Symposium NM09 : Novel Approaches and Material Platforms for Plasmonics and Metamaterials

In this paper, inverse design methodology, along with genetic algorithm (GA) optimization is employed for synthesis of optical metasurfaces. Despite many advantages of the ultimate forms of ultra-thin layers, the performance of metasurfaces is not yet satisfactory due to the limited range of either amplitude or phase gradients achievable with ‘units of canonical shapes’. In addition, the existing design methodologies by nature do not explicitly support incorporation of multi-functionality, multi-band and broadband applications within one unit cell. Utilizing inverse design methodologies, here we design ‘binary-digitized unit cells’ to overcome the above limitations by pushing the achievable phase and amplitude gradients to fundamental limits. We propose an optimization scheme based on our in-house developed Finite-Difference Time-Domain (FDTD) algorithm, genetic algorithm and GPU computing to optimize the ‘binary-digitized unit cells’ to achieve all possible combinations and independently controllable phase and amplitude gradients at a broad range of frequency spectrum. We present a library of such optimized unit cell patterns that can be utilized and transformed into ultra-thin metasurfaces with a variety of novel applications. In this fashion, conventional applications of metasurfaces, including beam-steering, lensing, and holography can be extended systematically by overcoming the inevitable limitations dictated by regular canonical unit cells. For example, by arranging the optimized unit cells in a unique fashion we present highly-efficient digitized plasmonic metasurface for directive radiation and beam-steering in space. Or by independently controlling the phase gradients at a finite set of frequencies one can obtain challenging functionalities such as achromatic bending and focusing of light irrespective of the frequency range. This will be of extreme benefit to the fields of photonics and metasurfaces, and the future of functional metasurfaces.

Chiral plasmonic metamaterials have been proposed as a promising platform for optoelectronic devices with exotic applications, such as negative index of refraction materials and superlenses that can break the diffraction limit. Chiral metamaterials have unit cells that lack mirror symmetry and inversion centers, and exhibit asymmetries in response to circularly polarized light that are orders of magnitude greater than the ones observed from naturally occurring chiral molecules. In the far-field, these asymmetries are manifested in the circular dichroism (CD) signal that quantifies the difference in absorption of left and right handed light. Despite the extensive literature on the far-field optical response of chiral metamaterials, our understanding of the chiral electromagnetic light-matter interactions remains limited. To develop design principles for chiral metamaterials, it is important to characterize and tune the optical chirality C of the electromagnetic fields in the vicinity of the nanostructures in the chiral unit cell, and elucidate the complex relationship between the near- and far-field optical response of chiral systems.
Here, we used Finite-Difference Time-Domain calculations to simulate the optical response of a stacked gold L-shape resonator system. In this system, the sign of the CD spectrum can be controllably changed through lateral shifts in the relative position of two gold L- resonators, which tunes the interaction strength. These small structural reconfigurations change the energetic ordering of the hybridized modes and alter the CD response of the system without changing its handedness. We examined the sensitivity of the system to small structural perturbations, and demonstrated that the CD spectrum can abruptly reverse under small (less than 1 nm) reconfigurations of the L-resonators. The abrupt change in the far-field response reflects the interesting evolution of the near-fields under small reconfigurations of the system.
Calculations revealed that superchiral fields around the plasmonic structures change their magnitude and localization based on the position of the upper L-resonator relative to the bottom one. The optical chirality, and volume of the superchiral fields are much larger for the mode with enhanced extinction cross section. In this way, we demonstrated that small structural modification abruptly change not only the cross section but also the chiral near field response of the L-shape system under illumination with circularly polarized light.
The ability of our chiral L-shape assembly to abruptly change the far-field chiroptical response makes it ideal for experimentally fabricating a fast switchable chiral platform with dynamically tunable CD response. Meanwhile, the tunable chiral local electromagnetic fields could be utilized to examine how the change in the chiral interactions between the nanostructures translates into a change in the far field response of chiral systems.

Breakthrough Starshot is an ambitious project with the goal to design and build a laser-propelled spacecraft that can reach Proxima Centauri b, an exoplanet 4.2 lightyears away from Earth, in just 20 years. In order to propel the spacecraft to relativistic speeds (~0.2c), an ultrathin, gram-sized, lightsail must be stably accelerated under MW/cm² laser intensities operating in the near-IR spectral range. Because radiative cooling in space is the only mechanism for nanocraft thermal management, the Starshot Lightsail requires multiband functionality: it must simultaneously exhibit very low absorptivity in the (Doppler-broadened) laser beam spectrum in the near-IR, and high emissivity in the mid-IR for efficient cooling. These engineering challenges present an opportunity for nanophotonic design. In this work, we show that optimized nanoscale optical structures could play an important role in the lightsail design due to their ability to achieve desired optical response while maintaining low absorption in the NIR, significant emissivity in the MIR, and a very low mass.

To address the issues of efficient propulsion and thermal management, we combine material properties of very weak sub-band absorption in semiconductors with phonon-polariton driven emission in the MIR in materials such as silica. By way of nonlinear optimization, we survey a range of canonical nanophotonic structures (including thin-film slabs, multi-layer stacks, and 2D photonic crystal slabs) to reveal a tradeoff between reflectivity, mass, absorptivity, and emissivity. Our analysis compares several relevant figures of merit for the interaction between the laser and the lightsail and points to optimal designs for propulsion and thermal management.

Detection and generation of circularly polarized (CP) light is an essential operation in optical communication, quantum computing, molecular spectroscopy, magnetic recording and imaging applications. Increasing demand for subwavelength and high efficiency detectors has intrigued numerous research groups to approach this problem by designing twisted optical metamaterial and helical structures, spiral plasmonic lens and chiral organic transistors. More recently, owing to the notion of chirality, the detection of handedness of light has been made possible in the context of hot electron injection in plasmonic metamaterials, as well as in all-dielectric metasurface [1-2]. However, complicated fabrication procedure, bandwidth limitation and low values of transmission and circular polarization dichroism are still deemed as impeding factors for most practical applications.
Here we have experimentally demonstrated a hybrid metal-dielectric metasurface for CP light detection in transmission mode. First, based on the birefringence effect we design a quarter waveplate (QWP) by patterning a PECVD-grown silicon layer in the form of a periodic array with rectangular unit cells. This can be achieved by electron beam lithography, followed by inductively coupled plasma etching. The degree of anisotropy in phase accumulation between the fast and slow axes of QWP can be tailored by engineering the aspect ratio, occupation factor and silicon thickness. These degrees of freedom, on the other hand, provide a significant versatility to tune the operation wavelength (visible to NIR) and bandwidth broadening (few hundreds of nanometers). Moreover, due to low loss in dielectric materials in contrast to plasmonic structures, the measured transmission is as high as 95%, associated with a remarkable degree of circular polarization (DOCP>98%). Depending on the handedness of incident light, the QWP output will be linearly polarized +45 or -45 degrees with respect to the major or minor axes of QWP. Therefore, integration of a metallic grating separated by a fused silica spacer layer can almost completely block or pass the output, hence forming a binary detector. The extinction ratios measured in experiment are up to 13, while the overall transmission is close to 90%. Further design optimization can lead to even higher extinction up to 400.
The proposed structure exhibits various advantages including scalability and CMOS compatibility, compact footprint (few tens of micron) and superior DOCP, high extinction ratio and transmission. Moreover, it shows robustness against imperfections in fabrication process which is deemed desirable in comparison with other chiral metamaterial designs in literature. Therefore, it can be a great candidate for imaging, sensing applications and communication systems.

[1] Wei Li, et al., Nature Communications 6, 8379 (2015)
[2] Jingpei Hu, et al., Sci Rep. 7: 41893 (2017)

Polarization detection is an essential topic due to enriching applications including safe optical communication, remote sensing, polarization imaging and biomedical applications¹. Polarization, unlike intensity of the light, cannot be directly detected by conventional photodetectors. Currently, the widely used polarization detection methods require bulky optical components such as polarizers and waveplates, which make it challenging for device integration and minimization. Flat optics based on plasmonic structure open a new path for polarization detection with ultra-compact size ^2-5. Polarization detection in MIR range is especially attractive due to wide applications in biomedical fields like cancer detection and molecule chirality detection. Yet, MIR polarization detection is even more challenging than that in visible and NIR due to the material absorption limitations. Here we present the theoretical modeling and experimental demonstration of MIR polarization detection based on integrated plasmoinc flat optics composed of optical antenna and nanogratings. Our technique provides complete measurement of full stokes parameters and thus enables the detection of light with any polarization state, including partially polarized light. Moreover, it has the advantages of being ultracompact, capable to work in MIR range with high extinction ratio and easy to integrate with photodetectors. The MIR polarization detector consists of 6 detection units, including 4 nanograting units and 2 circularly polarized light detection units. According to our theoretical modeling, the nanograting units and the CP detection units show high extinction ratio for linearly and circular polarized input light in MIR, respectively. We have also demonstrated experimentally circularly polarized light detection with extinction ratio of 6.2 and linearly polarized light detection with extinction ratio of 45.5. With all 6 elements, we have performed full-stokes polarization measurement of arbitrary polarization states. The measured Stokes Parameters are reasonably well consistent to the input polarization of the light. The average error of S1, S2, S3 is 0.035, 0.025, and 0.104, respectively. And the average error of DOLP and DOCP is 0.036 and 0.103, respectively. The device performance can be further improved by increasing the extinction ratio of the linearly and circular polarization detection units through optimization of design parameters as well as fabrication processes. The detector we proposed can be easily redesigned to any wavelength from NIR to MIR by changing the design parameters of the optical antennas, which is promising for multi-wavelength or broadband polarization detection.

1. Snik, F., et.al. SPIE Proceedings 2014, 90990B.
2. Afshinmanesh, F., et.al. Nanophotonics 2012, 1, (2).
3. Li, W., et.al. Nat Commun 2015, 6, 8379.
4. Pors, A., et.al. Optica 2015, 2, (8), 716.
5. Chen, W. T. et.al. Nanotechnology 2016, 27, (22), 224002.

We report the design of reconfigurable metamaterial consisting a large array of nanowire featuring U-shaped cross section. These nanowires, also named as nano-scale metamolecules, support co-localized electromagnetic resonance at optical frequencies and mechanical resonance at GHz frequencies with a deep-sub-diffraction-limit spatial confinement (~λ²/100). The coherent coupling of those two distinct resonances manifests a strong optical force, which is fundamentally different from the commonly studied forms of radiation forces, gradient forces, or photo-thermal induced deformation. The strong optical force acting upon the built-in compliance further sets the stage for allowing the metamolecules to dynamically change their optical properties upon the incident light. The all-optical modulation at the frequency at 1.8 GHz has thus been demonstrated experimentally using a monolayer of metamolecules. The metamolecules were conveniently fabricated using CMOS-compatible metal deposition and nano-imprinting processes and thus, offer promising potential in developing integrated all-optical modulator.

Non-Hermitian systems can possess real eigenvalues if their Hamiltonians have parity- and time-symmetries (PT-symmetry). Such systems have been actively studied because they demonstrate extensions of conventional Hermitian quantum mechanics into a more generalized framework. Recently, PT-symmetry has gained much attention, especially in optics because of ease of implementation. PT-symmetry in optics translates to a conjugate-symmetric refractive index distribution, i.e. a balanced loss-gain system. Implementing such systems is easier for larger-scale photonic systems than for nanophotonic systems. Thus, so far, experimental studies have primarily focused on larger-scale photonic systems, though there are proposals for nanoscale optical devices with PT-symmetry.

Nano-optical devices often have high losses, which require equally high gain to implement PT-symmetric potentials. Such high gains are impractical and alternative methods to circumvent this problem have been investigated. One such alternative is a passive PT-symmetric system, where the characteristics of PT-symmetry can be observed using lossless and lossy components. These passive systems are simpler to implement using various fabrication techniques and materials available to nanophotonics. Here, we demonstrate a PT-phase transition in a passive dimer system consisting of silicon and silver nanoparticle pairs fabricated using electron-beam lithography. We characterize the system’s PT-symmetric behavior by measuring the scattering spectrum and far-field radiation pattern as a function of coupling, or distance between the particles. From the scattering spectrum, we can deduce the real and imaginary eigenvalues by identifying resonant peaks and linewidths. At the same time, the far-field radiation pattern, observed from the back Fourier plane image, represents the eigenmodes of the system.

In the PT-symmetric phase, where coupling is strong, far-field radiation is dipolar symmetric and scattering spectrum shows two resonant peaks. As coupling is weakened by increasing separation distance, the resonances move closer together in frequency, with little change in linewidth. At the exceptional point, these resonances coincide, resulting in a degenerate mode. Decreasing coupling past the exceptional point leads to a PT-broken phase, where the system exhibits a single resonant peak, with smaller linewidths. In the PT-broken phase, the far-field radiation pattern becomes increasingly asymmetric as coupling lowers, with more scattering towards the lossless particle. This behavior in the transition from the symmetric to symmetry-broken phase demonstrates passive PT-symmetry breaking at the nanoscale. Our understanding of PT-symmetry in this nanoantenna dimer opens opportunities to explore the rich physics underlying PT-symmetric nanosystems.

Colloidal synthesis of doped metal oxide nanocrystals provides a great opportunity and easy route to generate materials that has unique optoelectronic properties with promising applications such as smart windows, displays, sensing and photo-catalysis etc. By introducing the free carriers with different type of dopants (n- or p-type) in the metal oxide nanocrystals, their surface plasmon resonance can be tuned precisely from near IR to mid-IR range. Similarly, like metals, the optical response of plasmonic metal oxide nanocrystals can be manipulated by controlling the shape, size of the nanocrystal and free electron concentration. The effect of nanocrystal shape, size on the enhancement of their local electrical field strength and surface plasmon resonance have paved the way for new technologies and better sensing opportunities. The sharp faceted nanocrystals exhibit enhanced electric fields at corners and edges, which give us an opportunity to explore different morphologies of the NC for sensing application. Here, we will be presenting a solution route to synthesize plasmonic metal oxide nanocrystal (doped Indium Oxide) with defined shape, size, dopant type and radial distribution of dopant in the nanocrystals. Also, with co-doping (cation, anion or both) in these nanocrystals, we can shift the surface plasmon resonance to higher energies and can also influence the shape of the nanocrystals. Further, we will present near field enhancement property of single nanocrystals via EELS mapping and quantify both near field and far field plasmon property via COMSOL electromagnetic simulations.

Through systematic tailoring of the optical properties of lithographically patterned plasmonic nanostructures it is possible to optimize solar absorption and thermal reemission for photo-thermal heating to temperatures well above ambient. We outline a method to take advantage of such resonant photothermal heating in addition to photo-excited hot electrons to promote electron emission from the metal with high efficiency. Due to the close relation to purely thermionic emission this process is termed Hot-Electron Enhanced Thermionic Emission (HEETE). This dual mechanism of electron emission may provide a technique to more efficiently utilize optical power and can theoretically out-perform traditional semiconductor based solar cells.

To address design of such nanostructures, we have developed a simple model of the photo-thermal response of a plasmonic absorber with allows us to explore features such as spectral width of absorbance and emittance as well as angular dependence of emission. Additionally, it allows us to examine the roll of non-radiative thermal loss pathways such as conduction and convection. While these pathways normally dominate, placing the structure in vacuum is a simple way to minimize this loss. In such conditions temperature increases of well over 900 K are achievable without additional optical concentration. The nanostructures that reach these temperatures have high absorption, greater than 90%, in the visible up to 1100 nm and emissivity of approximately 2% through the infrared as well as minimized emission at oblique angles.

Using full wave optical simulations (FDTD method) and particle swarm optimization algorithms, where we were able to use temperature as calculated by our model as the figure of merit to identify possible nanostructures. We found a range of structures that will have the desired absorbance and emittance properties which are made of a variety of noble metals such as gold, silver, and copper. When coated with a dielectric material such as aluminum oxide to increase the thermal tolerance of the nanostructures while minimally impacting the emission characteristics, our designs takes advantage of highly absorbing plasmon resonances in the visible as well as the naturally low emissivity in the infrared which are both characteristic to metals without losing the thermal stability of higher melting point refractory materials. The dielectric coating also allows for accurate temperature measurements of the structure via in-situ anti-stokes Raman thermometry. Test HEETE devices have been nanofabricated on thermally isolated Si₃N₄ membranes to minimize thermal conduction to the surrounding substrate. Initial temperature measurements demonstrate that these plasmonic arrays greatly exceed the temperatures of ideal blackbodies under solar fluence.

