing how these bounds can be achieved in specific nanophotonic designs. We first show that the most widespread category of nonlinear resonators based on a single nonlinear resonator are subject to a trade-off between transmission and isolation intensity range or bandwidth stemming from time reversal symmetry. We next demonstrate how this bound can be largely overcome in systems comprising multiple nonlinear resonators. We also show how such systems can be used to realize more advanced nonreciprocal devices, such as circulators. Furthermore, we present a new approach for the design of nonlinear isolators based on saturable absorption, which is less sensitive to loss compared to the most common approach based on the optical Kerr effect. In all cases, we provide examples of nanophotonic nonreciprocal devices with optical performance according to the derived bounds. Our results clarify several aspects of this important area in optics and provide guidelines for the systematic design of nonlinear nonreciprocal devices.

Plasmonic devices with the capability of generating coherent radiation at deep subwavelength scales have attracted great interest for diverse applications such as nanoantennas, single photon sources and nanosensors. However, the design of such lasing devices remains a challenging issue because of the long structure requirements for producing strong radiation feedback. Here, we present a near single-mode plasmonic laser by using a nanoscale hyperbolic metamaterial (HMM) cube, called hyperbolic metacavity, on a multiple quantum-well (MQW), deep-ultraviolet (DUV) emitter. The specifically designed metacavity merges plasmon resonant modes within the cube and provides a unique resonant radiation feedback to MQW. This unique plasmon field allows the dipoles of MQW with various orientations into radiative emission, achieving enhancement of spontaneous emission rate by a factor of 33 and of quantum efficiency by a factor of 2.5, which is beneficial for coherent laser action. Our approach shown here can greatly simplify the requirements of plasmonic nanolaser with a long plasmonic structure, and the metacavity effect can be extended to many other material systems.

Efficient deflection of light to large angles is important for realizing optical elements for high-resolution imaging or holography. Metasurfaces have enabled realizing these elements on a single, flat layer by controlling the propagation of light with great flexibility by means of changing its amplitude, phase and polarization abruptly on subwavelength scale¹. While this is typically achieved by making use of nanoscale optical building blocks that continuously shift the phase of the electric field at the surface in the range from 0 to 2π, it is difficult to efficiently introduce the large phase gradients required for large angle deflection of light². Here, by deviating from this commonly accepted approach, we introduce and experimentally demonstrate a new class of metasurfaces based on only partial control of the entire phase range (i.e. less than 0-2π). This relaxed phase requirement allows the realization of metasurfaces with more compact and less mutually-interacting scatterers, thus enabling the introduction of large phase gradients. We apply our concept to plasmonic and dielectric surfaces, report metasurface beam deflectors for anomalous refraction at extreme angles into dielectric substrates, and realize immersion metalenses with an unprecedented numerical aperture of NA = 1.4 for high resolution imaging at visible wavelengths. Furthermore, we show the application of our concept for realizing high-resolution metasurface holograms.

1. Yu, N. & Capasso, F. Flat optics with designer metasurfaces. Nat. Mater. 13, (2014).
2. Lalanne, P. & Chavel, P. Metalenses at visible wavelengths: an historical fresco. in Laser & Photonics Reviews 1600295, 101130F (2017).

Metal–insulator–metal (MIM) resonant absorbers comprise a conducting ground plane, a thin dielectric, and thin separated metal top-surface structures. Long-wave infrared (LWIR) fundamental absorptions are experimentally shown to be optimized for a ratio of dielectric thickness to top-structure dimension, t:L, of ~1:10. The fundamental resonance wavelength is predicted by different analytic standing-wave theories to be ~2nL, where n is the dielectric refractive index. Thus, for the dielectrics SiO2, AlN, and TiO2, L values of a few microns give fundamentals in the 8-12 micron LWIR wavelength region. Agreement with theory is better for larger t:L. Harmonics at shorter wavelengths are always observed. We show that there are additional resonances, in the far-infrared 20-50 micron wavelength range, well beyond the predicted fundamental. This may impact selectivity in spectral sensing applications.

Photoacoustically guided wave-front shaping provides unmatched capability to deliver focused light into strongly scattering media, such as biological tissue. It allows non-invasive optical imaging deep into brain, tissue, and many other optically opaque turbid media. However, the imaging resolution is intrinsically limited by acoustic diffractions. Increasing ultrasound frequencies addresses the problem at a cost of penetration depth due to the escalated attenuations of ultrasound signals in tissues. Here we introduce a super-resolved meta-lens for photoacoustically guided wave-front shaping and show single speckle imaging through turbid media. We have demonstrated > 3 photoacoustic enhancement factors and > 5 spatial resolution than that of the conventional ultrasound transducers, and more interestingly, > 3 convergence rate in genetic optimization of random phase fronts when one introduces super-resolving meta-lens.

Metal nanoparticles and their 2D sheets have been recognized as promising building blocks for the development of next-generation photoelectronics, metamaterials, and molecular electronic devices. Therefore, placing and arraying nanoparticles at the desired positions is a highly important technique for integrating these building blocks into devices. However, no single metal nanoparticle sheet currently exists with sufficient durability for conventional lithographical processes.
Here, we report the fabrication of large photo and/or electron-beam lithographic patternable metal nanoparticle sheets with improved durability by incorporating molecular cross-linked structures between nanoparticles. The cross-linked structures were easily formed by immersing a single nanoparticle sheet consisting of core metals to which capping molecules ionically bond [myristate capped silver nanoparticle (AgMy)] in a dithiol [1.16-hexadecanedithiol (DT16)] ethanol solution. The present Ag nanoparticle sheets with cross-linked structures via DT16 molecules (AgDT16 sheets) showed redshifts of the plasmonic resonance wavelength compared to that of AgMy sheets, despite having the same neighboring particle distances D in each sheet. This result indicates that the electronic states at the interface of the capping-molecule/nanoparticle-surface contribute to determining the plasmonic resonance wavelength of the sheets. In addition, the redshift exhibited a general exponential dependency on the number of Ag-S covalent bonds on the particle surface. To our knowledge, this is the first report of the contribution and the dependence of capping molecules on the plasmonic resonance wavelength of its sheets. Finally, the Ag nanoparticle sheets with improved durability for photo/e-beam lithography by incorporating molecular cross-linked structures between nanoparticles allowed us to fabricate micro/nano-scale line and space patterns of the sheet successfully. The details of the results will be discussed in the present talk.