Quantum dots (QDs) can lead either to the enhancement or to the quenching of their photoluminescence [1], provided that they are coupled with metallic nanoparticles (MNPs). Such MNPs, well known to sustain Localized Surface Plasmon (LSP) resonances, may indeed affect the QDs photoluminescence. The distance between QDs and MNPs is one of the switch parameters between both regimes. The goal of this study is to control the coupling distance (different from the physical distance) between QDs and MNPs by changing the refractive index of the surrounding medium using photochromic molecules. These molecules are optical switches, which move from a transparent state to a colored one by absorbing UV light. The spectral overlap and the lifetime of each optical phenomenon are the key parameters, since the photochromic molecules can couple to LSP to induce strong coupling [2] or couple to QDs to quench the photoluminescence [3].
In this study, the Fluorescence Lifetime Imaging Microscopy (FLIM) [4] has been performed to record QDs photoluminescence lifetime and intensity. We fabricated silver nanoparticles arrays covered with a protective SiO₂ layer and we spin-coated different mixtures on top of it. Firstly, we studied this sample spin-coated with QDs in a PMMA matrix and then, we studied the same sample spin-coated with QDs and photochromic molecules diluted in a PMMA matrix. The QDs photoluminescence lifetime and intensity have then been explored before and after the photochromic transition, above and nearby the MNPs arrays.
The analysis of the results shows a Förster Resonant Energy Transfer between the QDs (donors) and the colored form of the photochromic molecules (acceptors). In addition, it is observed an optical activation of the resonant coupling between QDs and MNPs due to the photochromic transition.
[1] Enhancement and quenching regimes in metal-semiconductor hybrid optical nanosources, P. Viste et al., ACS Nano, vol.4, n° 2, p. 759-764 (2010).
[2] Reversible strong coupling in silver nanoparticle arrays using photochromic molecules, A.-L. Baudrion et al., Nano Lett. 13, p. 282−286 (2013).
[3] Reversible Modulation of Quantum Dot Photoluminescence Using a Protein-Bound Photochromic Fluorescence Resonance Energy Transfer Acceptor, I. L. Medintz et al., J. Am. Chem. Soc., 126, p. 30-31 (2004).
[4] Munster, Erik B. van, and Theodorus W. J. Gadella. « Fluorescence Lifetime Imaging Microscopy (FLIM) ». In Microscopy Techniques, edited by Jens Rietdorf, 143 75. Berlin, Heidelberg: Springer Berlin Heidelberg, 2005. https://doi.org/10.1007/b102213.

Vanadium Oxide is well established for use in infrared bolometers because of its high temperature coefficient of resistivity (TCR). The metal-to-insulator transition of VO₂ has attracted recent interest for switchable infrared plasmonic devices. We demonstrate VO_x nanocrystalline thin films grown by aqueous spray deposition, which allows perfectly conformal coatings for convenient fabrication of (e.g.) plasmonic slot waveguides and metasurfaces. X-ray diffraction analysis shows that samples annealed post-growth in nitrogen have composition VO_x with x close to 2, while X-ray photoelectron spectroscopy on this film gives an x value of about 1.5. Its measured TCR is as high as -2.7 %/degC, which compares favorably with traditional sputtered films. Samples annealed in air have higher crystallinity in the more-oxidized insulating phases V₄O₉ and V₂O₅. Thus, for phase change applications, the degree of crystallinity can be increased, and value of x tuned, by post-growth annealing in oxidizing or reducing atmospheres.

The propagation of free-space electromagnetic signals is generally governed by time-reversal symmetry, meaning that forward- and backward-travelling waves will trace identical paths when being reflected, refracted or diffracted at an interface. Breaking time-reversal symmetry promises significantly improved photo-voltaic efficiencies and optical diodes, but is challenging to achieve in compact optical devices. Here, we introduce two nanophotonic designs that enable nonreciprocal transmission of visible and near infrared light within subwavelength optical paths. First, we design an all-dielectric, 100-nm-thick Si metasurface for non-reciprocal signal propagation. Owing to the high-quality-factor resonances of the metasurface and the inherent Kerr nonlinearities of Si, this structure acts as an optical diode for free-space optical signals. This structure also exhibits nonreciprocal beam steering with appropriate patterning to form a phase gradient metasurface. Secondly, we design a plasmonic metamaterial that exhibits broadband and wide angle nonreciprocity. A parity-time symmetric distribution of saturable loss and gain leads to nonreciprocal transmission over a 50 nm wavelength range and 60 degree angular range at visible frequencies. Compared to existing schemes, these platforms enable time-reversal-symmetry breaking for arbitrary free-space and modal optical inputs in a simple, robust materials platform.

Metal nanostructures are capable of plasmonic selective absorption in specific regimes of the electromagnetic spectrum, consequently leading to a higher ratio of absorption to re-radiation than that of a blackbody absorber. Selective absorbers are candidates for solar thermal energy generation, especially those materials with a large imaginary part of their dielectric function, such as gold. Titanium nitride (TiN) is a ceramic metal that absorbs in the visible and near-infrared range, with a large imaginary part of its dielectric constant—similar to gold and an ideal characteristic for absorption. It is also significantly less expensive, more chemically stable, and more thermally resistant than both gold and silver. TiN thin film shells (100-300 nm) have been grown on glass microbeads (3-10 μm diameter) on a heated boro-aluminosilicate glass substrate (300-600 K) by radio frequency (RF) sputtering physical vapor deposition (PVD) at 7 mTorr (ρ_Ar = 6, ρ_N = 0.5 mTorr) for 3-24 hours. Raman spectra for the films show consistent peaks at 210, 310, and 550 rel. cm^-1. X-ray diffraction (XRD) spectra confirmed crystallization of TiN at higher temperatures, with two peaks representing the (200) and (220) orientations. An analysis of the optical properties of the TiN nanoshells, obtained by spectroscopic ellipsometry (SE), will be presented.

Recently, multi-functional metasurfaces have been demonstrated based on polarization dependency, nonlinear optical effect and superposition of metasurfaces; nevertheless, conventional multi-functional metasurfaces have a severe limitation. We call them “coherent metasurfaces”. When incoherent light such as sunlight is shone on coherent metasurfaces, desired phase distribution cannot be developed; i.e., no information is obtained. In contrast, “incoherent metasurfaces” work only under incoherent light. They show colors or pseudo-holograms by controlling transmission or reflection spectra of incoherent optical waves. This incoherent response of the metasurfaces can be exploited for practical applications because incoherent light is more common than coherent light. However, no metasurface has achieved both coherent and incoherent functionality simultaneously.
Here we propose the first dual-mode metasurface that operates under both coherent and incoherent light simultaneously. The term “dual-mode” represents independent control of coherent response to transmitted light and of incoherent response to reflected light. Our dual-mode metasurface deploys parallel dielectric nanoantennas based on Pancharatnam-Berry phase to control spatial phase distribution of coherent light, and the reflection spectrum of incoherent light. Conventional metasurfaces based on Pancharatnam-Berry phase only control the orientation of each nanoantenna to manipulate phase distribution, but the reflection spectrum is also controllable by changing the sizes of nanoantennas. The nanoantenna sizes affect cross-polarization transmittance, so we find a pair of nanoantenna designs that have equal cross-polarization transmittance near the target wavelength of 635 nm. Based on the pair of designs, we design and experimentally demonstrate a crypto-display that contains encrypted information as an example of the dual-mode metasurface. Under incoherent white light the crypto-display works as a typical reflective display, whereas under coherent light, the encrypted information is revealed in the form of a hologram. Furthermore, the encrypted information does not affect the reflected image, so the information encoded in the crypto-display is not revealed unless coherent light is shone on it. Our device and design approach provide a way to develop novel security technologies such as steganography, anti-counterfeiting measures, and ghost imaging applications.

Nanostructures made of dielectric materials can have analogous properties to plasmonic structures for manipulation of light, with the advantage of having lower dissipative losses [1]. Wide bandgap phase change chalcogenides may be tailored to have a large refractive index with a low absorption in the visible and near infrared spectrum, and thus they are a promising platform for tunable metasurfaces in the visible and near infrared spectrum. For one of the most commonly used chalcogenides, Ge₂Sb₂Te₅ (GST), the refractive index at 840nm is rather large, approximately 4.5 for the amorphous state and 5.5 for the crystalline state. However, the extinction coefficient of GST at 840nm is approximately 1.5 and 3.6 for the amorphous and crystalline states respectively, which renders it very lossy and, therefore, impractical for realistic applications. In comparison, the properly designed wide bandgap phase change chalcogenide, which is used herein, has a refractive index of approximately 3.0 and 3.5 for the amorphous and crystalline states at 840nm, with an extinction coefficient near 0 at 840nm for both states, thus meeting the high index and low loss requirements for high efficiency devices.

We designed nanoantenna arrays metasurfaces based on a wide bandgap chalcogenide for the visible and NIR spectrum. The device operates in transmission mode and allows manipulation of the phase of the transmitted wave, and is tunable through structural phase transitions in the chalcogenide material. The proposed device exploits both the refractive index change in chalcogenide and the concept of Huygens’ metasurface [2] to exhibit a very high transmission (>80%) with full angular 2π phase control. Based on the structural phase change property of the chalcogenide, we designed a gradient metasurface using two different mechanisms: geometrical tuning and partial crystallization. Both designs allow dynamic control of the transmitted light at a wavelength of 840nm. The transmitted beam deflection could be tuned by adjusting the individual crystallization levels of the wide bandgap phase change material, with overall efficiencies exceeding 40% with respect to the incident power.

In conclusion, both simulations and experimental results will be presented that demonstrate nanoantenna array metasurfaces, which are based on a wide bandgap phase change materials, can achieve tunable control of light beams in the visible and NIR spectrum. These results suggest that phase change materials have a further application beyond data storage in high-speed spatial light modulator and phase arrays, with potential applications in dynamic holography.

Li Lu acknowledges his scholarship from Singapore Ministry of Education.

[1] Kuznetsov, Arseniy I., et al. "Optically resonant dielectric nanostructures." Science 354.6314 (2016): aag2472.
[2] Yu, Ye Feng, et al. ''High-transmission dielectric metasurface with 2π phase control at visible wavelengths." Laser & Photonics Reviews 9.4 (2015): 412-418.

Plasmonic chiral metamaterials with strong optical chirality and high tunability in visible and near-infrared light regimes have emerged as promising candidates for photonic sensors and devices. Here, we demonstrate a new type of chiral metamaterials, known as moiré chiral metamaterials (MCMs), to overcome limits in current chiral metamaterials that rely on local structural chirality or site-specific symmetry breaking. Consisting of two layers of identical achiral Au nanohole arrays stacked into moiré patterns, the ultrathin (~70 nm, which is only ≈1/10 of the operation wavelength) MCMs exhibit strong chiroptical effects. The optical chirality can be precisely tuned by the relative rotation between the lattice directions of the two Au nanohole arrays. We have further demonstrated that the MCMs can distinguish a therapeutic chiral drug, R-thalidomide, from its medically toxic enantiomer (S-thalidomide) at picogram level in a label-free manner.

Moreover, we have exploit Fano coupling as a new mechanism to achieve ultrathin active chiral metamaterials of highly tunable chiroptical responses by adding a dielectric spacer layer in MCMs. Our simulations and experiments reveal that spacer-dependent Fano coupling exists in the MCMs, which significantly enhances the spectral shift and line shape change of the circular dichroism (CD) spectra of the MCMs. We further use a silk fibroin thin film as an active spacer layer in the MCMs. With the solvent-controllable swelling of the silk fibroin thin films, we demonstrate tunable Fano coupling and chiroptical responses of the silk-MCMs using different solvents and their mixtures. Impressively, we have achieved the spectral shift over a wavelength range that is more than one full width at half maximum and the sign inversion of the CD spectra in a single ultrathin (1/5 of wavelength in thickness) MCM. Finally, we apply the silk-MCMs as ultrasensitive sensors to detect trace amount of solvent impurities down to 200 ppm, corresponding to an ultrahigh sensitivity of >10⁵ nm/refractive index unit (RIU) and a figure of merit of 10⁵ /RIU. With their strong and tunable optical chirality, in combination with robust cost-effective fabrication, the MCMs will become critical components for chiroptical devices. Our results also pave a way towards active chiral metamaterials of high tunability, ultrathin thickness and large-scale fabrication for a wide range of applications.

Hyperbolic metamaterials (HMMs) are highly anisotropic structures that exhibit metallic (i.e., Re (ε) < 0) and dielectric (i.e., Re (ε) > 0) response along orthogonal directions. They have been utilized to demonstrate various phenomena, including broadband light absorption, enhanced spontaneous emission, asymmetric light transmission, engineered thermal radiation, and sub-diffraction imaging. The key to the array of rich phenomena enabled by HMMs is their highly anisotropic permittivity. HMMs reported to date are often described by numerically calculated permittivity tensors based on effective medium theory (EMT), which utilizes constituent metal and dielectric permittivities reported in the literature or measured by spectroscopic ellipsometry. However, the accuracy of calculation is limited by the known precision of experimental layer thicknessness and local permittivities, as well as non-modelled effects such as layer roughness, strain, and inter-layer diffusion.

In this work, we demonstrate how both the in-plane and out-of-plane effective permittivities of an HMM operating at ultraviolet, visible, and near-infrared frequencies can be accurately extracted using a coupling-prism-enabled spectroscopic ellipsometry technique based on total internal reflection (TIR). For reference, this technique is compared to two other spectroscopic ellipsometry methods commonly used to date for HMM characterization, namely (1) interference enhancement (IE), in which reflection-mode ellipsometry exploits a substrate decorated with a silicon oxide layer to enhance light-HMM interaction, and (2) reflection plus transmission (R+T), which adds normal-incidence transmittance spectroscopy to standard reflection-mode ellipsometry. Although both IE and R+T techniques have been successfully used for characterizing isotropic thin absorbing films, we show here that neither method is able to robustly extract HMM out-of-plane effective permittivity. In contrast, the TIR method is demonstrated to provide robust permittivity extraction having well-converged fitting parameters. In particular, measurement sensitivity is improved compared to both the IE and R+T cases via prism-mediated enhancement of the out-of-plane electric field inside the HMM. The TIR technique requires neither modification of the HMM sample itself nor substantial re-configuration of a standard ellipsometer, and can therefore serve as a reliable and easy-to-adopt technique for the characterization of both HMMs and a variety of other anisotropic metamaterials.

With increasing concerns about environmental pollution, opiate abuse and terrorism, people have become more sensitive to hazardous substances that threaten public health and safety. Raman spectroscopy is a very useful analytical tool, giving molecular fingerprint information even with portable readers. However, since inelastic (Raman) scattering of light is inherently weak, the Raman-based sensors cannot detect the substances in trace amounts. The extraordinary enhancement of Raman signals from molecules adjacent to metallic nanostructures, which is called as surface-enhanced Raman scattering (SERS), has been discovered by Professor Van Duyne in 1976. Over the past 40 years, scientists have continuously expanded the theoretical understanding of the plasmonic phenomena and developed various nanomaterials to be SERS-active.

From a practical point-of-view, cost-effective high-throughput methods of fabricating SERS substrates are in great demand. In this regard, we introduce a novel approach for fabricating SERS substrates on plastic films. Maskless plasma etching of a plastic film produces nano-protrusions on the surface, which serving as selective growth sites for nanostructure development during the subsequent metal deposition step. These simple two steps result in ultrahigh density (>100/μm²) plasmonic nanopillar arrays. Besides of SERS performance (i.e., enhancement factor > 10⁷), the reproducibility and uniformity are thoroughly examined in 4 inch wafer scale. The detections of forensic drugs (heroin, Fentanyl, methamphetamine) and explosives (TNT, RDX, PETN) in low concentrations have been demonstrated on our SERS substrates (KIMStrates) using a portable Raman reader (Metrohm Raman Ltd.). We strongly believe that this economical and reliable SERS substrate can be a material platform for ultrasensitive Raman sensors in various applications of SERS technology.

Antireflective (AR) nanostructures observed in nature, such as the corneal surface of Moth eye, the wing scales of butterflies etc., exhibit an excellent AR performance compared to the quarter wave thin films over a wide range of incident angles and wavelengths, thus attracting great interests in the field of optical and optoelectronic devices. Especially, anti-reflectivity for wide angles of incidence is significant in display and photovoltaic applications to improve visibility and photo-conversion efficiency, respectively. Since the AR nanostructures with 3D geometry are fabricated on a substrate, it is necessary to consider the scattering and coupling of the light due to the geometry of the nanostructures on the substrate, in dealing with the anti-reflectivity by the incident angles.

In this work, we investigate effects on the AR properties of two geometries, nanocones (NCs) and inverted nanocones (INCs) which can be generated by geometric inversion in the nano-imprinting process. We fabricate the two geometries by repetitive polymer replication processes by using photo-curable polymers and nanostructured quartz molds and evaluate the specular reflectance for visible range with various incident angles from 6° to 75°. The measured spectra are analyzed in the view of Mie scattering and guided mode resonance.

We find that, unlike the INCs, the NCs enable to maintain Mie scattering efficiency against changes in the incident angles because the scattering fields are concentrated at the apex of the NCs. This phenomenon is verified by computational simulations based on finite-difference time domain methods. The concentrated scatterings on NCs allow the more propagation of incident fields and for this reason, the NCs provide better AR performance than the INCs. We observe the presence of guided mode resonance from the measured spectra and analyze it by considering the phase matching in 2D hexagonal nano-grating structures. Additionally, we find that INCs can exhibit stronger guided mode resonance and internal reflections, which can be another reason why AR performance is degraded in the INCs. By utilizing these findings on both-sided antireflective nanocones, we achieve extremely low average reflectance (5.4 %) at very high incidence angle of 75° for entire visible range.

This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2017R1D1A1B04033182)

Nanomaterials comprised of molecular excitons optically coupled to surface plasmon-polaritons at metal interfaces are considered. Using semiclassical theory we investigate optical properties of such systems under strong coupling conditions and high molecular concentrations. It is shown that exciton-plasmon materials in addition to conventional Rabi splittings exhibit collective optical exciton resonances. The results of theory are compared with experimental measurements for 3D opal plasmonic arrays. We also examine the radiative decay rates at high molecular concentrations. It is shown that the decay rates are significantly reduced due to strong exciton-exciton coupling.

In recent years, the development of two-dimensional transition metal dichalcogenides (2D TMDs) has aroused great interests in a variety of optoelectronic applications including photodetectors, optical chemical sensors, light-emitting diodes, lasers, and opto-valleytronic devises because of their high ON/OFF current ratios, low sub-threshold switching, strong photoluminescence, controllable valley polarization and high thermal stability. Despite their excellent optoelectronic properties, the light-matter interaction in 2D TMDs is weak due to their atomic thickness, thereby limiting their optoelectronic applications. Benefiting from the capability of surface plasmons (SPs) in concentrating light beyond the diffraction limit, plasmonic metal NPs have been applied to enhance light-matter interactions in quantum emitters including dye molecules and quantum dots through mechanisms such as Fano interference, strong coupling, plasmon-induced resonance energy transfer, and plasmon-enhanced emission. Therefore, there is an emerging trend of exploiting light-matter interactions in hybrid systems consisting of 2D TMDs and plasmonic metal NPs for boosting the performance of 2D TMD-based optoelectronic devices. Herein, we report two tunable plasmon-exciton interactions that are novel in 2D TMD-plasmonic NP hybrids: (1) tunable plasmon-induced resonance energy transfer from a single Au nanotriangle (AuNT) to monolayer MoS₂; (2) tunable Fano resonance and plasmon-exciton coupling in a single AuNT on monolayer WS₂ at room temperature. In the first case, we report the first observation and tuning of plasmon-trion and plasmon-exciton resonance energy transfer (RET) from a single AuNT to monolayer MoS_2. We achieved these phenomena by the combination of rational design of hybrid 2D TMD-plasmonic NP systems and single-nanoparticle measurements. By combining experimental measurements with theoretical calculations, we conclude that the efficient RET between SPs of metal NPs and excitons or trions in monolayer MoS₂ is enabled by the large quantum confinement and reduced dielectric screening in monolayer MoS₂. In the second case, we report tunable Fano resonances and plasmon-exciton coupling in 2D WS₂-AuNT hybrids at room temperature. The tuning of Fano resonances and plasmon-exciton coupling was achieved by active control of the WS₂ exciton binding energy and dipole-dipole interaction through controlling the dielectric constant of the surround medium. Specially, Fano resonances are controlled by the exciton binding energy or the localized surface plasmon resonance (LSPR) strength through tuning the dielectric constant of surrounding solvents or the dimension of AuNTs. Additionally, we observe a transition from weak to strong plasmon-exciton coupling when increasing the dielectric constant of surrounding solvents. Our results provide guidance on systematic tuning of the Fano line-shape and Rabi splitting energies at room temperature for 2D TMD-plasmonic NP hybrids.

Intense research on two-photon polymerization (2PP) processes has led to the development of sophisticated commercial apparatus capable of producing arbitrary 3D polymer scaffolds with spatial resolutions as high as 170 nm. Generally speaking, these polymer-based constructs do not interact with photons due to their low conductivity and low dielectric constants. Therefore, they do not make good optical metamaterials by themselves; however, metals and materials with high dielectrics constants such as polar dielectrics (e.g., Si, hBN and SiC) do. In this work we produced novel optical metamaterials by combining 2PP (or 3D-printed) structures with e-beam evaporation, atomic layer deposition and/or reactive-ion etching. Furthermore we compare optical measurements performed on these structures with full-wave electromagnetic simulations, demonstrating the strength of these fabrication methods of chiral/non-chiral structures suitable for applications such as SERS, SEIRA, light steering, and sub wavelength light focusing.

Conventional optical components are limited to size-scales much larger than the wavelength of light, as changes to the amplitude, phase and polarization of the electromagnetic fields are accrued gradually along an optical path. However, advances in nanophotonics have produced ultrathin, so-called “flat” optical components that beget abrupt changes in these properties over distances significantly shorter than the free space wavelength. While high optical losses still plague many approaches, phonon polariton materials have demonstrated long lifetimes for localized modes in comparison to plasmon-polariton based nanophotonics. Our work predicts a further 14-fold increase in the optic phonon lifetime and we experimentally report a ~3-fold improvement through isotopic enrichment of hexagonal boron nitride (hBN). We establish commensurate increases in the phonon polariton propagation length via direct imaging of polaritonic standing waves by means of infrared nano optics. Our results provide the foundation for a materials-growth-directed approach towards realizing the loss control necessary for the development of phonon polariton based nanophotonic devices.

Active control of the plasmonic properties in a dynamic and reversible manner enables applications such as plasmonic sensing and photovoltaic devices. Herein, we present electrochemically tunable hybrid nanostructures composed of gold nanorods encapsulated with directly polymerized poly[(3,4-propylenedioxy)pyrrole] (PProDOP) nanoshells with controlled thicknesses. This system displays narrow visible-near infrared absorption bands upon applying a variable electric potential due to the remarkable transmissivity of PProDOP at various oxidation states. The PProDOP, synthesized by in-situ chemical oxidative polymerization using a mild oxidizing agent, has demonstrated outstanding electrochemical performance such as reversible electroactivity, high transmissivity in visible-range at various oxidation states, as well as a low oxidation potential (-1.06 V vs. Fc/Fc+). It was revealed that the stable reversible modulation of the observed plasmonic response of the gold nanorods was caused by the variation of the refractive index of PProDOP shells at different oxidation states as confirmed by finite-difference time-domain (FDTD) simulations. A surface plasmon resonance (LSPR) band of gold nanorods at 800 nm was shifted reversibly by 24 nm upon multiple cycling of electric potential. Overall, these core-shell structures with electrochemical plasmonic tunability in the near-infrared region allow for tailoring of the optical and electrochemical properties of pre-programmed plasmon responses for active control of colorimetric appearance across the visible range and toward the near-infrared.

This research develops a plasmon-exciton system, which is composed of the fullerene film and the gold nanostructure, applicated in the optics and optoelectronics. Fullerene exciton can be excited and interacted with the surface plasmons produced from the gold nanostructure, and this interaction results in the plasmon energy transporting out of the near-field range. We demonstrate this effect by the cavity structure that sandwiches the fullerene films between a monolayer gold nano-islands and a gold film. The gold film act as a plasmonic mirror producing the image charges and its electromagnetic field couples with the extended plasmonic field from the nano-islands. The coupling phenomenon makes the reflection spectra exhibiting the asymmetric curve-lines, and it brings our cavity structure displaying a bright, saturated, and nearly omnidirectional visible colors. Furthermore, the plasmon-exciton interaction has a significant advantage in the plasmoelectric effect, which is an energy converting rout from plasmon to electronic. The strength of plasmoelectric potential is dominated by the effective temperature. Because of the extended plasmon energy by the plasmon-exciton interaction and the ultralow emissivity of the fullerene, the temperature of the hot spots may reach thousands of kelvins. The high temperature makes the output voltage is up to 277 mV under the UV illumination with the intensity of 10 mW/cm². The efficiency is hundreds of times as large as the voltage produced from the single layer of gold nanoparticles. With this advantage of high plasmoelectric voltage, fullerene films have broad use in many optoelectronic applications, such as solar cell, catalysis, and photovoltaic devices.

Today it is possible to engineer the building blocks of artificial materials (meta-materials) with feature sizes smaller than the wavelength of light. The ability to design meta-atoms in a largely arbitrary fashion adds a new degree of freedom in material engineering, allowing to create artificial materials with unusual electromagnetic properties rare or absent in nature. Achieving tunable, switchable and non-linear functionalities of meta-materials at individual meta-atom level could potentially lead to additional flexibility in designing active photonic devices. These include among others, meta-materials based on phase-change materials, whose properties could be altered by thermal or photo-thermal means. In this presentation, our recent results on developing appropriate numerical methods to study hybrid meta-material structures containing phase-change materials will be discussed. Meta-atoms based on plasmon polaritonic or phonon polaritonic materials are considered depending on the application spectral range. We develop appropriate phenomenological models of phase transition and self-consistently couple them with the full wave electromagnetic and heat transfer solvers. Developed methods are used to design meta-surface based tunable components.

Fundamental research in photonic materials has been one of the core drivers of progress in optical and photonic technologies, including photonic nanotechnologies, as well as in classical and quantum optical information processing. In particular, optical metamaterials - artificial composites with an engineered electromagnetic response - have recently emerged as a nascent class of novel materials that may offer the control and robustness required for a vast array of applications, ranging from precision wavefront sculpting through cloaking and to electromagnetic control at the single-quantum level. Using highlights of recent accomplishments and awards made through the Electronic and Photonic Materials Program at the NSF, I will address the current trends and recent breakthroughs in studies of optical metamaterials and metasurfaces, as well as their main materials challenges. I will additionally discuss recently identified gaps, both epistemic and technological, and the best approaches for addressing these challenges.

Surface-phonon polaritons (SPhPs) have emerged as attractive alternatives to plasmon polaritons because of the extremely high quality factors of their localized and propagating resonances (SPhPRs). The high quality arises from their dependence on concerted nuclear motion rather than electronic motion. These resonances can only be supported when the resonance frequency lies in the Reststrahlen band of a material, the region between the longitudinal and transverse optical phonons and characterized by metal-like optical constants. To date, most SPhPs have been observed in polar semiconductors, where the Reststrahlen band tends to occur at wavelengths longer than 6 micrometers. Shorter wavelength, higher frequency SPhPRs could potentially be useful for a variety of chemical applications like sensing or energy-transfer modulation. Here, we present infrared reflection spectroscopy of a number of molecular crystals that possess Reststrahlen bands in chemically relevant frequency ranges. We identify and assign resonances that appear within the Reststrahlen bands of some of these materials, specifically W(CO)₆, and comment on the range of resonances that can be supported on this class of SPhP materials.

The new design paradigm that metamaterials and metasurfaces provide, such as the ability to tailor the local near fields as well as the far field radiation patterns, are enabling new schemes for ultrafast control of light and nonlinear optical sources. In this talk I will describe some of these new developments when metasurfaces made from conventional and new oxide semiconductors are used combined with short pulse excitation using single and multiple pulses. I will describe i) recent results of multiple harmonic mixing spanning a wavelength range from the near infrared to the ultraviolet using highly nonlinear GaAs based metasurfaces, ii) ultrafast subpicosecond polarization switching of light with high contrast using plasmonic cavities containing high mobility CdO films.

Colloidal plasmonic nanocrystals (NCs) are known for their size- and shape-dependent localized surface plasmon resonances. Here we show these plasmonic NCs can be used as building blocks of mesoscale materials.¹ Chemical exchange of the long ligands used in NC synthesis with more compact ligand chemistries brings neighboring NCs into proximity and increases interparticle coupling. This ligand-controlled coupling allows us to tailor a dielectric-to-metal phase transition seen by a 10¹⁰ range in DC conductivity and a dielectric permittivity ranging from everywhere positive to everywhere negative across the whole range of optical frequencies. We realize a "diluted metal" with optical properties not found in the bulk metal analog, presenting a new axis in plasmonic materials design and the realization of optical properties akin to next-generation metamaterials. We harness the solution-processability and physical properties of colloidal plasmonic NCs to print NC superstructures for large-area, active metamaterials. We demonstrate quarter-wave plates with extreme bandwidths and high polarization conversion efficiencies in the near- to-mid infrared.² By fabricating colloidal NC superstructures on the surface of hydrogels, we fabricate optically-responsive sensors suitable for large-area monitoring of soil moisture. Finally, by combining superparamagnetic Zn_0.2Fe_2.8O₄ NCs and plasmonic Au NCs, we fabricate multifunctional, smart superparticles, that in suspensions, switch their polarization-dependent transmission in the infrared in response to an external magnetic field.³

(1) Fafarman, A. T.; Hong, S.-H.; Caglayan, H.; Ye, X.; Diroll, B. T.; Paik, T.; Engheta, N.; Murray, C. B.; Kagan, C. R. Nano Lett. 2013, 13 (2), 350–357.
(2) Chen, W.; Tymchenko, M.; Gopalan, P.; Ye, X.; Wu, Y.; Zhang, M.; Murray, C. B.; Alu, A.; Kagan, C. R. Nano Lett. 2015, 15 (8), 5254–5260.
(3) Zhang, M.; Magagnosc, D. J.; Liberal, I.; Yu, Y.; Yun, H.; Yang, H.; Wu, Y.; Guo, J.; Chen, W.; Shin, Y. J.; Stein, A.; Kikkawa, J. M.; Engheta, N.; Gianola, D. S.; Murray, C. B.; Kagan, C. R. Nat. Nanotechnol. 2016, 12 (3), 228–232.

The numerical design, synthesis and characterization of advanced colloidal structures and foams based on High Internal Phase Emulsions for application in plasmonics, nano- and macro- photonics has proven to be very attractive, especially in the fields of lasing and new single photon sources.
In this talk, we will report a review of our work in these domains and explain the salient features of the involved effects.
As such, i) we provide experimental evidence of plasmonic super-radiance of organic emitters grafted to Au@SiO2 nanospheres at room temperature. This observation of plasmonic super-radiance at room temperature opens questions about the robustness of these collective states against decoherence mechanisms which are of major interest for potential applications.
ii) We demonstrate both experimentally and theoretically how to manipulate strong coupling between the Bragg-plasmon mode supported by an organo-metallic array and molecular excitons in the form of J-aggregates dispersed on the hybrid structure [1]. We observe experimentally the transition from a conventional strong coupling regime exhibiting the usual upper and lower polaritonic branches to a more complex regime, where a third nondispersive mode is seen, as the concentration of J-aggregates is increased. Owing to numerical simulations, we could confirm the presence of the third resonance and attribute its physical nature.
iii) We demonstrate lasing oscillation in Colloidal Photonic Crystals (CPCs) based on a defect mode passband effect [2]. The spectroscopic measurements and theoretical simulations match well and reveal that the relatively low-threshold lasing exhibited by the structure can uniquely be attributed to the efficient coupling of the spontaneous emission of the dye to the defect mode of the CPC.
Finally, iV), we have numerically predicted and experimentally shown the coexistence and competition of random lasing (RL) and stimulated Raman scattering (SRS) in active disordered random media: foams based on silica HIPEs [3, 4]. We developed a simple model which includes both mechanisms coupled through diffusion equations. We found that the prevalence of a nonlinear mechanism over the other is determined by the degree of scattering. The competition was explained in terms of disorder-dependent pump depletion and fluorescence saturation.
References
[1] P. Fauché, C. Gebhardt, M. Sukharev, M., R. A. L. Vallée, Scientific Reports 7, 4107 (2017).
[2] K. Zhong, L. Liu, X. Xu, M. Hillen, A. Yamada, X. Zhou, N. Verellen, K. Song, S. Van Cleuvenbergen, R. Vallée and K. Clays, ACS Photonics 3, 2330-2337, (2016).
[3] N. Bachelard, P. Gaikwad, R. Backov, P. Sebbah and R. A. L. Vallée, ACS Photonics, 1(11), 1206–1211 (2014)
[4] P. Gaikwad, N. Bachelard, P. Sebbah, R. Backov and RAL Vallée, Advanced Optical Materials, 3(11), 1640–1651 (2015).

Metamaterials provide the ability to design material properties beyond those possible with conventional materials. Research programs at ONR and other agencies are pursuing fundamental and applied goals with the aim of mapping out the potential of metamaterials for a variety of applications. These programs have pursued optical, RF and acoustic materials for applications across the electromagnetic spectrum, from novel RF antennas to devices utilizing optical magnetism. Acoustic metamaterials are of unique interest to the Navy for underwater applications. Common challenges associated with loss, bandwidth, and scalability will be discussed along with the needs for innovative designs, materials, and fabrication approaches.