A technology for a rapid and accurate detection of toxic molecules disolved in complex biological mixtures is highly demanding. Raman spectroscopy provides a non-destructive and ultra-fast molecular analysis, thereby being suitable for a point-of-care. However, Raman spectrum is difficult to measure for a low concentration of molecules as the probability of Raman scattering is as low as 10^-7. Nevertheless, Raman intensity can be drastically amplified by metal nanostructures as electromagnetic field is strongly localized on the surfaces; the amplication of Raman intensity is referred to as surface-enhanced Raman scattering (SERS). However, it is still difficult to directly detect molecules dissolved in most biological fluids because surfaces of SERS-active nanostructures are prone to contamination by adhesive molecules, especially for proteins. Even if large proteins are removed by pretreatment of samples through dialysis or centrifugation, the concentration of target small molecules adsorbed on the metal surfaces is somewhat limited in the absence of active-capturing mechanism, lowering sensitivity of detection.
In this work, we report charged microgels containing agglomerates of gold nanoparticles (Au NPs) for direct detection of small charged molecules dissolved in biological fluids without pre-treatment of samples. Toxic molecules frequently carry charges in aqueous solution. For example, most pesticides are positively charged to promote adhesion on the surface of negatively-charged soil. The charged molecules can be concentrated by using oppositely-charged microgel through electrostatic attraction. When the charged microgels contain agglomerate of Au NPs, Raman intensity of the concentrated molecules can be highly enhanced by SERS. Moreover, the microgels possess consistent sizes of mesh, which enables the autonomous exclusion of large proteins while allowing infusion of small molecules. Therefore, it is possible to detect the charged small molecules through SERS with high sensitivity in the absence of interruption from adhesive proteins. To produce such microgels in a controlled manner, we use a droplet microfluidics. Two aqueous streams of gel precursor containing Au NPs and agglomerants of sodium chloride are simultaneously emulsified into oil phase in a capillary microfluidic device. In the resulting emulsion drops, Au NPs are destabilized by the agglomerants. The agglomerates are captured in a microgel matrix by photocuring the gel precurors. To render the microgel positively or negatively charged, acrylamide or acrylic acid are copolymerized with a gel precursor of poly(ethylene glycol) diacrylate. The charged microgels concentrate the oppositely-charged small molecules while excluding the same-charged molecules and larger molecules than mesh size. Therefore, we can directly detect charged pesticides dissolved in a protein solution. Moreover, we demonstrate the direct detection of fipronil sulfone dissolved in egg without pre-treatment.

Regular arrays of colloidal particles, or colloidal crystals, shows striking structural colors due to diffraction by their periodic nanostructures. The structural colors are iridescent and never fade as long as the periodic structures persist. Moreover, the color is tunable by adjusting interparticle separation or refractive index without changing the set of materials. These unique features of structural color, distinguished from chemical pigments, render the colloidal crystals promising for various applications, including aesthetic coating, anti-forgery patches, colorimetric sensors, and encoded microcarriers for biological assays. To use the colloidal crystals as an alternative to chemical colorants, it is required to design the colloidal crystals to have a powder format suspendable in liquid media. Although microgranules of colloidal crystals have been prepared by droplet templating and other methods, it is still challenging to produce photonic microparticles with well-controlled shape and internal structure in a scalable manner.
In this work, we employ a facile micromolding to create photonic microcylinders with pronounced structural colors. To compose a regular array of colloids, silica particles are dispersed in a photocurable resin that forms a solvation layer on the surface of silica particles. The solvation layer exerts disjoining pressure, rendering the silica particles repulsive in a short separation. Therefore, the particles spontaneously form colloidal crystals at volume fraction above 0.1. The photocurable dispersion of silica particles is molded by cylindrical holes in an elastomer. The silica particles form non-close-packed structure by aligning their hexagonal array along the cylindrical wall, which are captured by photopolymerization. The micropillars containing the regular array of silica particles are released from the substrate, resulting in composite microcylinders with a low reflectivity at stop band wavelength. The reflectivity can be enhanced by selectively etching out silica particles to make a regular array of air cavities. The porous microcylinders show pronounced structural colors with high reflectivity, which can be dispersed in a polymer solution to formulate photonic inks for coating. In addition, the microcylinders can be further functionalized by making multiple compartments. We use the photocurable dispersions diluted by a volatile solvent to form compartments that partially fill the cylindrical holes. The remaining volume of the hole can be further subjected to molding process to make two or more compartments. Each compartment can be independently rendered to be either structurally-colored, magneto-responsive, or transparent so that multicompartment microcylinders can be functionalized. As the micromolding technique is compatible with a roll process, photonic microgranules can be produced in high throughput. Moreover, the advanced functionality of the multicompartment microgranules further expands application.

Quantum dot (QD) is a promising material for next-generation display devices due to its superior optical properties such as size-dependent narrow emission and broad absorption. Although it can be used for display in many ways, it is most successfully used in the device of LED-backlit LCDs. In particular, many researchers are paying attention to devices that use QD for patterned polymer film at the top of liquid crystal layer. It has advantages over other types because QDs are located away from heat source (blue LED) to prevent reduction in efficiencies. Therefore, it is expected to increase efficiencies of the device by 50% compared to the conventional LCDs. However, there still remain challenges. Although photoresist must be used for patterning, bare QDs cannot be dispersed in the photoresist. To overcome this limitation, we used silica nanoparticles as template particles for dispersing QDs. Furthermore, we synthesized silica nanoparticles with wrinkled surface (WSNs) for purpose of enhancing efficiencies. WSNs could be synthesized using the water-oil surfactant system. This unique surface structure makes WSNs scatter more light than the silica nanoparticles with smooth surface. Then, we prepared QD-WSN hybrid particles (WSQs) by embedding QDs on WSN through swelling method. Using the WSQs, we could disperse QDs into photoresist film in the form of WSQs. We fabricated the display devices by attaching the nanocomposite onto white OLED. Using various techniques such as electron microscopy, UV-vis spectrophotometer, and fluorometer equipped with integrating sphere, we characterized particles and nanocomposites. We believe this study suggest new way to disperse QDs in photoresist film and enhance the efficiencies of nanocomposites.

In this work, ZnO polycrystalline nanoparticles have been intentionally doped with rare earth (RE) elements (Er, Yb) and co-doped with Na by conventional sol-gel process to study the effect of varying doping concentration on structural, dielectric, magnetic, optical (photoluminescence and cathodoluminescence) and photomagnetic properties of ZnO. The structure, morphology, particle size, RE ion doping and incorporation in ZnO matrix of three different samples (Zn_0.96Na_0.02Er_0.01Yb_0.01O, Zn_0.97Na_0.015Er_0.0075Yb_0.0075O and Zn_0.98Na_0.01Er_0.005Yb_0.005O) were observed by X-ray diffraction (XRD), Raman spectroscopy, Scanning Electron Microscopy (SEM), Energy Dispersive X-ray (EDX) analysis and X-ray Photoelectron Spectroscopy (XPS). Frequency dependence of the dielectric constant, dielectric loss and ac conductivity at room temperature with and without the influence of UV light and/or a magnetic field will be discussed. The light control of magnetic properties of ZnO due to doping with Er³⁺ and Yb³⁺ ions is shown These results will stimulate the possible application of doped ZnO for optoelectronic and spintronic devices.