The delay-bandwidth limit refers to the trade-off between the time delay that can be applied to a signal traveling through a device and its bandwidth. Recently, there have been several studies showing that this bound can be broken in nanophotnics, including time-modulated photonic crystals, nonreciprocal cavities and terminated unidirectional waveguides. In our talk, we will discuss various approaches to overcoming the delay-bandwidth limit, their relation to Lorentz reciprocity, and a novel approach to slow down waves using temporally modulated network, which enables a quasi-electrostatic signal transport with large group velocities over broad bandwidths of operation.

Based on their ability to provide control over wavefront, polarization and spectrum of light fields while having just nanoscale thickness, optical metasurfaces are promising candidates for flat optical components. Typically, metasurfaces consist of two-dimensional subwavelength arrays of designed metallic or dielectric scatterers. So far, deviations from a periodic, ordered arrangement were usually associated with a deterioration of the metasurface optical properties. However, more recently researchers started recognizing the introduction of controlled disorder as a new handle to engineer the optical response of metasurfaces. For example, the introduction of disorder can decrease unwanted anisotropy in the optical response [1] and it can enhance the channel capacity of wavefront shaping metasurfaces [2].
Here we investigate two different types of disordered metasurfaces. In a first study, we consider a chiral plasmonic metasurface consisting of twisted gold-nanorod dimers. Chiral metasurfaces and metamaterials were intensively studied in the past. Most prominently, they can exhibit huge optical activity [3] and were suggested for applications as polarizing elements [4,5] or nanophotonic sensors. Using polarization spectroscopy and interferometric white-light spectroscopy, we demonstrate that the introduction of rotational disorder at the unit-cell level enables the realization of chiral plasmonic metasurfaces supporting pure circular dichroism and circular birefringence. Importantly, we show experimentally that the polarization eigenstates of these metasurfaces, which coincide with the fundamental right- and left-handed circular polarizations, do not depend on the wavelength in the spectral range of interest. Thereby, our metasurfaces closely mimic the behaviour of natural chiral media, while providing a much stronger chiral response.
In a second study, we concentrate on disordered silicon metasurfaces exhibiting electric and magnetic dipolar Mie-type resonances [6]. Silicon metasurfaces exhibit very low absorption losses in the near-infrared spectral range, thereby opening the door to long-range in-plane interactions between the individual nanoresonators. We systematically investigate how the introduction of different types of positional disorder influences the complex transmittance spectra of these metasurfaces, showing that disorder provides an independent degree of freedom for engineering their spatial and spectral dispersion.
[1] S. S. Kruk et al., Phys. Rev. B 88, 201404(R) (2013).
[2] D. Veksler et al., ACS Photonics 2, 661 (2015).
[3] M. Decker et al., Opt. Lett. 35, 1593 (2010).
[4] J. K. Gansel et al., Science 325, 1513 (2009).
[5] Y. Zhao et al., Nat. Commun. 3, 870 (2012).
[6] I. Staude & J. Schilling, Nature Photon. 11, 274 (2017).

Random metamaterials present several advantages compare to their periodic counterparts. (i) The circular symmetry of the elements is statistically restored by the randomness, which allows to design polarization independent metalenses with very anisotropic elements, providing new opportunities to design a. (ii) The random design process optimizes the area of the metasurface. In a periodic metasurface, small and large elements have the same footprint. On the contrary, in a random metasurface, the random design finds more easily a spot for a small element than a large one. This comes to optimize the local density and the footprint of the elements. (iii) The absence of periodicity eliminates any possible spurious diffraction order that can arise for large periods, due to large footprints of elements as in dielectric metasurfaces, or for large numerical aperture lenses.
We will consider arrays of gold plasmonic nanorods in the infrared domain (1500 nm). Such rectangular element is very anisotropic and only polarizable along its longer dimension. Varying the nanorod length from 150 to 500 nm changes the resonant frequency of the element, which allows us to tune the phase-shift provided to an incident plane wave which electric field is parallel to the long axis. On the contrary, the nanorod is transparent to an incoming plane wave with a polarization perpendicular to its main axis. The question that arises is: can we use this simple but anisotropic element to design an isotropic and all polarization metalens?
Here we propose to discuss metasurfaces made of such nanorods that have random positions and orientations. The density of elements is the only tunable parameter in the case of random metamaterial. It influences the phase shift through the scattering cross section of the resonators as well as the near field coupling between them. Hence, we will discuss the parking problem of rectangular elements in 2 dimensions in the context of metasurfaces.The focusing efficiency strongly depends on the density if nanorod per wavelength but also of the dimensionality and of the symmetry of the metasurface Using full wave simulation with CST, we design ordered and nematic 1D and 2D metalens and compare their characteristics.
Finally, we present a experimental realizations of 2D random metalens. The latter are made with conventional top-down fabrication techniques and e-beam lithography. We will show that the resulting lens focus light on diffraction limited focal spots for any polarization.
As a conclusion, this communication will show that random metasurfaces are can be used to realize devices usually made within a periodic framework, opening new perspectives to design metasurfaces.

The concept of diffusion is widely used to study propagation of light through multiple scattering media such as clouds, interstellar gas, colloids, paint, and biological tissue. Such media are often called random. This terminology is, however, misleading. Notwithstanding its complexity, the process of wave (e.g. light, sound, electron wave, etc) propagation is deterministic – i.e. given the exact position of scattering centers and the amplitudes of the impinging waves, one can uniquely determine the precise pattern of wave field throughout the system. This pattern can be represented in the basis of eigenchannels of multiple-scattering medium that are based on singular value decomposition of a suitably defined transmission/reflection/absorption matrix, and coupling into different eigenchannels can lead to such a diverse transport behaviors as perfect transmission/reflection/absorption.
The universal bimodal distribution of singular values of transmission matrix in lossless diffusive systems underpins such celebrated phenomena as universal conductance fluctuations, quantum shot noise in condensed matter physics, and enhanced transmission in optics and acoustics. In contrast to the distribution of singular values, the corresponding eigenchannels, are sensitive to the geometry – a specific choice of boundary conditions, placement of macroscopic inhomogeneities in the system, etc. In lossy systems, absorption and its spatial distribution represent the additional degrees of control. In this talk, we will demonstrate effective approaches to modify the eigenchannels in a deterministic way, opening up new opportunities for controlling energy distribution inside complex media via wave-front shaping.

Rare-earth doped photon upconversion (UC) nanomaterials have numerous applications in different fields such as wavelength conversion layers in solar cells, security inks, and autofluorescence-free fluorescent labels for bioimaging. However, these materials suffer from inherent limitations; the small excitation cross section and the low quantum efficiency are the obstacles for the practical applications. A promising approach to overcome these problems is the formation of nanocomposites composed of UC nanomaterials and metal nanostructures and utilizing the enhanced electric fields accompanied by the localized surface plasmon (LSP) resonances. A variety of nanocomposite structures has been proposed and tested so far, and more than 100-fold enhancement of the UC intensity has been reported [T. Hinamoto et. al, JPCC, 121 (2017) 8077].
In order to maximize the UC enhancement by the LSP resonance, a metal nanostructure has to satisfy some criteria. First, it should have multiple resonances at largely separated wavelengths, because the upconverted photon energy is usually 1.5-3 times larger than the excitation one. A LSP mode at the excitation wavelength is preferably a dark mode with a large absorption cross-section and a small scattering cross-section, while that at the emission wavelength is vice versa. Furthermore, the electric field distribution has to be optimized to maximize the overlap between the field enhancement region and a volume of an UC material. Apparently, metal nanostructures satisfying all these criteria are not simple.
In this work, we develop a metal nanostructure composed of a metal core and a metal nanocap, and placed an UC material in between. In this structure, the LSP modes split into bonding and antibonding ones due to the plasmon hybridization, and the resonance wavelengths can be controlled in a wide wavelength range by the strength of the hybridization. Furthermore, a strong enhancement of the electric fields is expected in the gap, where an UC material exists.
We fabricated the nanocomposites as follows. First, a shell of an UC material (Er and Yb doped Y₂O₃) about 10 nm in thickness was formed around a Au nanoparticle core (64 nm in diameter) by a homogeneous precipitation method. The composite nanoparticles were placed on a fused silica substrate, and then Ag about 20 nm in thickness was deposited for the formation of a Ag nanocap. The structure of the nanocomposites was characterized by HAADF STEM observations and EDS element mappings. The scattering and UC of single nanocomposites were studied by a dark-field microscopy coupled with a monochromator and visible (CCD) and near-infrared (InGaAs diode array) detectors. The measured scattering spectra and electromagnetic field simulations based on a boundary element method revealed that the nanostructure has two scattering peaks due to the bonding and anti-bonding modes. In this work, an UC enhancement of 5-fold was achieved in not well-optimized structures.

Recently, it has been established that polar semiconductor materials offer a foundation to build novel long-wavelength photonic devices. For instance, their ability to support phonon-polariton resonances that provide highly sub-diffractional electromagnetic fields with exemplary resonant quality factors suggest these systems would be ideal as nanoscale THz emitters. Polar semiconductors also offer wide spectral tunability, with the constituent atomic basis and crystal structure defining the active frequency region. The vast library of these materials allows for photonic applications from the mid-infrared to THz regions. In this work, we investigate the Raman active nature of phonon-polariton modes in selectively grown epitaxial GaN nanowire arrays. We observe strong Raman peaks within the Reststrahlen band of GaN that are hypothesized to originate from localized monopolar and transverse dipolar phonon-polariton modes. These modes occur around 700 cm^-1 (~ 14.3 μm), opening a unique spectral region for device applications; which is currently not accessible by other phonon-polariton enabled materials, e.g., SiC and h-BN. Tuning of the apparent resonances are a function of nanowire pitch and diameter for the expectant phonon-polariton modes. Additional measurements were performed using an FTIR microscope that further establish the physical properties of the resonances observed in the Raman microscopy measurements. Interestingly, since the tuning of the modes is geometry dependent, this scheme lends itself well to engineering of the phonon-polariton resonances using the high-precision selective epitaxy process used here.

Surface phonon polaritons (SPhPs) have seen a recent surge in interest due to their ability to bring the optical confinement of plasmonics in to the Mid-IR/THz regime, without any of the associated losses. By creating metamaterials and multicomponent superlattices made out of polar dielectric materials (i.e. SiC) SPhPs can be manipulated to achieve active tunability, second harmonic generation enhancement, and structure-induced hyperbolic permittivities. For nanoscale complex structures such as these localized SPhP measurements are key to understanding the interplay between the different components.

Electron energy loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM) has been a highly successful method of locally measuring optical properties of nanostructures. However, up until recently the Mid-IR/THz regime was inaccessible due to the energy spread of the field emission electron guns. Recent breakthroughs in STEM monochromation have reduced the background in the Mid-IR by orders of magnitude, allowing for low-efficiency signals to be tracked and measured through EELS. Here we examine nanostructured SiC metamaterials in a monochromated STEM to map the excitation of SPhPs directly at the nanoscale.

This work is supported by the Center for Nanophase Materials Sciences (CNMS), which is sponsored at ORNL by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy

We will focus on using light-matter interactions in an effort to alter the chemical behavior of a target molecular species. This is done through cavity coupling to a molecular vibration. Coupling vibrational transitions to resonant optical modes creates vibrational polaritons shifted from the uncoupled molecular resonances and provides a convenient way to modify the energetics of molecular vibrations and to explore controlling chemical reactivity[i] and energy relaxation.[ii] Here, we experimentally and numerically describe strong coupling between a Fabry-Pérot cavity and several molecular species (e.g., poly-methylmethacrylate, thiocyanate, hexamethyl diisocyanate)[iii] and investigate the transition from the strong to weak coupling regimes. Furthermore, we map the influence of molecule/cavity mode overlap by systematically altering the position of a molecular slab throughout the first and second order cavity resonances with results agreeing well with analytical and transfer matrix predictions.
In the time domain, we use pump-probe infrared spectroscopy to characterize the dynamics of vibration-cavity polaritons for the CO vibrational band of W(CO)₆.² At very early times, we observe quantum beating between the two polariton states, which may account for a lower degree of vibrational excitation observed. After the quantum beating, we interpret our observations as excited-state absorption from polariton modes and uncoupled reservoir modes. The polariton mode relaxes ten times more quickly than the uncoupled vibrational mode and it exhibits a cavity tuning-dependent lifetime which we believe is a result of modifying the relative fractions of cavity and molecular character comprising the polariton. We show that energy relaxation times depend on cavity-vibration coupling and thereby may be a viable way to control the frequency and lifetime of vibration-cavity polaritons and, therefore, may provide opportunities to influence chemical reactivity. This work points out the possibility of systematic and predictive modification of the excited-state kinetics of vibration-cavity polariton systems. Opening the field of polaritonic coupling to vibrational species promises to be a rich arena amenable to a wide variety of infrared-active bonds that can be studied in steady state and dynamically.


[i] A. Thomas, J. George, A. Shalabney, M. Dryzhakov, S. J. Varma, J. Moran, T. Chervy, X. Zhong, E. Devaux, C. Genet, J. A. Hutchison, T. W. Ebbesen, Angew. Chem. Int. Ed. Engl. (2016)

[ii] A.D. Dunkelberger, B.T. Spann, K.P. Fears, B.S. Simpkins, and J.C. Owrutsky, “Modified Relaxation Dynamics in Coupled Vibration-cavity Polaritons”, Nature Communications 7, 13504 (2016)

[iii] B. S. Simpkins, K. P. Fears, W. J. Dressick, B. T. Spann, A.D. Dunkelberger, and J. C. Owrutsky, , ACS Photonics, 2, 1460 (2015)

The recent approach to utilize metasurfaces made from resonant nanostructures has been revolutionizing our perception of nonlinear optical processes. Due to the relaxed phase-matching requirements, simultaneous generation of various nonlinear optical processes can be expected from them. In this work, we show that seven nonlinear processes, including fourth-harmonic generation, four-wave mixing (FWM), and six-wave mixing (SWM) processes can occur simultaneously in GaAs dielectric metasurfaces.
The dielectric metasurfaces used in this work consist of a square array of GaAs nanocylinder resonators that are spatially separated from a GaAs substrate by AlGaO. The nanocylinders have diameters of ~420 nm and support magnetic and electric dipole resonances at ~1520 nm and ~1250 nm, respectively. To explore frequency mixing processes, the GaAs metasurface sample was pumped by two near-infrared femtosecond beams. The optimization of the frequency-mixing signal was achieved by overlapping the pumps’ wavelengths with the two dipole resonances of the nanoresonators. When the two pump pulses spatially and temporally overlap, eleven spectral peaks are observed spanning from UV to near-infrared wavelengths. We divide the newly generated frequencies into two groups: those relying on only one of the two pump beams such as second-, third- and fourth-harmonic generation and two-photon absorption induced photoluminescence; and those relying on both pump beams such as sum-frequency generation, three types of FWM processes, and SWM process. We identify these mechanisms by measuring the power dependence, as well as by matching the photon energy. For example, the observed fifth-order nonlinear effect, SWM, was verified by tunning the wavelengths of the two pumps. To confirm the resonantly enhanced behavior, we also measured the generated nonlinear spectra on an unpatterned sample and observed at least two orders of magnitude smaller signal intensities for most the spectral peaks.
Our demonstration of frequency-mixing in dielectric metasurfaces combines strong material nonlinearities of GaAs, enhanced electromagnetic fields in resonators, and relaxed phase-matching conditions in nanostructures, to allow simultaneously generate of eleven new frequencies. Use of III-V metasurfaces paves a road for realizing ultra-compact optical frequency-mixer for various applications, such as telecommunication technologies.
This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering and performed, in part, at the Center for Integrated Nanotechnologies, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science. Sandia National Laboratories is a multi mission 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.

A conventional thermal emitter exhibits a broad emission spectrum with a peak wavelength depending upon the operation temperature. Recently, narrowband thermal emission was realized with periodic gratings or single microstructures of polar crystals such as SiC [1, 2]. These polar crystals support Surface Phonon-Polaritons (SPhPs) [3], which offer lower losses and higher resonance quality factors due to longer lifetime than the commonly used Surface Plasmon Polaritons (SPPs). Due to the strong confinement of SPhPs, subwavelength resonators can host different, spectrally narrow modes depending on geometry and period.
Here, we go one step further and investigate the coupling of adjacent phonon-polaritonic nanostructures, specifically deeply sub-diffractional bowtie-shaped silicon carbide nanoantennas. We experimentally demonstrate that the nanometer-scale-gaps can control the thermal emission frequency while retaining emission linewidths as narrow as 10 cm^-1[4].
To prove that the thermal emission originates from of nanoantenna structures and for an unambiguous assignment of the strongly localized SPhP resonant modes, we employ infrared far-field reflectance spectroscopy and compare it with full-wave electromagnetic simulations and near-field optical nanoimaging. The latter is based on scattering-type scanning near-field optical microscopy (s-SNOM) and enables us to directly visualize the rather complex modes of our 3-dimensional nanostructures. We also observe slight differences between individual bowties in our array, again indicating the strong influence of the nanoscale gaps on some of the narrow emission lines.
We believe that the observed narrow emission linewidths and exceptionally small modal volumes will provide new opportunities for the user-design of near- and far-field radiation patterns for advancements in infrared spectroscopy, sensing, signaling, communications, coherent thermal emission, and infrared photo-detection.

[1] J. J. Greffet et al, Nature 416, 61 (2002).
[2] J. A. Schuller et al, Nature Photon. 3, 658 (2009).
[3] J. D. Caldwell, et al., Nanophotonics 4, 44 (2015).
[4] T. Wang et al., ACS Photonics 4, 1753 (2017).

The decay dynamics of excited carriers in graphene have attracted wide scientific attention, owing to the much lower relaxation rate of excited ‘hot’ carriers than that seen in many three-dimensional materials, owing to the Dirac electronic dispersion of graphene. Plasmons in graphene can significantly reduce the lifetime of photoexcited charge carriers, and this plasmon effect on excited state decay increases with increasing carrier density, as indicated by theoretical calculations and ARPES experiments ^[1,2,3,4]. In a recent theoretical study, it was also shown that plasmons can be amplified in an inverted graphene and be spontaneously emitted on ultrafast time scale^[5].
We report experimental demonstration of gate-tunable mid-infrared plasmon-coupled radiation from graphene under ultrafast optical pumping, and our experimental results suggest that graphene plasmons excited by ‘hot’ carriers affect the radiative emission rate. We have measured emission for several sample geometries: planar graphene, and non-resonant and resonant gold nanodisks(NDs) on graphene. In infrared emission spectroscopy measurements taken under optical excitation with a Ti:sapphire laser operating at 850nm with 100fs pulse duration, we observe broad radiative emission with features across an energy range of 150meV to 430meV (2.8-8um). The randomly distributed gold NDs on graphene facilitate out-coupling of plasmon excitations to free space light by accommodating the momentum mismatch. In addition, when NDs are resonant with the incoming laser frequency, ultrafast plasmon emission is enhanced in portion to the field intensity concentrated at the location of the graphene sheet. With a surface coverage of 1% for the resonant NDs and less than 3% for the non-resonant NDs, the collected plasmon-coupled light emission intensity is at least a factor of 8 and 4 larger, respectively, than that collected from planar graphene. In all cases, the emission intensity increases for higher graphene carrier density, which is controlled via the changes in applied gate voltage. For a given, moderate laser power, the emission intensity is approximately 1%, 6%, and 70% larger when the graphene Fermi level is at 0.4eV compared to the charge neutral point for planar graphene, and non-resonant and resonant NDs on graphene, respectively. This work has important implications for achieving ultrafast optical control of mid-infrared light emission. Our results are indicative of a spectral modification of plasmon-mediated emission arising from ‘hot’ plasmons in graphene created by ultrafast optical excitation.
1. A. Bostwick et al., Nature Phys., 2007, 3(1), pp.36-40.
2. A. Bostwick et al., Science, 2010, 328(5981), pp.999-1002.
3. F. Rana et al., Phys. Rev. B, 2011, 84(4), pp.045437
4. J. M. Hamm et al., Phys. Rev. B, 2016, 93(4), pp. 041408
5. A. F. Page et al., Phys. Rev. B, 2015, 91(7), pp. 075404

Two-dimensional (2D) transition metal dichalcogenides (TMD) functionalized with plasmonic metal nanoantennas (NA) exhibit rich energy conversion capabilities as a material platform for optoelectronics and sustainable energy. This work examined (i) plasmon-enhanced nonlinear second harmonic generation (SHG) and (ii) injection of plasmonic hot electrons into 2D TMD via coordinated multi-photon microscopy, hyper Rayleigh Scattering (HRS), electron energy-loss spectroscopy (EELS), and discrete dipole computation. Augmented local fields by NA surface plasmon resonance enhanced SHG from monolayer MoS₂ at efficiencies of up to 0.025 %/W. Hyper Rayleigh scattering (HRS) assessed the second-order nonlinear susceptibility for WS₂ monolayers to be 250±12 pm/V. Quantum yield of plasmonic hot electrons transported to 2D TMD was measured locally for two NA-TMD hybrids by EELS, revealing dependence on bonding characteristics at the metal-TMD junction. Highest measured efficiency was 11±5% for NA physicochemically bonded to WS₂ edge disulfides via redox-directed self-assembly.

The emergence of two-dimensional (2D) monolayer transition metal dichalcogenides (ML-TMDC) as direct bandgap semiconductors has rapidly accelerated the advancement of room temperature, 2D optoelectronic devices. Optical excitations on the TMDCs manifest from a hierarchy of electrically tunable, Coulombic free-carrier and excitonic many-body phenomena. Investigating the fundamental interactions underpinning these phenomena presents challenges, however, due to a complex balance of competing optoelectronic effects and interdependent properties. We show how optical detection of bound- and free-carrier photoexcitations is used to directly quantify carrier-induced changes of the quasiparticle band gap and exciton binding energies [1]. Pushing to the nanoscale, we demonstrate that a model hybrid architecture, a nano-optical antenna and a ML-WSe₂ nanobubble, activates the optical activity of BX states at room temperature and under ambient conditions. These results show that engineered bound-exciton functionality as, in this case, localized nanoscale light sources, can be enabled by an architectural motif that combines localized strain and a nano-optical antenna, laying out a possible path for realizing room-temperature single-photon sources in high-quality 2D semiconductors.
[1] Kaiyuan Yao, et al., Phys. Rev. Lett. 119, 087401 (2017)

We have developed and experimentally demonstrated an actively tunable infrared filter that enables modification of the amplitude of reflected long-wave-infrared light. Tunability results from plasmons excited in an unpatterned sheet of chemical-vapor-deposition grown graphene. Through conventional gating using a periodic metal grating, the Fermi level of the graphene can be modified to change the plasmonic response, resulting in changes to reflectance. The filter enables simultaneous modification of two distinct spectral regions between 600 and 1600 cm^-1, whose positions are controlled by the device geometry and graphene plasmon dispersion. Within these bands, the reflected amplitude varies by over 15% and reflectance minima can be shifted over 90 cm^-1. As demonstrated though electromagnetic simulations, tuning arises from graphene plasmons excited within the graphene via coupling through the metallic grating. The tuning range is determined by a combination of graphene properties, device structure, and the surrounding dielectrics. Using these parameters, the device architecture demonstrated here is applicable to a broad range of infrared wavelengths.

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.

Topological insulators (TIs) are layered materials that ideally exhibit an insulating bulk with conducting surfaces. The electrons in these surface states are two-dimensional, linearly dispersing, and exhibit spin-momentum locking. Light can couple to these surface electrons, exciting two-dimensional Dirac plasmons, similar to those excited in graphene. Unlike graphene, however, TI plasmons are expected to be spin-polarized. This causes them to be protected from non-magnetic backscattering, potentially leading to extremely long propagation distances. There are a variety of potential applications for TI plasmons, including THz sensors, optically-driven spintronics, and THz metamaterials. In this talk, I will discuss our recent results measuring the dispersion relationship of coupled Dirac plasmons in topological insulator films. Because the TI films are extremely thin, plasmons excited on the top and bottom surfaces of the film will couple, resulting in Dirac plasmon acoustic and optical modes. By patterning the films into stripes, localized plasmons can be excited. Since we are exciting the optical mode in our films, by changing both the stripe width and film thickness, the coupled plasmon dispersion relationship can be mapped out. Our results show that we are indeed exciting coupled 2D Dirac plasmons and not massive 2D plasmons from either the bulk or from a band-bending two-dimensional electron gas.

Finally, I will discuss recent results on TI films grown with molecular beam epitaxy on high-quality buffer layers. One of the challenges when studying TI films is that the Fermi energy is usually pinned near the bottom of the conduction band, leading to a large density of trivial carriers in the bulk states. These trivial carriers can open up additional scattering pathways for the topological carriers, reducing plasmon lifetimes. We have found that growth of TI films on high-quality buffer layers brings the Fermi energy closer to the Dirac point, reducing the density of trivial carriers. I will close by showing data for Dirac plasmons excited in these high-quality TI layers. Overall, TIs represent an exciting new material class for studies of Dirac plasmon physics as well as plasmonic applications in the THz.

Polaritons in materials with free charge carriers (surface plasmon polaritons) or polar optic phonons (surface phonon polaritons) offer a route to beating the diffraction limit for compact mid- and far-infrared optoelectronics. The latter of these two quasi-particles has received intense scrutiny in recent years due to inherently low losses from phonon scattering, albeit at the limitation of relatively low spectral tunability. One particularly interesting class of polaritons are hyperbolic modes – which occur in highly anisotropic crystals, such as the 2D materials or artificial layered metamaterials. This phenomenon arises from the layered structure, with significantly different vibrational energies in- and out-of-plane directions. The techniques that have been used to study these modes involve nano-structuring, scattering type scanning nearfield optical microscopy (s-SNOM) and photothermal induced resonance (PTIR) techniques. Whilst highly successful these techniques have drawbacks, such as the complex effect of scattering from a nanoscale particle or tip which makes data analysis complex. As a result, accurately measuring the dielectric response of 2D crystals Is extremely difficult using these techniques, but the small size of samples precludes the use of infrared spectroscopic ellipsometry. Here we discuss how prism coupling techniques can be used to measure hyperbolic polaritons and extract dielectric function data from two dimensional crystals.
Specifically, we discuss how the choice of appropriate substrate is critical for successful measurements on two dimensional crystals such as hexagonal boron nitride. Prism coupling requires attenuated total reflection (ATR) between the boundary and substrate, but many low-index materials are not suitable for 2D material preparation. By using a combination of simulations and experiments, we show that by thickening the Si/SiO₂ layers conventionally used for mechanical exfoliation of two dimensional materials, we can measure ATR spectra even on high index Si. The measured response is highly sensitive to the thickness of the flakes, as predicted by the dispersion of hyperbolic modes. Specifically, we are able to observe multiple dips in reflection, corresponding to different modes of identical wavevector. Finally, we comment on the repeatability of this technique under both varied measurement and different exfoliation conditions, including the importance of substrate adhesion. One major advantage of this approach is that by using an appropriate prism it is possible to access frequencies that are difficult to measure using both s-SNOM and PTIR techniques (for example the far-IR, where laser sources are limited). Furthermore, by comparison with numerical simulations it is possible to extract dielectric data, like spectroscopic ellipsometry. This can then inform the design of nanostructures to create efficient far-IR thermal emitters and optical components.

The advent of commodity single crystal silicon wafers enabled the growth of the electronics industry through the extreme lithographic fidelity and improved device yeild that these substrates afford. The field of plasmonics would benefit from access to low-cost, single crystal metal surfaces in an analogous fashion. Enhanced nanostructure fidelity and film morphology eliminates many fabrication challenges and improves experimental throughput through better agreement between simulation results and device performance. In this work, the fabrication of smooth single crystal metal films of gold, silver, and platinum, on silicon substrates through a modified PVD process was demonstrated. Nanostructuring of these metal surfaces was explored through the use of several novel, low-cost, techniques to generate smooth epitaxial nano-features with excellent plasmonic properties. We will present and discuss our fabrication techniques as well as examples of plasmonic devices with application to photovoltaics, electrochemistry, and non-linear optics.

The ability to deposit and pattern noble metals to form moncrystalline thin films and high-definition subwavelength nanostructures represents a significant challenge in the development of next generation plasmonic, nanophotonic, and metamaterial technologies. Typical physical vapour deposition-based methods result in the deposition of polycrystalline features characterized by structural inhomogeneity that reduces feature quality and increases losses due to grain boundaries. Alternatively, strategies based on the solution phase synthesis of crystalline nanparticles of controlled size, shape, and composition face difficulties in patterning and registering these structures in well-defined locations onto substrates and are of limited utility for electronic applications due to the presence of capping agents. Here, we describe a new method for the deposition of epitaxial, single crystal, noble metal thin films and nanostructures from solution. We demonstrate that epitaxial electrochemical deposition (EED) enables fabrication of large-area, atomically flat, single crystal metal films of desired thickness that are ideal for nanopatterning through ion beam milling. The resulting structures are smooth, homogeneous, manufacturable in high yield, and display thermal and mechanical stability at least one order of magnitude greater than their polycrystalline counterparts. While this chemistry allows for the subtractive manufacture of nanostructure through ion beam milling, EED also enables additive crystalline nanostructure using standard lithographic techniques such as electron beam lithography to enable novel, large area, metamaterial arrays and high aspect ratio crystalline nanostructure. Epitaxial electrochemical deposition represents a new, easily accessible, and cost effective, bottom-up approach to high fidelity nanostructure that will help to enable new plasmonic research and application.

To date, there is a keen interest in the photonics community to control the optical properties of metal, at both thin film and nanoscale level. While the size, geometry and spatial distribution are often used to modify the localized surface plasmon resonance (LSPR) of nanostructures; the chemical composition is a powerful knob to tune their response in the Vis-NIR range of the spectrum. Here, we demonstrate how the alloying of metals (Ag, Au, Cu, Al) enables the tenability of the dielectric function for applications ranging from energy harvesting to superabsorbers. First, we build a library of the optical response of alloyed thin films, and show that in some cases a mixture can offer a material with dielectric function not achieved by its pure metal counterparts [1]. We numerically and experimentally demonstrate how Al-Cu, potentially CMOS-compatible, can be implemented in a superabsorber thin film stack, providing dual-band near-unity absorption (> 99%) in the Vis-NIR [2]. At the nanoscale, we map the near- to far-field optical response of alloyed nanoparticles, which present local field enhancements at wavelength not accessed by pure metals [3]. We anticipate the investigation of materials beyond noble metals to enable the design and realization of optical systems with superior performance and on-demand response.


[1] ACS Photonics 3, 507 (2016) – COVER. [2] Adv. Optical Materials, in press (2017). [3] Adv. Optical Materials 5, 1600568 (2017) – COVER.

Semiconductor metasurfaces represent a unique platform for linear and nonlinear optics. Because semiconductors typically have a high refractive index, the resulting meta-molecules can support a wide selection of sharp Mie resonances that can be used for shaping their optical response to incident light. I will discuss two applications of Si metasurfaces: (a) the development of highly efficient diffraction gratings based on super-wavelength bianisotropic metasurfaces, and (b) demonstration of rapidly time-varying metasurfaces that can capture and blue-shift mid-infrared photons. In the case (a), I will present an experimental demonstration of a metasurface that directs over 90% of the transmitted energy into a single diffractive order, and describe a semi-analytic theory predicting that only four Mie resonances are needed to achieve such performance. In the case (b), I will describe how ultra-intense femtosecond laser pulses rapidly create free carriers in a spectrally-selective metasurface, thereby blue-shifting its resonant frequency during the lifetime of a trapped mid-IR photon. As the result, the photon is “accelerated”, and can contribute to spectrally blue-shifted and broadened harmonics generation. To our knowledge, this is the first experimental demonstration of a metasurface that is simultaneously time-dependent and nonlinear. The fundamental importance of photon acceleration is that it overcomes what was considered a key limitation of the resonant efficiency enhancement of nonlinear processes using spectrally-selective metasurfaces: that it must come at the expense of the bandwidth. Our results demonstrate that both the efficiency and the bandwidth can be simultaneously enhanced.

As the direction of technological advancement pushes towards miniaturization of devices with high operating speed and efficiency, germanium (Ge) as compared to silicon (Si), is more superior in terms of performance. Ge has higher carrier mobilities and large intrinsic carrier concentrations which makes it highly suitable for photonic devices. The fundamental energy band gap of Ge can be narrowed down through strain engineering so that it becomes a direct band gap semiconductor, which has enabled the realization of electrically pumped Ge-based lasers. Several low-loss waveguides and modulators have also been demonstrated on Ge integrated Si-based photonic systems. In addition, Ge has the added advantage of CMOS compatibility in microelectronics. The integration of semiconductors as active media into metamaterials offers vast opportunities for a wide range of innovative technologies enabled by strong light-matter interactions within the semiconductors.

Despite its wide applications in microelectronic and optoelectronic devices, there does not exist any demonstration of ultrafast flexible Ge thin-film based metaphotonic devices. In the previous demonstrations of photoswitching on GaAs and other semiconductors (Si on Sapphire), the recombination time of the carriers is >1 ns, which indicates a slow switching time. In order to achieve an ultrafast photoswitching time, superlattices were implemented but lattice matching is crucial to achieving short carrier lifetimes. The fabrication of superlattices requires the use of MBE which is a complicated and precise process as many growth factors must be considered.