Recently developed super resolution fluorescence microscopes have provided quite important new information concerning molecular scale of hierarchical structure of biological systems [1]. However, these techniques are inadequate for imaging of rapid dynamics of living systems, and also their axial resolution is not as high as their lateral resolution [2]. In our previous study, we have developed a simple and effective method to visualize nanointerface of the adhesive cells using localized surface plasmon resonance (LSPR) excited on a two-dimensionally assembled metal nanoparticle (NP) sheet [3, 4]. The confined light in a few ten nanometers region from the sheet enables to improve axial resolution of fluorescence images drastically, which provides even better SN ratio (high contrast) images compared with the total internal reflection fluorescence (TIRF) microscopy, where the exposure time became even shorter owing to the plasmon-enhanced fluorescence. This technique is suitable for imaging the cell/substrate contact regions where cellular dynamics and molecular reaction occur.
In this study, we conducted detailed analysis of cell dynamics via the local movement of focal adhesion visualized by our LSPR method. Oleylamine-capped gold NPs (AuOA) sheets were fabricated at the air-water interface by using Langmuir-Schaefer method and transferred onto a hydrophobic cover slip. A 10-nm-thick SiO₂ layer was deposited on AuOA sheet to avoid fluorescence quenching by FRET. The 3T3 fibroblasts with stably expressed venus-paxillin (Ex: 520 nm; Em: 540 nm) were trypsinized, seeded onto the sheet and maintained in the CO₂ incubator overnight prior to imaging. The imaging was performed in a humidified temperature-controlled chamber at 37°C. The laser intensity was 1 mW, the excitation wavelength of the laser was 514 nm, and the incident angle was 75° (TIR). We used a super-resolution digital CMOS camera (65 nm/pixel, ORCA-Flash 4.0, Hamamatsu, Japan). The video images were obtained as a 50-tuple speed without image processing. The frame size is 2048 x 2048 pixels (133 µm x133 µm).
The live-cell image on SiO₂-covered AgMy sheet exhibited the morphology identical to that on glass, and revealed quite detailed, high quality of fluorescence image. The dynamics of each focal adhesion were clearly visualized on an AuOA sheet, which enabled the observation of the behaviors of the pseudopodium and cell shape. Movement of the cell body could also be monitored via Venus-paxillin, which was distributed in the cytosol. Paxillin is known to be located approximately 30 nm from the plasma membrane [1]. Because the LSPR field intensity decays sharply according to the distance from the sheet, we could investigate not only lateral movement but also vertical movement of focal adhesion through the brightness of the focal adhesion spots.
[1] Nat. 2010, 468 (7323), 580-U262.
[2] J. Cell. Biol. 2010, 190 (2), 165-175.
[3] Sci. Rep. 2017, 7, 10.
[4] Plos One 2017, 12 (12), 10.

Photonic crystal which has periodic structure comparable to the wavelength of visible light displays vivid structural color due to the photonic band gap. Its coloring phenomenon don’t rely on any toxic dyes or pigments, so it is expected to be applied to environmental friendly colored materials or optical filters. One convenient method to prepare photonic crystals is the utilization of colloidal crystals. Monodisperse charged colloidal particles in dispersion can self-assemble to produce three-dimensional colloidal crystals. The lattice scale of the colloidal crystals is determined by the amount and the size of the particles. As a result, the wavelength of photonic band gap arising from the colloidal crystals is easily controlled. However the elastic modulus of the colloidal crystals is very weak and easily destroyed by external forces. Therefore, we developed a tough structural colored material by fixing colloidal crystals in elastomers.
Di (ethylene glycol) methyl ether methacrylate (MEO 2) and the fine silica particles having a surface negatively charged and a diameter of 180 nm were used as raw materials for the composite elastomer. First, various volume fractions of the fine silica particles were added in the mixed solution of MEO 2 and the crosslinking agent: poly (ethylene glycol) dimethacrylate. After the fine silica particles were sufficiently dispersed into the solution, a small amount of Azobisisobutyronitrile was added as an initiator and the composite elastomers were prepared by free radical polymerization. The elastomer containing 22 vol% or more of the fine silica particles exhibited a reflection peak in the visible light region. From the scanning electron microscope (SEM) image, it was confirmed that the fine silica particles formed hexagonal structure in the section near the elastomer’s surface and reflection peak was derived from the 111 planes of the FCC structure. The structural color from the elastomer can be controlled by changing the length of the lattice spacing of the 111 planes of the FCC structure due to the deformation of the elastomer. As we expected, the position of the reflection peak continuously blueshifted when the elastomer was stretched. When the elastomer was deformed repeatedly, the relationship between the stress generated in the elastomer and the reflection wavelength matched each times. Further, we carried out the tensile test of the elastomer and found that Young's modulus was improved without decreasing the rupture strain by increasing the addition amount of the fine silica particles. In summary, we succeeded to develop a tough inorganic-organic composite material that can visualize stress generated in the elastomer.

Gold nanoparticles (AuNPs) have been widely used in biosensing due to their localized surface plasmon resonance (LSPR), especially when integrated with fiber-optic probes, it brings the feasibility of miniaturized, remote and even in vivo biosensing. The surface modification of gold nanoparticles has been proven effective to meet goals of both biocompatibility and specific molecular recognition. As a macrocyclic molecules, cyclodextrins can specifically recognize cholesterol through host-guest interaction.
In this work, biocompatible, high-quality and monodisperse b-CD-capped AuNPs 15-20 nm in diameter are synthesized based on complete “green” synthesis method. The microfiber is fabricated by tapering a standard single-mode optical fiber using the heating and pulling method. The synthesized nanoparticles are decorated on the surface of a microfiber via electrostatic interaction to realize a highly sensitive and highly integrated biosensing device.
After immersed in aqueous cholesterol solutions, the attenuation band on the transmission spectrum of b-CD-capped AuNPs decorated microfiber shows a distinguishable deepening. Our proposed biosensor reaches an ultralow cholesterol detection limit of 5X10^-18 M, which is the most sensitive among the state-of-the-art cholesterol detections. Given that cholesterol is in mM level in human serum, this device can achieve extremely low consumption of clinical samples. Meanwhile, the interference study of cholesterol sensing with the existence of common interfering substances in human serum is carried out, which proves the good selectivity of our proposed biosensor.

The recent increase in counterfeited documents, banknotes, and medications impacts society, calling for new security technologies that are difficult to reproduce. Nature mimicking materials with unique optical properties are well-suited for security applications.
Aqueous suspensions of Cellulose Nanocrystals (CNC), extracted from cellulose, self-assemble into chiral nematic structures creating, upon drying, free-standing films. These films exhibit unique optical properties: structural color, iridescence, and light polarization for covert and overt encryption. The films are also suitable for anti-counterfeiting materials and can be enhanced to provide security features beyond optical properties.
It was hypothesized that combining CNC with sub 50 nm metal and non-metal nanoparticles yields composite materials that retain the CNC optical properties. It was further hypothesized that the added nanoparticles influence the materials’ electrical and dielectric properties, which could be detected and validated as non-optical properties.
The experiment created new, CNC-based composite materials with various nanoparticles: copper, aluminum, cobalt, iron, carbon single-walled nanotubes, graphene, titanium dioxide, titanium carbide, fluorescent rhodamine nanoparticles, and gold colloid.
All composite films retained the CNC unique optical properties. Adding the nanoparticles shifted the structural color and reduced iridescence, measured with a custom-designed setup using a Fiber Spectrometer. The film morphology, examined using a Polarized Optical Microscope and Scanning Electron Microscope, was uniform with the nanoparticles evenly dispersed.
The film’s non-optical properties (resistivity and dielectric constant) were measured using a custom-built setting composed of plates and electrodes connected to Digital Multimeters. Adding the nanoparticles uniquely impacted the CNC non-optical properties.
The functionalization of CNC films beyond optical properties makes these materials even more suitable for anti-counterfeiting materials.