In our work, we have designed a Ge-based metaphotonic device by evaporating 310 nm thickness of Ge thin film onto the terahertz metamaterial arrays. The terahertz metamaterial arrays were fabricated onto a flexible Kapton film substrate via photolithography. Optical-pump Terahertz-probe spectroscopy was used to study the relaxation dynamics of Ge and to optically pump and modulate the strength of the resonances.

From our results, we achieved a transmission modulation of ~ 90 % with a switching speed at ultrafast picosecond timescale of ~ 17 ps. A sub-picosecond decay time constant of ~670 fs is obtained from theoretical fitting of our relaxation dynamics which we attribute to the defect states present in the evaporated germanium thin film.

This is the first demonstration of Ge-based ultrafast flexible photoswitch. Our fabrication is simple, cost-effective, and involves thermal evaporation of a thin-film single element semiconductor material (Germanium) that shows such an ultrafast photoswitching of Fano resonant metamaterial. The simplicity of our concept suggests that it is universally applicable to the current state-of-the-art photonic devices. Our device could function as an ultrafast modulator or active filters. It could also pave the path for the realization of flexible electronic and photonic devices based on Ge.

Hyperbolic metamaterials (HMMs) are artificial materials with an engineered subwavelength structure. The permittivities of HMMs in the plane versus out of the plane are of opposite sign, resulting in an open hyperbolic isofrequency surface. HMMs possess novel properties, like negative refraction and an enhanced Purcell effect, which are difficult to find in natural materials. One simple way to create HMMs is by growing a multilayer structure with alternating metal and dielectric layers. Previously, researchers have used traditional metals (silver, gold) and dielectrics (silica) to create HMMs for the ultra-violet and visible spectral ranges. We are interested in working in the infrared, so we choose to use semiconductor building blocks. It has been demonstrated that doped semiconductors grown by molecular beam epitaxy act as infrared plasmonic metals with optical properties tunable across the infrared and low optical losses [1]. We have demonstrated the designer infrared HMMs comprising Si:InAs (metal) and intrinsic InAs (dielectric) [2]. Using Fourier transform infrared spectroscopy, we observed discontinuity of the Brewster angle and negative refraction for our samples, both hallmarks of HMM behavior. Another interesting property of HMMs is that they can support the propagation of light with large wavevectors (volume plasmon polariton, or VPP, modes) which are not allowed in normal materials. These collective modes in the HMM arise from the coupling of surface plasmon polaritons at each metal/dielectric interface. We investigated the VPP modes in Si:InAs/InAs HMM and Si:InGaAs/InAlAs HMM by depositing gold gratings with different periods on top of their surface. We found that the detailed distribution of electrons at metal/dielectric interface strongly affects the signal of the collective modes. Conversely, the strength and full width-half maximum of these features indicates the quality of the interface [3]. Studying the details of the VPP mode shape and dispersion gives important information about the interface quality and subwavelength structure with in an HMM. This information cannot be obtained any other way and is necessary for the design of devices using these collective modes. The study of the novel optical properties of HMM and their collective modes will lay the foundation for the applications such as enhanced infrared detectors, superlens, hyperlens and other optical devices.

[1] S. Law, L. Yu, and D. Wasserman, J. Vac. Sci. Technol. B Microelectron. Nanom. Struct. 31, 03C121 (2013).
[2] D. Wei, C. Harris, C. C. Bomberger, J. Zhang, J. Zide, and S. Law, Opt. Express 24, 8735 (2016).
[3] D. Wei, C. Harris, and S. Law, Opt. Mater. Express 7, 2672 (2017).

Changing the colour of light is one of the most fundamental processes of nonlinear optics and can in principle be achieved by mixing light beams in nonlinear crystals. However, such processes are considered unrealistic in small nano-crystals due to the negligible conversion efficiency, related to their short length. Nevertheless, for more than three decades [1] researchers have been actively looking for ways of increasing the efficiency of nonlinear frequency conversion in ultra-thin surfaces. Plasmonic (metallic) nanostructures were considered as a possible solution, due to their strong field enhancement, however, up to now, there has been limited progress, mainly due to their dissipative losses and low mode volume. A major breakthrough in increasing the efficiency was enabled by the use of high-refractive-index resonant dielectric nanoantennas [2] to demonstrate third harmonic generation (THG) with an efficiency several orders of magnitude higher than what is possible in plasmonics [3]. Even further frequency conversion enhancement can be achieved in the case of second harmonic generation (SHG) in AlGaAs nanoantennas due to their large quadratic nonlinear susceptibility, reaching conversion efficiencies of ~10⁻⁴ [4-6]. These record-higher efficiencies open a wide range of possible applications, including nonlinear imaging and holography. In this work, we review the recent progress of nonlinear frequency mixing in all-dielectric nanoantennas and metasurfaces and explain the underlying physics behind the enhancement of the nonlinear processes in high-refractive-index nano-crystals, including AlGaAs. Importantly, we demonstrate the ability to design the radiation pattern of SHG emission from the nanocrystals, to create complex beam radiation shapes with high conversion efficiency, including nonlinear images.

[1] M. Kauranen and A. V. Zayats, Nat. Photon. 6, 737 (2012).
[2] A. I. Kuznetsov, A. E. Miroshnichenko, M. L. Brongersma, Y. S. Kivshar, and B. Luk’yanchuk, Science 354, 846 (2016).
[3] M. R. Shcherbakov, D. N. Neshev, B. Hopkins, A. S. Shorokhov, I. Staude, E. V. Melik-Gaykazyan, M. Decker, A. A. Ezhov, A. E. Miroshnichenko, I. Brener, A. A. Fedyanin, and Y. S. Kivshar, Nano Lett. 14, 6488 (2014).
[4] V. F. Gili, L. Carletti, A. Locatelli, D. Rocco, M. Finazzi, L. Ghirardini, I. Favero, C. Gomez, A. Lemaître, M. Celebrano, C. De Angelis, and G. Leo, Opt. Express 24, 15965 (2016).
[5] S. Liu, M. B. Sinclair, S. Saravi, G. A. Keeler, Y. Yang, J. Reno, G. M. Peake, F. Setzpfandt, I. Staude, T. Pertsch, and I. Brener, Nano Lett. 16, 5426 (2016).
[6] R. Camacho-Morales, M. Rahmani, S. Kruk, L. Wang, L. Xu, D. A. Smirnova, A. S. Solntsev, A. Miroshnichenko, H. H. Tan, F. Karouta, S. Naureen, K. Vora, L. Carletti, C. De Angelis, C. Jagadish, Y. S. Kivshar, and D. N. Neshev, Nano Lett. 16, 7191 (2016).

The early screening and diagnosis of various infectious diseases is critical to public health and homeland security. The application of plasmonic nanoantennas on early-stage disease detection has attracted considerable attentions due to its ultra-high sensitivity, design flexibility and low cost. Recently the incorporation of sensitive plasmonic nanoantennas into fluidic channels (plasmofluidics) have proven advantageous in fast, label-free, and multiplexed detection of biological entities. However, challenges exist to achieve high-sensitivity plasmofluidic sensing of biological nanoparticles such as viral particles and exosomes. The refractometric resonance shifts are generally small, typically <5 nm. This is due to a small refractive index contrast of the biological nanoparticles with the buffer background and also lack of high-sensitivity antenna designs to probe the large particles (40-150 nm).
Here we present a novel and generic plasmofluidic sensor design to achieve high-sensitivity detection of various antigens (e.g., influenza, Zika viruses and exosomes). Our sensor consists of strongly coupled antennas, i.e. nanobar dimers (bright mode) and U shaped bar (coupling to dimer, dark mode), that creates a sharp Fano resonance with a very narrow FWHM (e.g. 70 nm at resonance wavelength 990 nm). Our gold sensor array is surface functionalized with streptavidin, which further binds to biotinylated antibodies that selectively capture the antigen to be detected. The combination of Fano resonance design and the antibodies allow high-sensitivity and high-specificity detection, and the incorporation of different antibodies in multiple fluidic channels allow multiplexed detection of different antigens on one single fluidic chip.
Our sensor design has a number of advantages. First, it enables high-sensitivity dual mode sensing by simultaneously detecting the resonance shift and the spectral intensity modulation. Second, the plasmonic hotspots can be designed at different dimensions, as large as >100 nm, with a uniform electric field intensity enhancement to probe the biological nanoparticles. Third, nanobar dimers and U shaped bar synergistically behave as effective traps in fluidic channels to capture the antigen virus into hot spots.
Our full-wave simulation shows that the captured virus particles (assumed ~80 nm in diameter) can lead to 3-6 nm shift and 4.2-5.5% reflection intensity modulation at the same time – 5 to 10 times better than demonstrated plasmonic sensors. The resonance shift and intensity modulation can be further amplified by 2-3 times by binding the captured antigen with a second antibody conjugated with gold nanoparticles. Our simulation indicates that our proposed structure can be a promising high-sensitivity optical detector of a variety of antigens, with an estimated limit of detection about two orders of magnitude better than ELISA.
The sensor fabrication and antigen detection are ongoing and will be presented at the conference.

Directionally solidified eutectics exhibit important mechanical, and for emerging systems, interesting magnetic and optical properties. The goal of this work is to understand and apply directional solidification of anisotropic eutectics to form structures with structural motifs, which behave as optical metamaterials. The eutectic’s unique two-phase repeating structures can be controlled by adjusting thermal gradients and cooling rates to finely tune the structure and shape, producing distinctive optical properties with potential for new applications in photonics. This work investigates a eutectic with a eutectic phase that forms facets due to the anisotropy of one of the phases, which restricts the solidification in certain directions. If the solidification direction and facet directions are aligned, the eutectic will uniformly solidify along the thermal gradient. Due to the faceted nature of anisotropic phase, once the eutectic is uniformly directionally solidified the interfaces will be anatomically smooth, providing great benefits to the optical responses.

The microstructures necessary to provide the desired properties may require more complexity than eutectics naturally provide. We and others have observed that a template can be used to guide the phase separation into unique structural motifs. However, it remains experimentally challenging to study the effects of confinement in traditional metallic and ceramic systems. One possible solution is to study the microstructure development in low temperature, optically transparent organic eutectics. In our experiments, organic eutectics were solidified through an assortment of 2D and 3D templates ranging in size and shape. The effects of the templates can be seen in optical images and videos. These effects also were modeled using phase field methods, starting with 2D models of lamellar eutectic solidification through confining geometries. These results were compared to the experimental data.

Assembling optical active nanostructures into polymer matrices holds promise for the design of functional materials with controlled light-matter interactions, which finds applications such as sensors and flexible photovoltaics. We present various methodologies for the control of the plasmonic properties of gold nanorods and the photoluminescence of quantum dot (QD). First, electrochromic hybrid systems with electrochemical modulation of plasmon resonance were designed by direct polymerizing electroactive polymers around gold nanorods. The plasmon tuning behavior, resulted from polymer’s refractive index change, varied when different electroactive polymers were used. For example, a dual-responsive system with the plasmon mode reversibly modulated through electric potential and pH was realized when polyaniline served as the outer shell and a maximum shift of the longitudinal plasmon mode of 149 nm was obtained. Another electrochemical modulated plasmon tuning system with narrow visible-near infrared absorption bands was also demonstrated using a transmissive polymer poly[(3,4-propylenedioxy)pyrrole]. Overall, these core-shell nanostructures with electrochemical plasmonic tunability allow for the fine control of the optical and electrochemical properties of plasmon response. Second, we developed a new method for enhancing photoluminescence from QDs/polymer nanocomposite through the control of the degree of film dewetting. The dewetted films were found to have increased amounts of scattering, which resulted in up to a 5-fold enhancement of the film emission. A unique photopatterning strategy was also presented based on the aforementioned method.

Thermal dissipation of plasmon energy from gold nanoparticles (AuNPs) dispersed in transparent polymers is important to biotherapeutics, optoelectronics, sensing, and chemical separations. This work evaluated heat dissipated from power extinguished by 16 nm AuNPs with negligible Rayleigh scattering cross-sections dispersed into sub-wavelength, 70-nm polyvinylpyrrolidone (PVP) films at interparticle spacings much less than the resonant wavelength. Compared to super-wavelength films with interparticle spacing near the resonant wavelength, measured optically extinction and temperature increase on a per NP basis decreased as AuNP concentration increased. Change in temperature per NP decreased 22% and optical extinction per NP decreased 35% as AuNP concentration increased from 1.01 to 5.06 x 10¹⁵ NP/cm³. The trend and magnitude of measured values were consistent with those from a priori description of optical extinction per NP derived from Maxwell Garnett effective medium theory (EMT) and from coupled diode approximation (CDA). Thermal dissipation measured from the films at particle separations of 130 to 76 nm. Comparison of EMT, CDA, and finite element analysis (FEA) measured results showed the contributions to plasmon-resonant optical extinction and heat dissipation. These results support design and adaptive control of thermal dissipation from plasmonic films.

The chemical sensing has important applications in gas sensing^[1], noninvasive disease diagnosis^[2], security monitoring^[3] and others^[4]. In past several decades, significant research efforts have been devoted to the development of miniaturized chemical sensors based on chemiresistors, surface plasmonic polaritons and microelectromechanical resonators for on-site chemical detection and point-of-care disease diagnosis. However, these sensors often suffer from low sensitivity and poor chemical selectivity. The state-of-arts technology for chemical sensing is based on the spectral analysis since most chemicals, in particular organic chemicals, have signatures of spectral absorption in mid-infrared region. But the analysis can be only performed in scientific laboratories using large instruments.

Here, we aim to realize a miniaturized spectral analyzer by developing a multispectral filter. In recent years, multispectral filters based on plasmonics^[5], nanowire waveguides^[6] and metamaterials^[7] have been extensively investigated. However, these filters are operating in visible spectrum and no filters in mid-infrared range are reported as far as we know. In this work, we report that an integrated spectral analyzer can be constructed by using Cr microhole arrays as multispectral filters. The microhole arrays were fabricated with CMOS compatible processes. The transmission peak of the microhole arrays can be continuously tuned from 3 μm to 8 μm by linearly increasing the periodicity of the microholes in each array. Fourier transform infrared (FTIR) microscopy was applied to spatially map out the transmission of the microhole arrays. The results show that each microhole array can selectively allow for transmission at a specific wavelength. We further constructed an IR spectral analyzer model based on the microhole multispectral filters to retrieve IR spectral information of two test samples. Our experimental results show that the spectra from the integrated spectral analyzer follows nearly the same pattern of the FTIR spectra of the test samples, proving the potential of the miniaturized spectral analyzer for chemical analysis.

References:
[1] P. Werle, F. Slemr, K. Maurer, R. Kormann, R. Mücke, B. Jänker, Optics and Lasers in Engineering, 37 (2002) 101-114.
[2] A.B. Seddon, Optical Fibers and Sensors for Medical Diagnostics and Treatment Applications XIII, International Society for Optics and Photonics, 2013, pp. 85760V.
[3] U. Willer, M. Saraji, A. Khorsandi, P. Geiser, W. Schade, Optics and Lasers in Engineering, 44 (2006) 699-710.
[4] B. Van Eerdenbrugh, L.S. Taylor, International journal of pharmaceutics, 417 (2011) 3-16.
[5] S. Yokogawa, S.P. Burgos, H.A. Atwater, Nano Letters, 12 (2012) 4349-4354.
[6] H. Park, K.B. Crozier, Scientific reports, 3 (2013).
[7] I.J. McCrindle, J. Grant, T.D. Drysdale, D.R. Cumming, Optics express, 21 (2013) 19142-19152.

Conventionally, pigment or dye based color filters have been used for commercialized display panels and image sensors. However, due to their chemical / ultraviolet / thermal instabilities, plasmonic color filters, which have control of the color by adjustment of the shape, size, and lattice configurations of their sub-wavelength unit structures without alteration of the intrinsic properties of constituent materials, have been suggested as an alternative. Compared to the conventional counterpart, plasmonic color filters have orders of thinner magnitude profiles and excellent stability, making them a good potential candidate for application on displays and image sensors. However, the plasmonic color filters proposed thus far have low transmittance and narrow color gamut, mainly due to the Ohmic loss of metal.
Here, we presented the bright and saturated transmissive plasmonic RGB color filters using the orthogonality control of multiple resonance modes. We started by calculating ideal spectra for color filters showing the highest brightness, while satisfying a given standard chromaticity such as that of sRGB, DCI-P3, or BT-2020. While it would be practically impossible to realize these ideal spectra either by plasmonic color filters or conventional dye filters, they served as the ultimate reference against which actual color filters can be measured. We considered new plasmonic color filter designs that utilized the Ohmic loss of metal as an advantage by introducing two resonance modes to block unwanted wavelengths. Based on coupled mode theory (CMT), it is shown that two resonance modes with proper orthogonality can closely approximate the ideal spectral response. It was derived that the red and the blue filters needed orthogonal modes and the green filter needed non-orthogonal modes, according to CMT. Metal-Insulator-Metal (MIM) nanodisk arrays for excitation of orthogonal modes and metal di-atomic arrays of square-cross structures for excitation of non-orthogonal modes were adopted. Specifically, it was identified that the artificial magnetic resonance in MIM nanodisk arrays played a significant role in producing excellent transmittance and color purity. The structural parameters were optimized to obtain bright and saturated optical properties using particle swarm optimization with the Finite-Difference Time-Domain method.
After being optimized, RGB color filters were successfully designed. These color filters had transmittances of over 71% for the green and over 80% for the other two, which were higher than that of any plasmonic transmissive RGB color filters proposed thus far. The 62.2% sRGB color space area coverage by these filters also exceeded that of their proposed predecessors. Furthermore, the optical properties of the color filters were insensitive to the incident angle of the light. In summary, we proposed and demonstrated the brightest and the most saturated plasmonic transmissive RGB color filters with good angular performance.

Nano-plasmonic sensing applications have been investigated over the years due to the strong EM field confinement and the nature of localized surface plasmon resonances. The sensing mechanism relies on the ability to shift the resonance in the presence of a perturbation. We propose and demonstrate a passive plasmonic nanostructures sensor operated at an exceptional point (EP), that fundamentally shift more than conventional resonances. Exceptional points are degeneracies in open wave systems where at least two energy levels and their corresponding eigenstates coalesce. They manifest themselves by the simultaneous degeneracy of both resonant frequencies and its linewidths. These points are highly sensitive to the external perturbations as even a sufficiently small variation will lift the degeneracy and cause splitting of both resonant frequencies and linewidths. The plasmonic EP sensors pave the way to highly sensitive plasmonic devices with applications ranging from biological to electro-optical sensing.

Optically excited plasmonic nanostructure display remarkable electron dynamics in the form of coherent electron displacement motion, as well as efficient generation of non-thermal ‘hot electrons’ at room temperature with kinetic energy substantially greater than kT. Here, we provide a theoretical framework of our studies of photo-enhanced charge transport across plasmonic tunneling junctions composed of nanoscale metallic gaps, as a strategy for taking advantage of such electron motion for optoelectronic energy conversion.
In a symmetric plasmonic tunneling gap the redistribution of electrons due to photo-induced thermalization and hot electron generation is not sufficient to provide significant electrical currents, either through injection over the interface potential barrier, or via electron tunneling effect. However, asymmetric resonant structure can provide uneven optical absorption and photo-excitation across metallic tunneling junction that induce significant temperature gradients and local variations in the hot electron population. Such asymmetry can be used to promote unidirectional tunneling transport currents with significant enhancement compared with conventional photoelectron and thermionic emission, and thus comprises an intriguing mechanism for providing electrical work. We will introduce the theoretical frame work of tunneling phenomena associated with photo-excited hot electrons in plasmonic structures, including principles of hot electron distribution under photon excitation, strategies for amplifying hot electron generation (e.g. manipulating hot spots in nano-antennas) and provide a mechanistic quantum model of electron transport and power conversion based on unidirectional electron tunneling across nanoscale plasmonic junctions.

The refractive index of natural transparent materials in visible wavelength range is limited to 2~3. However, for advanced optical applications like nanoimaging and integrated photonics, refractive index need to be controlled in a wider range. We fabricated metamaterials with period and symmetry-tunable self-assembled nanopatterns which can control refractive index for broad wavelength range, including visible light. Nanoscale objects smaller than skin depth allow independent control of permeability and permittivity. The effective refractive index increased up to 5.10 by precisely adjusting the interobject distance through the shrinkage of the block copolymer nanopatterns, and maintained above 3.0 over more than 1,000nm wavelength bandwidth. spatially graded and anisotropic refractive indices. Also, spatially graded and anisotropic refractive indices were obtained by modification of transitional and rotational symmetry design.

Broadband and incoherent thermal radiation from hot surfaces may be turned into narrowband and quasi-coherent light using selective thermal emitters. Selective thermal emitters operating at high temperatures enable many applications including gas sensing and thermophotovoltaic energy conversion. The narrow emission bandwidth of selective emitters is accomplished by some resonant phenomenon, and often built using refractory metals because of their thermal stability. However, refractory metals have high optical losses at high temperatures and shorter wavelengths limiting the spectral selectivity of the emitter. Thus, high quality factors (> 100) are not possible for metallic resonators when operating at temperatures > 700 K and center wavelengths < 4000 nm. On the other hand, nearly lossless semiconductors provide high spectral selectivity, but with very small thermal emission. Thus, there remains an open question – what material makes the best platform for high temperature (> 700 K) selective thermal emitters? Answering this question requires us to evaluate temperature dependent optical properties of metals and semiconductors, and identify the right material platform for any given application.

In this work, we model the temperature dependent optical properties of materials using the physics of microscopic processes. Modeling metals is relatively straightforward as their optical properties are dominated by free carriers. However, semiconductors pose a tougher challenge as their sub-bandgap optical properties are dominated by the interaction between free carriers, phonons, impurities and bandstructure changes. We develop a semi-empirical model to predict temperature dependent optical constants of semiconductors for any doping concentration and at any temperature. We show that our model predicts the optical constants with reasonable accuracy by comparing our predictions against experiments and more complicated theoretical methods such as variational method and second order perturbation theory. Using the temperature dependent optical properties of metals and semiconductors, we attempt to answer the question of what makes a good material platform for high temperature selective thermal emitters. Our analysis shows that in the temperature range 300-1200 K, semiconductors with tunable optical properties are promising and for high temperatures, refractory metals are promising. Our investigation guides the choice of materials and their processing condition such as doping concentration, and provides insight into new material discovery for high temperature selective thermal emitters.

The field of nanophotonics utilizes surface plasmon polaritons (SPPs), and epsilon near zero (ENZ) modes to realize sub-diffractional confinement of light. SPPs are hindered by high electron-phonon scattering rates and thus have short lifetimes and high optical loss. ENZ modes can be excited where the real part of the dielectric function approaches zero. When loss is present, ENZ modes can offer extreme light confinement to ultra thin dielectric films through the excitation of leaky Berreman modes. However, the losses in plasmonic metals are too high to support this mode. Recently much attention has been placed on the use of transparent conductive oxides (TCOs) for nanophotonics in the mid-infrared. Unlike traditional metals that are not tunable in the IR and THz due to fixed, high carrier densities, TCOs allow for tuning of both carrier density and electronic mobility. One highly promising TCO is highly doped CdO, which has been shown to achieve electron mobilities extending upwards to 500 cm²/V-s with carrier densities ranging from 10¹⁹ to 10²⁰ cm^-3. Unique to this material is a range of carrier densities where increasing values result in increasing mobilities. In addition to supporting SPPs, thin films of CdO also support leaky Berreman modes near the ENZ condition. These modes offer a unique approach towards tailoring emissivity due to their extremely strong absorption/emission and narrow resonant linewidths that can be achieved without the need for nanostructuring, making this a potentially scalable technology. By modifying the carrier density in the thin layers of doped-CdO, broad spectral tunability of the ENZ condition can be achieved while limiting additional losses. Therefore, by placing two thin CdO layers, with one supporting an SPP and the other a leaky Berreman mode that spectrally overlap, a SPP-ENZ hybrid mode will result from the strong coupling induced.
Here we utilize this SPP-ENZ hybridization within three-dimensionally confined cavities to monitor the impact of nanostructuring upon this strong coupling phenomena and how this can be used as two independent knobs for tuning the thermal emissivity of the structures. This can be achieved through the design of layered CdO films with varying carrier densities, for instance a bottom SPP (higher carrier concentration) and upper ENZ (lower carrier concentration) layers. Previously we have shown that this strong coupling actually imparts SPP-like properties (evanescent, propagating surface fields) upon the so-called ‘ENZ layer’, while the reverse is also true, effectively combining the key properties of each distinct mode in the hybrid system. By fabricating such hybrid materials into nanostructure arrays, direct control of the emission polarization, spatial coherence and divergence is also engineered through careful design of nanostructure geometry and periodicity. This approach can potentially lead to the possibility of realizing a narrow-band, spectrally tunable thermal emitter.

Control of thermal radiation at high temperatures is vital for waste heat recovery and for high-efficiency thermophotovoltaic (TPV) conversion. Previously, structural resonances utilizing gratings, thin film resonances, metasurfaces and photonic crystals were used to spectrally control thermal emission, often requiring lithographic structuring of the surface and causing significant angle dependence. In contrast, here, we demonstrate a refractory W-HfO₂ metamaterial, which controls thermal emission through an engineered dielectric response function. The epsilon-near-zero frequency of a metamaterial and the connected optical topological transition (OTT) are adjusted to selectively enhance and suppress the thermal emission in the near-infrared spectrum, crucial for improved TPV efficiency. The near-omnidirectional and spectrally selective emitter is obtained as the emission changes due to material properties and not due to resonances or interference effects, marking a paradigm shift in thermal engineering approaches. We experimentally demonstrate the OTT in a thermally stable metamaterial at high temperatures of one thousand centigrade.

We will discuss opportunities for light trapping and manipulation offered by creation of topological defects (e.g., vortices) in optical fields. Topological nature of an optical vortex is revealed through its quantized topological charge – the winding number of an optical wavefront around the vortex core, where the electromagnetic field is zero and the phase is undetermined. The topological charge of a vortex remains constant under continuous system deformations and in the presence of noise, although it can be altered abruptly, e.g. in the process of mutual annihilation with another vortex of the opposite charge. We exploited these intriguing properties by developing plasmonic nanostructures with ‘pinned’ topological defects in their optical near-fields. These nano-structures exhibit superior light trapping and tuning/switching properties over those of conventional single-, dimer-, or array-based plasmonic elements (1–4).
We will also report on engineering topologically-protected optical states on material interfaces by using boundary-bulk correspondence principle adopted from the solid state physics of topological materials. Instead of engineering nano-patterned surfaces, we engineer the meta-material ‘bulk’ to guarantee formation of topologically-protected states on planar interfaces, which are amenable to large-scale fabrication, are less prone to post-fabrication contamination and deformations, and provide strong uniform field enhancement across the surface accessible by target molecules and optical probes. Light coupling to topologically-protected interfacial states can result in its complete absorption, which in turn can be used to realize sensitive singular-phase optical sensors (5). We have developed planar singular-phase sensors and tested them as temperature detectors with a remote optical readout, confirming high sensitivity of the new sensing approach. We will also discuss applications of plasmonic nanostructures with interfacial states for engineering thermal emitters with spectral- and angular selectivity.
1. W. Ahn et al, Electromagnetic field enhancement and spectrum shaping through plasmonically integrated optical vortices. Nano Lett. 12, 219–27 (2012).
2. S. V. Boriskina, Plasmonics with a twist: Taming optical tornadoes on the nanoscale, Plasmonics: Theory and Applications, T. Shahbazyan, M.Stockman (Eds) Ch. 12 (2013).
3. S.V. Boriskina and N.I. Zheludev, Singular and Chiral Nanoplasmonics, Pan Stanford, 2014.
4. S. V. Boriskina et al., Losses in plasmonics: from mitigating energy dissipation to embracing loss-enabled functionalities. Adv. Opt. Photonics. 9, 775 (2017).
5. Y. Tsurimaki et al, Topological darkness of interfacial optical Tamm states for highly-sensitive singular-phase optical detection, submitted (2017).

The US industries reject nearly 20%-50% of the consumed energy into the environment as waste heat. Harvesting this huge amount of heat can substantially improve the energy usage efficiency. For waste heat in the medium temperature range (~500-900 K), traditional solid-state waste heat recovery techniques like thermoelectric generators and thermophotovoltaics (TPVs) are still suffering from relatively low efficiency or power density. In this work, we analyze a near-field TPV system based on a plasmonic emitter (ITO) and a narrow-bandgap thin-film cell (InAs) that are brought to deep sub-wavelength distances for high-efficiency and high-power-density waste heat recovery. The calculations are based on a detailed balance analysis and the formalism of fluctuational electrodynamics. The thermal radiation spectrum from ITO is reshaped and enhanced toward the bandgap of the InAs cell by the photon tunneling effect between the ITO and InAs cell. We find that the near-field photon tunneling probability can be greatly enhanced by thermally excited surface plasmon resonances and waveguide modes in the thin InAs cell. We show that despite the inclusion of realistic nonradiative recombination rates and sub-bandgap heat transfer, such a near-field TPV system can convert heat to electricity with up to nearly 40% efficiency and 11 W/cm² power density at a 900 K emitter temperature. While the dominant enhancement effect comes from the surface plasmon resonances, the waveguide modes in the thin cell play a significant role as well, especially at relatively large gap distances. We further demonstrate that the power density can be further enhanced by placing a thin metal film on the cell. This is a somewhat counterintuitive result since one might think that the thin metal film might serve to block the near-field radiation. Thus, we show that for waste heat recovery, near-field TPV systems can have performances that significantly exceed typical thermoelectric systems.

Polarization is an important characteristic of electromagnetic waves that has a significant impact on number of applications such as molecular analysis, sensing, and quantum communications [1]. Conventionally, the polarization state of an electromagnetic wave is tailored via propagating the wave through a birefringent crystal or polymer [2]. As a result, the optical elements that perform polarization conversion are typically bulky. Alternatively, the polarization conversion can be achieved by using low-profile nanophotonic components based on metasurfaces. Metasurfaces are ultrathin nanophotonic structures composed of artificially designed arrays of optical scatterers, which introduce abrupt changes to the amplitude and phase of the scattered light within a subwavelength spatial region [3].
Here, we demonstrate that the polarization state of the reflected light can be actively controlled by using indium tin oxide (ITO)-based tunable metasurfaces. The proposed metasurfaces consist of an aluminum back reflector, a 20-nm-thick gate dielectric layer followed by a 5-nm thick ITO layer on which we fabricate an aluminum nano-antenna array. The period of the suggested metasurface is 400 nm while the operation wavelength is 1580 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 the change of the effective index of the ITO layer [4]. The epsilon-near-zero (ENZ) mode, which is accessed under applied external DC bias, alters the interaction between the induced plasmonic modes (which corresponding to the orthogonal polarization components), leading to the modulation of polarization state of the reflected light. By suitably biasing the metasurface structure, the linearly-polarized incident light can be converted to a 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.

References
1. T. B. Freedman, X. Cao, R. K. Dukor, and L. A. Nafie, "Absolute configuration determination of chiral molecules in the solution state using vibrational circular dichroism," Chirality 15, 743-758 (2003).
2. G. Ghosh, "Dispersion-equation coefficients for the refractive index and birefringence of calcite and quartz crystals," Opt. Commun. 163, 95-102 (1999).
3. P. Genevet, F. Capasso, F. Aieta, M. Khorasaninejad, and R. Devlin, "Recent advances in planar optics: from plasmonic to dielectric metasurfaces," Optica 4, 139-152 (2017).
4. Y.-W. Huang, H. W. H. Lee, R. Sokhoyan, R. A. Pala, K. Thyagarajan, S. Han, D. P. Tsai, and H. A. Atwater, "Gate-tunable conducting oxide metasurfaces," Nano Lett. 16, 5319-5325 (2016).

Optically resonant nanostructures allow for the deterministic design of their optical properties through material properties, dimensions, and dielectric surrounding. Due to their large scattering and absorption cross section, such nanostructures are promising for a wide range of applications, including sensors, photodetectors, and color filters. However, each of these properties is typically static, fixed at the time of synthesis and/or fabrication. The ability to dynamic control the optical response of nanostructured elements enables new functionalities and structures, and is a grand challenge.

One way to create actively-tunable structures is to modulate the optical index of the dielectric media. Electrochemical ion insertion is a well-established method to dynamically tune the relative permittivity, but the spectral response of electrochromics is generally defined by their intrinsic material properties. Here, we combine electrochemical ion insertion in electrochromic tungsten trioxide (WO₃) with engineered gap plasmon resonances in subwavelength aluminum nanoparticle arrays to achieve tunable reflection spectra. The aluminum particles have a diameter of 60-100 nm, 30 nm height, and 250 nm periodicity. The gap plasmon resonance of these particles allows for precise control of the spectral response during fabrication, and shows high sensitivity to the refractive index of the material in the gap. Changing the refractive index of the 30 nm WO₃ in the gap enables active tuning. To this end, a solid dry polymer electrolyte and a Li_XFePO₄ counter-electrode were used to enable reversible electrochemical lithium insertion into WO₃, modifying the refractive index of the WO₃ dielectric media and thereby the resonant response of the gap plasmon. Using this solid-state electrochemical device, we demonstrate continuous tuning of the spectral response by up to 100 nm in visible wavelengths within 10 seconds, giving rise to drastic color changes. Such spectral tuning is obtained with <1 V switching voltages, non-volatile behavior, high reversibility, and the ability to address individual pixels, and provides a powerful platform for displays, sensors, and other actively-tunable plasmonic devices.