Natural optical activity of plasmonic metal nanoparticles has been studied in terms of molecular chirality. The nanoparticle is commonly assumed to be chiral when the nanoparticle lacks inversion symmetry or is parity-odd (P₋). While optical activity generally requires the violation of parity and time-reversal symmetry (PT-symmetry), the time-reversal symmetry has typically been neglected in nanomaterials. Here, we demonstrate that time-reversal symmetry can be broken in the plasmonic nanoparticles by longitudinal volume plasmons. Without applying an external magnetic field we experimentally observed circular dichroism (CD) of gold, silver, and copper nanoparticles at the plasmon excitation energy. Spatial variation of light absorption in the nanoparticle is attributed to the origin of longitudinal plasmons that light cannot excite but an optical force may be able to. We propose that optical forces can generate the optical activity of achiral metal nanoparticles. The polarization rotation of surface plasmons is interpreted as orbital angular momentum that is time-odd (T₋). The sum rule observed for the dipolar and quadrupolar surface plasmons may indicate the conservation of orbital angular momentum in the surface plasmon oscillation. We show that, rather than only parity, the PT-symmetry fully describes the optical activity of nanomaterials.

Zinc oxide (ZnO) semiconducting nanorods/nanowires are very attractive for fabrication of novel optoelectronic nanodevices, due to their excellent optical properties (e.g., a large excitonic binding energy of 60 meV) combined with their ease in synthesis. ZnO has a direct wide bandgap of ~ 3.4 eV, leading to efficient light emission in the ultraviolet (UV) region. An extension of the ZnO nanowire emission beyond the UV spectral region, and in particular, to the technologically important near infrared range around 1.5 μm would greatly enhance these devices’ applications.

Erbium (Er) has offered a means towards optical amplification around 1.5 μm due to the intra-4f transitions of Er³⁺ ions. In this work, we present an approach to achieving strong light emission around 1.5 μm by combining ZnO nanowires with nanostructured Er oxides. ZnO nanowire arrays are first synthesized on a silicon wafer via the hydrothermal method, and a layer of 100 nm Er is deposited on the ZnO nanowires by electron beam deposition. The resultant nanowire composites are thermally annealed at 900 °C in an oxygen gas. Strong photoluminescence around 1.5 μm is observed at room temperature. We discuss possible mechanisms for energy transfer between ZnO nanowires and nanostructured Er oxides.

Hybrid nanostructures based on semiconductor-plasmonic nanomaterials are receiving extensive attention due to the synergic exciton-plasmon interaction between linked units.¹ These characters are essential for constructing functional heterojunctions by which the exciton-plasmon coupling has vital effect on plasmon-based molecular sensors, photocatalysis, photoluminescence, and active nanophotonic devices.^2-4
On this basis, we develop an Au-ZnO nanomaterial relevant for photocatalysis using a simple method, which exhibits high quantum yield and good stability nanomaterials.
First, ZnO nanocrystals (ZnO NCs) were thermally synthesized in an aqueous medium, where the annealing was indeed to crystallize our products and to control their sizes. In the second step, the obtained ZnO NCs were added to various sizes and volume fractions of dispersions of gold nanoparticles (AuNPs). The optical properties of hybrid Au-ZnO nanostructures have been probed by UV-Vis absorption, photoluminescence and photocatalysis processes, and the system’s morphology was evaluated by HR-TEM imaging.
Owing to the fact that photo-generated charge and/or energy transfer between the constituents in the hybrid system can enhance the optoelectronic properties of both units, we show in this communication how tuning the structural properties of ZnO NCs and AuNPs affects the plasmon-coupled-emission interaction, tailoring the photoemission of the ZnO NCs, and enhancing photocatalytic activity of the whole system.
References
¹ Zhao, W. W., Yu, P. P., Shan, Y., Wang, J., Xu, J. J., & Chen, H. Y. (2012). Analytical chemistry, 84(14), 5892-5897.
² Djurišić, A. B., Leung, Y. H., & Ng, A. M. C. (2014). Materials Horizons, 1(4), 400-410.
³ Cheng, C. W., Sie, E. J., Liu, B., Huan, C. H. A., Sum, T. C., Sun, H. D., & Fan, H. J. (2010). Applied Physics Letters, 96(7), 071107.
⁴ Chen, Z. H., Tang, Y. B., Liu, C. P., Leung, Y. H., Yuan, G. D., Chen, L. M., ... & Lee, C. S. (2009). The Journal of Physical Chemistry C, 113(30), 13433-13437.

By changing the morphology of a poly(3-hexylthiophene) (P3HT) conductive polymer via self-assembly from a planar thin film to a nanowires mesh mesostructure, we demonstrate hybrid graphene-conductive polymer photodetectors with an experimentally observed 600% improved photo-responsivity when compared to their analogous hybrid photodetectors based on graphene and a planar P3HT thin film of similar thickness. At least two reasons stand behind such a dramatic increase in photoresponsivity: (i) the polymer nanowire mesh architecture with cell unit dimensions comparable to the wavelength of light, which produces light scattering and increased light absorption in the polymer compared to the case of a planar polymer thin film of similar thickness and (ii) the crystallization of P3HT molecules within the nanowires which reduces the density of charge trap states, which in turn provides improved charge transport and charge transfer compared to a P3HT thin film.

Surface-enhanced Raman scattering (SERS) has attracted researchers towards developing high sensitive molecular sensing systems. The enhancement factor (EF) by SERS phenomenon can be estimated by measuring the extent of scattering enhancement via finite-difference time-domain (FDTD) simulations. The enhancement factor (EF) is taken as (│E│/│E₀│)⁴ where E is the local maximum electric field, and E₀ is input source electric field [1,2].
In this paper, uniformly distributed self-organized nanocones on the gold surface induced by argon (Ar⁺) ion beam sputtering (IBS) is utilized, and high EF of SERS is predicted for hybrid gold-nanocone/graphene/gold-nanohole tri-layers system. The uniformly distributed self-organized gold nano-cones are formed by grazing-angle irradiation (at 82^o to the surface normal) of argon ion beam (with energy 400 V and beam current 42 mA) on the gold surface. The base diameter of the typical cone is ranging from 110 to 250 nm, and the height of the typical cone is ranging from 150 to 200 nm. Utilizing the fabricated nanocones, we propose the tri-layer system of gold-nanocone/graphene/gold-nanohole. The thickness of gold nanohole layer is 10 nm and diameter of nanohole is 30 nm. Though the theoretical thickness of graphene is 0.35 nm, considering natural wrinkles of graphene at room temperature, 1 nm thick graphene was modelled using Drude-like surface conductivity approximation [3]. A hybrid gold-nanocone/graphene/gold-nanohole based SERS sensor is shown to exhibit enhancement factor of 10⁹ via FDTD simulations. The graphene layer ensures ~1 nm spacing between cone tip and hole, such a close proximity provides enhanced LSPR (localized surface plasmon resonance) coupling between cone tip and hole and huge SERS enhancement is achieved. The theoretical results reveal that the proposed structures give SERS enhancement factor of the order of 10⁹ which is two orders higher than that of 1 nm spaced gold nanoparticles [4].

References:
(1) M. Banik, A. Nag, P. Z. El-Khoury, A. Rodriguez Perez, N. Guarrotxena, G. C. Bazan, and V. A. Apkarian, "Surface-Enhanced Raman Scattering of a Single Nanodumbbell: Dibenzyldithio-Linked Silver Nanospheres," J. Phys. Chem. C 116, 10415–10423 (2012).
(2) J. K. Yoon, K. Kim, and K. S. Shin, “Raman Scattering of 4-Aminobenzenethiol Sandwiched between Au Nanoparticles and a Macroscopically Smooth Au Substrate: Effect of Size of Au Nanoparticles,” J. Phys. Chem. C 113, 1769–1774 (2009).
(3) L. A. Falkovsky, “Optical properties of graphene,” J. Phys. Conf. Ser. 129, 012004 (2008).
(4) P. Zheng, X. Shi, K. Curtin, F. Yang, and N. Wu, "Detection of mercury(II) with a surface-enhanced Raman scattering sensor based on functionalized gold nanoparticles," Mater. Res. Express 4, 055017 (2017)

Radiative cooling exploits the high transparency of the earth’s atmosphere at mid-infrared wavelengths (8-13 µm), often referred to as the atmospheric window, to radiate heat to the cold outer space and passively cool terrestrial objects. While most of the past work has focused on developing spectrally selective surfaces to maximize radiative heat transfer and cooling performance, low conduction and convection thermal resistances between the radiative cooler and the environment have severely hindered the device-level performance. We have developed infrared-transparent and thermally-insulating polyethylene aerogels (PEAs) optimized for radiative cooling applications that could significantly improve the cooling power and minimum temperature of current radiative coolers by minimizing parasitic heat gains from the environment while allowing radiative exchange with outer space. Highly-porous PEAs of varying densities (20 to 120 kg/m³) were fabricated by thermally-induced phase separation of polyethylene/decalin and polyethylene/paraffin oil solutions followed by a solvent extraction by CO₂ supercritical drying. We tuned the porous structure of the PEAs by optimizing the fabrication process to minimize scattering and absorption in the atmospheric window and thus maximize transparency. We measured an atmospheric-window-weighted IR transparency of up to 92% along with a solar-weighted reflectance of 87% for a 2.3 mm thick sample with a density of 25 kg/m³. The optical measurements were compared to our theoretical model which solves the radiative transfer equation and assumes Mie scattering. We also characterized the thermal properties of the PEAs experimentally as well as theoretically, accounting for solid, gaseous and radiative contributions, and report an effective thermal conductivity of less than 0.05 W/mK at room temperature. Finally, we demonstrate that PEAs can significantly improve the performance of existing radiative cooling systems due to their combined high IR and low solar-weighted transmittance as well as ultra-low thermal conductivity.

An object is chiral if it cannot be superimposed to its mirror image. The existence of chirality in nature is universal, ranging from galaxies to gastropod shells to biomolecules. While a pair of chiral molecules, termed as enantiomers, exhibit identical scalar physical properties, they take part in many chemical reactions in biological processes differently, acting in a desirable or harmful way. Therefore, discriminating between enantiomers is of vital importance, especially in pharmacology and life sciences.

Light can be chiral as well, and chiral light-matter interactions provide an opportunity to noninvasively identify chiral molecules with superior precision. For instance, circular dichroism (CD) spectroscopy that measures the differential absorption of left- and right-circularly polarized (L/RCP) light is widely used to reveal the structural information of biomolecules. However, CD signals are usually very weak due to the intrinsically weak chirality of the molecules. In order to enhance CD signals for characterizations at low concentrations, different platforms have been proposed, such as partially reflecting mirrors, plasmonic nanostructures, and dielectric nanoparticles, etc.[1,2] Despite improved differential absorption by chiral molecules, these methods suffer from either strong background absorption by the platforms or non-uniform chiral fields that hinder the global CD enhancement. Both issues impose limitations on the measurement sensitivity. Therefore, novel platforms exhibiting low loss and uniform chiral fields are highly desirable.

In this work, we numerically study the generation of chiral fields in achiral dielectric nanocube dimers and demonstrate its beneficial role in enhancing CD signals.[3] With practically achievable designs, we show that under the illumination of circularly polarized light, the local magnetic and electric fields are simultaneously enhanced and properly overlapped in the gap area, resulting in an accessible and uniform chiral hotspot. A volume-averaged chirality enhancement factor exceeding 15 is demonstrated for silicon nanocubes in the visible region around 550 nm wavelength. We further investigate the CD enhancement by positioning chiral molecules at the chiral hotspot. Thanks to the achiral and low-loss properties of the proposed platform, very weak absorption from the nanocubes will be present, which allows effective characterization of the chiral molecules and an over-10 fold enhancement of CD signals is demonstrated. We also consider other materials such as titanium dioxide, and the desired performance is successfully achieved in the violet region. Being able to engineer chiral fields without complex nanostructures or plasmonic materials, we expect that our findings can open up a new avenue to CD spectroscopy, chiral sensing and photolysis.

[1] J.T. Collins et al., Adv. Opt. Mater. 5, 1700182 (2017).
[2] C.-S. Ho et al., ACS Photonics 4, 197 (2016).
[3] K. Yao and Y. Liu, Nanoscale 10, 8770 (2018).

Direct bandgap semiconductors on Si is a key technical challenge for Si-based integrated photonics. A promising approach is to alloy Ge with Sn, which effectively causes the energy of the direct Γ valley to decrease faster than the indirect L valleys, leading to indirect-to-direct gap transition. One of the main issues with this approach is that the equilibrium solubility of Sn in bulk Ge is <1 at % according to the existing Ge-Sn phase diagram for bulk materials (extrapolated at <400 C) [1]. Although substitutional Sn compositions of ~9-10 at % have been achieved by CVD, MBE and direct crystallization of amorphous GeSn [2], higher Sn composition is needed for stronger direct bandgap semiconductor behavior and better optoelectronic properties in the mid infrared regime. In these cases, Sn segregation is the biggest concern. Here we present a counterintuitive approach that achieves a much higher equilibrium solubility with up to 26 at.% Sn in GeSn nanodots directly crystallized on dielectric layers towards monolithic 3D mid-infrared (MIR) photonic integration on Si. The approach involves sequentially depositing a layer of metallic Sn nanodots with a diameter of 20-60 nm and a thin layer of Ge using physical vapor deposition (PVD), followed by interdiffusion/crystallization annealing at 300-500 C. In this case the alloying of Ge and Sn is driven by interfacial energy minimization between the Sn nanodots and the Ge capping layer, which allows for higher equilibrium solubility of substitutional Sn into the Ge lattice. It can also be viewed as dissolving the nanoscale Sn nuclei below the critical size for nucleation into the Ge lattice. X-ray diffraction and Raman spectroscopy analyses indicate that up to 26 at.% Sn has been substitutionally incorporated into the Ge lattice, increasing the lattice constant by 4% compared to pure Ge. The initial metallic Sn nanodots exhibit a strong (200) orientation, while after interdiffusion and crystallization the resulting GeSn nanodots show a strong (111) orientation without observable diffraction peaks from metallic Sn. This result indicates that Sn nanodots are completely dissolved into the lattice of Ge after crystallization annealing, and that the solubility limit of Sn in GeSn nanostructures is at least 30x higher than that in bulk materials. To our knowledge, this is also the first observation of converting metallic Sn into diamond cubic lattice via interdiffussion. Preliminary studies on these high Sn composition GeSn nanodot samples also show photoluminescence (PL) at both low and room temperature, suggesting good crystallinity. These results offer a new and facile approach to fabricate high Sn composition GeSn nanostructures for MIR integrated photonics.

[1] Springer Materials Database, https://link.springer.com/content/pdf/10.1007/BF02868550.pdf
[2] J. Liu, Photonics 1, 162-197 (2014)

Bulk metals are naturally opaque to light due to the large index mismatch between metals and dielectrics. Making metals transparent for white light, which could achieve various fascinating applications in optoelectronics, has been expected for a long period. In this talk, we present our recent work on making structured metals transparent for white light via surface excitations, such as surface plasmons. First, we show that periodic, quasiperiodic, and disordered metallic gratings can become transparent and completely antireflective for extremely broadband electromagnetic waves at oblique incidence. Second, we significantly develop oblique metal gratings transparent for broadband electromagnetic waves (including optical waves and terahertz ones) under normal incidence. In the third, we present that the principles of broadband transparency for structured metals can be extended from one-dimensional metallic gratings to two-dimensional cases. Moreover, similar phenomena are found in sonic artificially metallic structures, which present the transparency for broadband acoustic waves. These investigations provide guidelines to develop novel broadband metamaterials, and have potential applications on transparent conducting panels, antireflective conducting solar cells, broadband acoustic imaging and sensing, and so on.

Mechanically tunable lasers are critical components in responsive optical displays, ultra-sensitive strain sensors, and flexible photonic devices. Compared to laser architectures that rely on photonic cavities having microscale device footprints and limited strain sensitivity, plasmonic nanocavities can confine light to subwavelength scales. Nanolasing has been realized from coupled arrays of metal nanoparticles surrounded by gain media, with wavelengths tailorable by varying refractive index environment and lattice spacing. However, their use in stretchable lasing has been precluded because the mode quality of dipolar coupling cannot be preserved under applied strain.
Here we focus on an entirely new architecture to achieve stretchable nanolasing based on our ability to harness hybrid quadrupole plasmons for robust lasing feedback. Large metal nanoparticles in a lattice produced hybrid quadrupole lattice plasmons with sharp resonances under different strains. We patterned Au nanoparticle arrays on elastomeric substrates to achieve mechanical control over the plasmonic nanocavity modes. Stretching the device along the polarization direction resulted in out-of-plane standing waves at the band edge that were tolerant to lateral changes in lattice spacing. We achieved reconfigurable, mechanical control of nanolasing with ultra-sensitive strain responses (31 nm for 0.03 strain) by stretching and releasing the substrate. Our semi-quantum modeling demonstrates that lasing build-up occurs at the hybrid quadrupole electromagnetic hot spots, which provides a route towards mechanical modulation of light-matter interactions on the nanoscale.

Artificial plasmonic molecules, i.e. nanoparticles that are separated by a nanometer gap, show strong near-field coupling. The gap ‘hot-spot’ exhibits new resonance features,¹ and is used for a variety of plasmonic applications including sensing, imaging, and lithography.² However, all of these applications require highly uniform gap structures, which are challenging to grow using existing nanofabrication methods, since material, size, gap, and geometry need to be engineered with nanometer accuracy. Here we report the successful fabrication of plasmonic molecules in large numbers (billions), including dimers and trimers, with precise control over their material composition (e.g. gold and silver), size (< 5 nm resolution), gap dimension (atomic resolution), and orientation on the wafer.
We demonstrate that a physical shadow growth method, which is described in earlier reports,^3,4is, in conjunction with atomic layer deposition, able to grow plasmonic dimers with a nanometer-sized dielectric at the wafer scale. We describe how we can precisely engineer the structural parameters of the plasmonic molecules and thus tune their corresponding coupled resonances across the entire visible spectrum. Our fabrication method can even progressively control the symmetry of the dimer and trimer structures across an entire wafer while preserving a nanogap. Furthermore, we spectroscopically map the resonances and qualitatively model the spectra. Theory and experiment are in good agreement with numerical calculations. While thus far dimer structures have largely been obtained by random drop casting, we show that we can systematically obtain identical dimers and also identical dimers where one of several parameters is systematically changed across the wafer. We expect that the structures will be important for spectroscopic studies.

1. C. Yi et al., PNAS, 2017, 114, 11621-6.
2. N. Zohar, L. Chuntonov, and G. Haran, J Photochem Photobio C 2014, 21, 26-39.
3. A. G. Mark, J. G. Gibbs, T.-C. Lee and P. Fischer, Nature Materials, 2013, 12, 802-807.
4. H.-H. Jeong, M. Alarcoon-Correa, A.G. Mark, K. Son, T.-C. Lee, and P. Fischer, Adv Sci, 2017, 4, 1700234.

The efficiency of absorption and emission in organic semiconductors can be significantly improved by near-field optical interactions with resonant metallic plasmonic nanostructures. However, these improvements do not often translate to significantly enhanced optoelectronic device performance. Practical metals that exhibit strong plasmonic properties don’t have stable work functions suitable for effective charge collection or injection from most organic semiconductors. To address this problem, in this project, we investigate the application of surface oxides to both planar and structured metallic plasmonic metasurface electrodes and determine the impact on the charge transport and stability of organic semiconducting conjugated polymer devices. The properties of metasurface electrodes and surface oxides can be elucidated by comparing with planar devices and devices without surface oxides through electrical measurements, such as current-voltage measurements, and capacitance-voltage measurements. In addition, we determine the charge collection barrier height at the conjugated polymer-metal electrode interface to understand the impact of surface oxides and nanostructures on charge injection and charge collection from the polymer. The overall goal is to make better performing electrodes for organic photovoltaics that are also capable of exhibiting effective light trapping below the diffraction limit.

Silver nanoparticle (AgNP) metasurface electrodes were fabricated using a thermally-assisted dewetting method. Then hole-only conjugated polymer-based devices were fabricated directly on top of AgNP metasurface electrodes. Devices with and without a silver oxide (AgOx) hole transport layer on the AgNP metasurface electrode were prepared. MoO₃ is used as an interlayer between the conjugated polymer and a top Au electrode to block electrons and, hence, to achieve hole-only operation. This device configuration allows us to investigate the barrier to hole injection on the AgNP metasurface electrode side. The barrier to hole injection/collection from the conjugated polymer is expected to change when the AgOx layer or plasmonic metasurface is used compared to when only planar Ag is used. UV-visible extinction spectra and dark-field scattering spectra of AgNP electrodes with different sizes show different optical properties (such as scattering intensity and color) between AgNP metasurface electrodes with different nominal AgNP diameters. Current-voltage measurements show that the turn-on voltage of devices with an AgOx layer is lower than the devices without AgOx, which is attributed to the lower workfunction of AgOx relative to Ag. Ultraviolet photoelectron spectroscopy (UPS) is used to obtain the work function, which can be used to verify origin of the lower turn on voltage. Meanwhile, X-ray photoelectron spectroscopy (XPS) is also used to analyze the surface composition of the AgOx electrodes.

Harvesting hot carriers from photo-excited metal nanoparticles on ultrafast (~fs) timescales holds great promise for the operation of non-equilibrium optoelectronics. While the ultrafast dynamics of hot electrons in metal nanostructures have been extensively studied, the temporal evolution of plasmon-induced hot holes remains unexplored. Here, we monitor ultrafast hot-hole injection from gold (Au) nanoparticles into the valence band of p-type gallium nitride (p-GaN) via ultrafast transient absorption spectroscopy. By spatially separating hot electron-hole pairs across the metal-semiconductor interface via plasmon excitation, Au/p-GaN heterostructures offer an ideal platform for spectrally distinguishing the dynamics of the hot holes, which can be probed in the infrared regime via free-carrier absorption within the p-GaN valence band, from the hot electron dynamics occurring on the metal, which are monitored across the visible spectrum with a white-light supercontinuum probe. We observe plasmon-induced hot-hole injection from Au to p-GaN within the 200 fs instrument response of our experimental setup, placing hot-hole transfer from a plasmonic metal to a p-type semiconductor on similar timescales as hot-electron injection into an n-type semiconductor. Moreover, hot holes persist within the p-GaN valence band for several nanoseconds before recombination of the charge-separated state occurs across the Au/p-GaN heterojunction. We further evaluated the influence of hot-hole injection on the temporal dynamics of hot electrons left behind on Au by comparison with Au/SiO₂ heterostructures for which hot-hole injection is prohibited. Interestingly, we found that the ultrafast (t < 200 fs) injection of hot-holes into p-GaN on timescales commensurate with electron-electron scattering exerts a profound influence on the spectral and temporal dynamics of hot electrons on the Au nanoparticles. Taken together, these ultrafast studies offer a comprehensive picture of hot-carrier dynamics across the Au/p-GaN heterojunction and suggest the potential for manipulating the energy distributions and subsequent thermalization dynamics of hot electrons above the metal Fermi level by controlling the collection of hot holes on ultrafast timescales.

Plasmonic hot-carrier devices offer below the band-gap charge generation, ultrafast response times and spectral and polarization control over absorption at the nanoscale. These features could result in transformative advances in photon detection devices, especially in the near-infrared regime. However, the dominant route for hot-carrier generation from current noble metals (e.g. Au) is interband transitions from high-energy (≥2 eV below Fermi level) d-band states. There is a need for plasmonic materials which offer efficient hot-carrier generation from intraband transitions utilizing low energy (~1 eV from Fermi level) infrared excitations. Our work focuses on identifying promising alloys and their associated composition which could result in efficient plasmonic hot carrier generation from infrared excitation.

We report the measured and calculated electronic properties of various binary metal-alloy thin films deposited at room temperature using co-thermal evaporation. Our deposition technique is amenable to modern lithographic approaches and therefore can be easily utilized to deposit nanostructured alloys for future devices. Both noble-noble and noble-transition metal alloys, where the transition metal exhibited a high electronic density of states (EDOS) near the Fermi energy, were characterized. We show experimental and calculated EDOS for 50 nm thick Ag-Cu, Ag-Pd, Ag-Ni, Ag-Ti, Au-Ni, Au-Ti, and Au-Pd films with various compositions. EDOS were collected using angle-resolved photoemission spectroscopy (ARPES) and film composition was measured using X-ray photoemission spectroscopy (XPS). Results of 50 nm thick Ag-Cu (25/75%) deposited at RT reveal shifts in d-bands due to hybridization above 2eV binding energy. However, upon annealing above 400°C, phase separation occurs. These results will be correlated with the local structure of the alloy film employing XAS and EXAFS. In the case of Ag-Cu, current EXAFS results reveal a nanocrystalline single phase material. Lastly, we will report the visible and near-infrared optical properties of these films and discuss them relative to the measured and calculated EDOS.

Plasmonic nanoparticles are of interest due to their novel therapeutic and diagnostic application. Recently, there has been increased interest in the potential for designed nanomaterials for non-equilibrium studies and materials processing. The optical properties of metal nanoparticles strongly depend on their electronic responses which are different than bulk metals. In our study, the electronic behavior of mellatic nanoparticle are investigated using femtosecond transient absorption measurement to elucidate laser exposure effects on their spectroscopy and non-equilbrium dynamics. .Femtosecond laser pulses generated by a regeneratively amplified Ti-sapphire laser are used to excite localized surface plasmon resonances in synthesized gold nanorods with a plasmon resonance close to 805 nm. The transient absorption spectra of the nanoparticles are obtained by recording their absorption at different probe delays. Our experimental findings indicate that the amplified femtosecond excitation of the surface plasmon resonance band in gold nanorods leads to transient absorption spectra changes which reveals that the electron-phonon relaxation dynamics is happening on early picosecond time scale. This relaxation time depends on the pump peak power indicative of a temperature dependent nonlinear relaxation back to equilibrium. Spectroscopy, light scattering and Finite element method simulations are used to characterize non-equilibrium dynamics. The findings expand the knowledge about the ultrafast electron processes in nanoparticles and provide information for controlling their time response, facilitating prospects for plasmonically enhanced cavitation for materials processing.

Localized surface plasmons render metal nanoparticles efficient light absorbers at their resonance frequencies. After its excitation, the plasmon resonance decays into high-energy electrons and holes, which, combined with the large absorption cross section, makes metal nanoparticles attractive hot-carrier generators for photocatalysis. In this work, we study the femtosecond real-time dynamics of localized surface plasmons in noble metal nanoparticles by using time-dependent density-functional theory (TDDFT). By extending the frequency-space Kohn-Sham decomposition analysis [1] to real time, we follow the plasmon formation as a collective excitation, track its subsequent decay into incoherent electron-hole transitions, and finally, obtain the corresponding hot-carrier distributions. Our work paves the way for addressing the spatiotemporal dynamics of hot-carrier generation in catalytically-relevant edge and corner sites of nanoparticles.

[1] T. P. Rossi et al., J. Chem. Theory Comput. 13, 4779 (2017).

Localized surface plasmon resonances (LSPRs) occur in certain metals where electrons confined to the metal surface oscillate with a frequency similar to that of the perturbing source, giving rise to localized electromagnetic fields. In this study we employ experimental and theoretical analyses to characterize LSPRs in Al_core(Al₂O₃)_shell nanoparticles with controlled morphologies. We perform simulations of LSPR using boundary element methods, where the electron beam is passing at a distance of 2.5 nm from the surface of an icosahedral Al_core(Al₂O₃)_shell nanoparticle. The electron energy loss probability spectra show that, for the mode located at an energy ≈ 7 eV, the magnitude of the LSPR shift towards smaller energy values is higher compared to other modes located at lower energies, when the impact parameter is near a facet, an edge, or a corner of the nanoparticle, respectively. This agrees with our experimental data, where we collected electron energy-loss spectroscopy (EELS) data in the low-loss region, near the surface of the nanoparticles, using a monochromated 80 keV electron source with 100 meV energy resolution. The experimental spectra appertaining to the edge and corner of the nanoparticle display an energy shift as a function of position of the electron beam with respect to the nanoparticle. By applying a non-negative matrix factorization algorithm, we deconvolve the LSPR signals and thus determine the effects of geometry and composition on coupling efficiency as a function of beam position. This allows us to map the coupling efficiency of the electron beam to the surface electrons, and reveals the energy transfer path from the excitation source to the nanoparticles. This study paves the way for a better understanding of the localization of LSPR in catalyst nanoparticles with engineered morphologies, and highlights the capabilities of EELS as a tool for probing surface plasmons of metallic nanostructures with high spatial and energy resolution.

Surface enhanced Raman scattering (SERS) has been a key tool for molecular detection and identification with single-molecule sensitivity. However, creating high spatial resolution SERS maps of sophisticated surfaces such as cellular membranes and biological tissues remains a significant challenge because of the diffraction limit of light. In this work, we introduce a novel imaging technique that utilizes light excitation within an electron microscope to perform high spatial resolution Ramanmapping. Unlike conventional stimulated Raman measurements, our technique uses a laser source as the pump and the electron beam as the broadband Stokes excitation. Particularly, we leverage the electron beam as a highly-localized Angstrom-scale source to locally excite the plasmonic resonances of individual nanoparticles. The resonant wavelength of the nanoparticles is tuned such that it is red-shifted from the pump laser to resemble the Stokes excitation when excited with the electron beam. Accordingly, the Raman scattering is locally enhanced by the electron beam and the spatial resolution is determined by the electron beam spot size and the nanoparticle size.
To compute the local nonlinear Raman enhancement under dual optical and electron beam illumination, we first use finite difference time domain (FDTD) simulations. Attention is given to gold nanorods (NRs) with plasmonic resonances between 800nm and 900nm; we assume 785nm laser illumination and 80keV electron beam excitation. We study the enhancement of the 992cm^-1 benzene line, showing that laser pump intensities less than 3mW/µm² can locally enhance the Raman peak. We compute the pumping threshold and the Raman gain achieved under various electron-beam and nanorod excitation positions, showing sub-2nm resolution of molecular fingerprints.
Experimentally, we use aberration corrected environmental transmission electron microscope combined with light excitation. We synthesize colloidal gold NRs functionalized with4-Nitrothiophenol (4NTP) as Raman reporter. The lengths and radii of the rods are 105nm and 12nm, respectively, with transverse and longitudinal plasmon resonances at 510nm and 830nm. Upon electron beam excitation of the NR plasmon modes, we observe an increase in the Raman intensity of the1335cm^-1 and 1570cm^-1 lines of the 4NTP molecule. Monochromated electron energy loss spectroscopy (EELS) confirms the plasmon modes we excite during each individual Raman acquisition. We investigate and locally map the stimulated Raman enhancement upon electron-beam excitation of different NP resonances and with distinct nanorod lengths. Our results demonstrate the ability to perform in-situ Raman interrogation with single nanometer resolution inside an environmental TEM while simultaneously imaging the nano-to-atomic-scale structure of the sample.

Super-Planckian thermal emission refers to emission from a material that exceeds the blackbody limit. Within the past two decades or so, there has been research on the transfer of thermal radiation between two materials that are separated by only a short distance (within the near-field). The findings indicate thermal transport greater than that predicted by Stefan-Boltzmann’s Law is a result of tunneling of evanescent waves between the mediums. While this is of fundamental scientific interest, the practical applications of such designs are limited due to the small spacing requirements. It would be beneficial to observe super-Planckian thermal emission at large distances in the far-field. While there have been a few papers that report observing this effect in the far-field, the topic is still fairly contested. It has been suggested that an anisotropic material, such as a hyperbolic metamaterial (HMM), could exhibit super-Planckian thermal emission.
Hyperbolic metamaterials (HMMs) are artificial materials composed of subwavelength metal and dielectric structures. They are extremely anisotropic with the permittivities of opposite signs in the parallel and perpendicular directions. This results in an open isofrequency surface that can support theoretically infinitely large wavevectors. The access to large wavevectors also results in a large photonic density of states (PDOS), which make HMMs of great interest for thermal emission. Traditional artificial HMMs use noble metals like gold or silver for the metallic layer, and silicon for the dielectric layers. Although these materials work well in the visible spectral range, they are not appropriate for the mid- to long-wave infrared. It is possible to create an infrared HMM using alternating layers of doped and undoped semiconductors. In this case, the doped semiconductor acts as the metallic layer while the undoped semiconductor acts as the dielectric layer. Specifically, a Si:InAs/AlSb HMM has been shown to act as a good, low-loss infrared HMM.
The high wavevector modes comprising the large PDOS in the HMMs are called volume plasmon polaritons (VPPs). VPPs result from the coupling of surface plasmon polaritons at the interfaces between the metal and dielectric layers in the HMM. We previously demonstrated highly efficient coupling from free space into VPP modes using gold grating couplers. To look for super-Planckian thermal emission, we heated our grating-patterned HMM samples to 185°C and measured polarized thermal emission. Although super-Planckian emission was not observed, we did observe significantly enhanced thermal emission by outcoupling the VPP modes. This emission was found to be almost completely angle-independent up to 50 degrees. This is the first report of thermal emission leveraging VPP modes, which could be used as a component of engineered thermal emission devices.