Dynamic metasurfaces could enable a number of exciting optical devices including tunable lenses, free-space modulators, and holographic displays. In this talk, I will discuss our recent efforts towards this goal using a number of different approaches. I will first discuss how we can use dynamic metasurfaces to manage thermal radiation. Dynamic control is based on optically generated carriers in zinc oxide thin films which are embedded in a metasurface architecture. Importantly, a long carrier lifetime in the zinc oxide allows us to achieve modulation of the metasurface’s emissivity using low power light emitting diodes as the illumination source opening the door to large area modulation. I will also discuss the use of phase change materials, specifically vanadium dioxide (VO₂), to dynamically control the properties of metasurfaces. In this approach, we take advantage of the field concentration within the metasurface to utilize nanoscale patches of VO₂. The small patch size and thermal mass of the VO₂, compared to past demonstrations using thin films, allows us to greatly decrease both the switching time and power consumption of the device. Lastly, I will discuss the use of intercalation to modulate the color of metasurfaces. This approach is extremely low power and allows spectral tuning across the visible regime.

Narrow lattice resonances result from dipole coupling in periodic nanoparticle arrays, and their spectral positions are defined by the array period in a particular direction. In the electric or magnetic dipole approximation, the collective resonances involve corresponding moments of the nanoparticles oriented perpendicular to the lattice wave propagation. Recently, it has been shown that adjusting array periods independently in each direction, one can completely overlap the electric and magnetic dipole lattice resonance and achieve the resonant lattice Kerker effect, i.e. back-scattering suppression or near-zero reflection [1]. In this work, we study collective resonances in nanoparticle arrays with electric dipole moment oriented along the lattice wave propagation, and we analyze roles of three multipoles, that is an electric dipole, electric quadrupole (EQ), and magnetic dipole (MD), in the lattice resonant feature. We have performed both semi-analytical calculations of coupled equations and rigorous numerical simulations and considered different particle sizes and shapes to vary contributions of the electric and magnetic multipoles. We have found that non-zero multipole moments of a single particle are significantly enhanced in the periodic lattice at the wavelength of collective lattice resonance. We demonstrate that EQ and MD resonances are coupled in the periodic lattice and affect each other resonant contributions. We show that at the lattice-resonance wavelength, the enhanced EQ and MD moments have contributions to reflection comparable to the electric dipole contribution resulting in a significant drop of reflection that is corresponded to the generalized Kerker effect realization.

[1] V.E. Babicheva and A.B. Evlyukhin, "Resonant Lattice Kerker Effect in Metasurfaces with Electric and
Magnetic Optical Responses," Laser & Photonics Reviews, doi: 10.1002/lpor.201700132 (2017).

Tuning optical properties in the long wave infrared (LWIR) has been overwhelmingly dominated by semiconductors where plasmon interactions are modulated by electrostatically induced changes in the carrier concentration. Charge is not the only mechanism by which LWIR properties can be changed, however. Rather, any mechanism by which the dielectric permittivity of the plasmonic medium is affected can be leveraged. Here, lead zirconate titante (PZT) ferroelectric bilayers are instead employed and shown to possess a combination of LWIR tuning advantages—speed, multistate operation, and scalable feature size—unavailable in approaches demonstrated heretofore. Mechanistically, field-induced domain reconfiguration alters the phonon energies defining PZT's AC permittivity thereby altering the gap plasmon formed between the ferroelectric and surrounding metals resulting in reflectance changes of ~10% at 800 cm^-1 and a multistate unpowered response controlled by the remanent polarization. The utility of ferroelectrics for tunable plasmonics is thus demonstrated.

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.

Mid-infrared light has attracted attention in the fields of material science and sensing applications. However, the large mismatch in the scale between the light with a wavelength of several micrometers and nanometer-sized materials makes the light-matter interaction inefficient. To enhance this interaction, the light field should be concentrated into a small volume while overcoming the diffraction limit. Such a field concentration is possible by using surface polaritons. The sub-wavelength confinement of the surface phonon polariton was observed in cavities on SiC [1] and phase-changed spots in a phase change material on quartz [2].
In this study, we demonstrated the high lateral confinement of the mid-infrared surface phonon polariton (SPhP) with metal circular cavities fabricated on 30-nm-thick germanium-antimonide-telluride (GST) deposited on SiC. Scattering-type scanning near-field optical microscopy showed that the SPhP is launched at a metal edge and propagating in plane with a reduced polariton wavelength. In a 1-mm-diameter metal hole, a single bright spot was observed at the center of the hole. The width of the spot was ~250 nm that is 1/44 of the wavelength of incident light (11.1 mm). This result indicates the high concentration of the SPhP field. The large momentum of the SPhP in GST/SiC and the metal circular cavity achieve the highly vertical and lateral confinements. In addition, the heating of the sample induced the changes in both the material phase and dielectric constants of GST, which enable a large detuning from a cavity resonance to switch the SPhP confinement off, in contrast to other polariton devices tuned by external fields or fabrication.

1. T. Wang, P. Li, B. Hauer, D. N. Chigrin, T. Taubner. Optical Properties of Single Infrared Resonant Circular Microcavities for Suface Phonon Polaritons. Nano Lett. 13, 5051 (2013).

2. P. Li, X. Yang, T. W. W. Mass, J. Hanss, M. Lewin, A-K. U. Michel, M. Wuttig, T. Taubner. Reversible optical switching of highly confined phonon-polaritons with an ultrathin phase-change material. Nature Mater. 15, 870 (2016).

A silicon based metasurface is designed and implemented to transmit only one circular polarizatiom (optical spin) and reflect the other. Achieving differential response between opposite optical spins typically require chiral structures which don’t superimpose on their mirror image. Chiral structures are very common in biological materials and organic compounds, therefore manipulating optical spin has many applications in biosensing, stereochemistry and DNA structural analysis.
Spin control of light typically requires multi-layer bianaisotropic structures or biaxial crystals. We propose a new methodology using Silicon metasurface that enables all forms of optical operations implemented using 3D bianisotropic or biaxial structures to be implemented on a single planar layer. We utilize an array of highly anisotropic rectangular silicon nano-antennas with high aspect ratio. These nano-antennas induce two kinds of optical phase-shifts which are independently controlled. One of them is the phase-shift induced by electric and magnetic Mie resonances excited inside the Silicon nano-antennas which is controlled by antennas’ dimensions, while the other one is the Pancharatnam-Berry phase-shift controlled by the geometric orientations of the nano-antennas. A planar array of Si nano-antennas with different dimensions (i.e, different Mie Scattering phase-shifts), and different orientations (i.e, different geometric phase-shifts) is judiciously designed to achieve the spin based performance. The Mie phase and geometric phase coherently interact to interfere constructively for one optical spin and destructively for the other, leading to the differential optical response between opposite spins. The effect is experimentally demonstrated in the visible and NIR spectral range.

Surface phonon polaritons (SPhPs) are quasiparticles consisting of a strongly coupled phonon-photon pair resulting from the interaction between light and optic phonons. They occur naturally in polar crystalline solids. SPhPS are an appealing alternative to surface plasmon polaritons due to the longer scattering lifetime of phonons, which subsequently results in substantially lower losses. Hexagonal boron nitride (hBN) is an especially appealing polar crystal due to high crystal and optical anisotropy, making it a naturally hyperbolic material.
A hyperbolic material is one in which the real part of the dielectric function is opposite in sign along orthogonal crystal axes. In the case of hBN, these are the in-plane and out-of-plane axes. This results in two spectral bands where either the in- or out-of-plane permittivity is negative, while the other is positive. Within these spectral bands, hyperbolic phonon polaritons (HPhPs) may be supported, which differ from SPhPs in that they can propagate through the deeply sub-diffractional volume of the material rather than being confined to the surface. Because they are confined in all three dimensions, they must be described using three quantum numbers, resulting in several distinct sets of polaritonic modes appearing in the HPhP dispersion. However, due to optical selection rules, only a subset of these modes has been observed through far-field reflectance and scattering-type scanning near-field optical microscopy (s-SNOM).
Photothermal induced resonance (PTIR) is an emerging technique in which incoming light is scattered off of a metallized AFM tip to excite resonant modes in a sample. Unlike in s-SNOM that measures scattered light, the AFM tip is typically operated in contact mode and is used to directly measure local thermal expansion of the material or nanostructure resulting from resonant optical absorption. Here, PTIR is leveraged for the first time to observe “dark” HPhP modes, which do not radiate to free space, within hBN nanocones. Through careful resonance lineshape analysis and comparison with far-field and nano-FTIR spectra, we have identified ~20 resonant dispersive modes not observed using the more conventional techniques. These resonances were confirmed as HPhP resonances by their dispersion and by comparison with analytical predictions of these modes. Our results show the first clear observation of these predicted, but previously unreported dark HPhP modes. Control of these modes could present a new avenue to novel nanophotonic devices and deeper understanding of nanophotonic materials and devices.

We present the design and realization of optical metasurfaces for applications in enhanced photovoltaics, controlled spontaneous emission and optical computing, and present a unique tool to characterize these geometries at deep-subwavelength spatial resolution.

The building blocks in our work are dense periodic and aperiodic arrays of crystalline silicon nanocylinders made using soft-imprint or electron beam lithography. In photovoltaic metasurfaces resonant light scattering from Si Mie scatterers causes strong (30%–40%) specular light scattering on resonance creating photovoltaic mini-modules with well-defined colours, while maintaining 90% of the photocurrent. We also demonstrate photovoltaic metasurfaces composed of pixelated arrays composed of Si Mie scatterers that create a white appearance. The colored/white solar minimodules can find application in building-integrated photovoltaics.

Next, we use a novel arbitrary-wavefront transformation concept based on metagratings composed of specially tailored light scatterers, developed by Alu et al., to create a Lambertian angular scattering response. Lambertian surfaces can find many applications in optical components, photovoltaics and more. We demonstrate how these tailored metasurfaces can be used to control spontaneous emission at distances further than the optical near-field. Further advanced optical metasurfaces can serve to perform mathematical operations on optical field distributions as we will demonstrate.

Finally, we present angle- and polarization-resolved cathodoluminescence spectroscopy as a tool to characterize optical metasurfaces using a 30 keV electron beam as the excitation source. The beam is raster-scanned over the surface and the collected optical radiation provides invaluable information on optical resonances and near-field modal coupling in the metasurfaces at nanometer spatial resolution.

Silicon particles exhibit electric and magnetic Mie resonances enhancing the light matter interaction [1-4]. The resonant interaction of light with high refractive index nanostructures offers novel opportunities to design dielectric optical antennas or dielectric colored metasurfaces.

Mie resonators can be coupled to enhance the near electric or magnetic fields [5,6]. Dimers of silicon particles with nanogaps of 20nm were designed and fabricated to enhance the fluorescence emission of molecules. By increasing the excitation rate and by strongly decreasing the excitation volume, silicon nanogap antennas permit to detect individual molecules at micromolar concentration [7]. The low losses of silicon make silicon optical antennas good candidate to inhibit the spontaneous emission by decreasing the electric or magnetic local density of states [8]. Magnetic Mie resonances open novel routes to promote magnetic spontaneous emission of trivalent lanthanide ions [9]. The current challenge is to accurately control the location of the emitters compared with the silicon antenna and to locate the emitters at magnetic LDOS maxima. The shape of the antenna has to also to be optimized in order to enhance specifically the magnetic density of states outside in the vicinity of the silicon antenna [10].

In the far field, the resonant light scattering features by silicon nanostructures can be used to encode structural colors [11]. By controlling the shape and the size of the silicon resonators, one can tailor the resonant scattering spectra and therefore the structural colors. A large palette of colors can be created with silicon particles and colored metasurfaces can be designed [12-13].

Mie resonances featured by silicon particles raise theoretical questions such as the derivation of accurate polarizability expressions [14], analogy between plasmonics and Mie resonances [15]. Another question of interest deals with the quasi-normal mode theory which includes the calculation of the effective volume of electric and magnetic modes [16].

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[2] A. García-Etxarri et al., Opt. Express 19, 4815 (2011)
[3] A. I. Kuznetsov et al., Science 354, aag2472 (2016)
[4] I. Staude, J. Schilling, Nature Photon. 11, 274 (2017)
[5] G. Boudarham et al., Appl. Phys. Lett. 104, 021117 (2014)
[6] R. M. Bakker et al, Nano Lett. 15, 2137−2142 (2015)
[7] R. Regmi et al., Nano Lett. 16, 5143−5151 (2016)
[8] D. Bouchet et al., Phys. Rev. Applied 6, 064016 (2016)
[9] B. Rolly et al., Phys. Rev. B 85, 245432 (2012)
[10] M. A. van de Haar et al., Optics Express 24, 2047-2064 (2016).
[11] A. Kristensen et al., Nature Rev. Mat. 2, 16088 (2016)
[12] J. Proust et al., ACS Nano 10, 7761–7767 (2016)
[13] V. Flauraud et al., ACS Photon. 4, 1913 (2017)
[14] R. Colom et al., Phys. Rev. A 95, 063833 (2017)
[15] A. Devilez et al., Phys. Rev. B 92, 241412(R) (2015)
[16] X. Zambrana-Puyalto et al., Phys. Rev. B 91, 195422 (2015)

Low-loss optical antennas based on high index dielectric nanostructures have been attracting much attention as an alternative to plasmonic antennas that often suffer from losses of noble metals. They have several characteristic features such as the enhancement of a magnetic field of incident electromagnetic wave and the directional scattering. These properties provide an opportunity to realize new types of metasurfaces and metamaterials. Among several high-index dielectrics, silicon (Si) has some advantages. It is abundant and the most commonly used material in semiconductor industry. Furthermore, most importantly, the imaginary part of the permittivity of crystalline Si is very small in the optical regime due to the indirect bandgap nature.
In this work, we focus on Si nanospheres 50 to 250 nm in diameter as a dielectric nanoantenna. In the size range, Si nanospheres exhibit the electric and magnetic Mie resonances in the visible range. So far, several techniques have been developed to fabricate submicron size Si particles. However, all the previously developed methods have problems in the control of the size, shape and crystallinity and in the scalability for mass production. In this work, we develop crystalline 50-250 nm Si nanospheres dispersible in alcohol by applying inorganic surface modification technique developed in our group.[1] Thanks to the perfect dispersion of Si nanospheres, they can be placed on an arbitrary substrate, and the interaction between a single sphere and a substrate can be studied in detail. We show that forward and backward scattering spectra obtained from single Si nanospheres are very well reproduced by analytical Mie calculations, which guarantees high quality of our Si nanospheres. The important application of the developed Si nanosphere is an antenna for fluorescence enhancement of nearby materials. To demonstrate that, we placed a Si nanosphere on a thin dye layer (>1 nm in thickness) coated on a glass substrate. We observed up to 200 times enhancement of the dye fluorescence. Numerical simulations revealed that the observed fluorescence enhancement is mainly due to the enhancement of the incident electric field by the sphere. We also fabricated a hybrid nanoantenna structure composed of a Si nanospheres and a gold film separated by a very thin dielectric spacer. This structure can confine electric fields in the gap very effectively at the resonance wavelengths of the gap modes, and thus can further enhance the optical responses of a material in the gap. We demonstrate fluorescence enhancement by the hybrid nanoantenna by placing a monolayer of luminescent quantum dots in the gap. We also observed strong modification of the spectral shape due to the large Purcell enhancement.
[1] Fujii et al., Nanotechnology, 27, 262001 (2016). [2] Sugimoto, et al, Adv. Opt. Mater. 5, 1700332 (2017).

We illustrate the applications of magnetic phenomena at optical frequencies sustained by semiconductor and metal nanostructures for improving nanophotonic technologies, ranging from increased efficiency and performance in lightning and displays to the modification of the intrinsic optoelectronic properties of materials.
In this context, high-refractive index optical antennas have emerged as promising tools for the control of light at the nanoscale, benefitting from mature fabrication technologies and potential integration with on-chip optoelectronic systems. In this presentation, we exploit two of the advantages offered by silicon nanostructures. First, thanks to the narrow Mie resonances of silicon nanobeams, we demonstrate strong coupling to a molecular J-aggregate, with a Rabi frequency of 150 meV. Both hybrid polariton branches are visible in photocurrent measurements, therefore tailoring the optoelectronic response of both materials. Second, thanks to the coexistence of magnetic and electric dipole response in silicon nanowires, we demonstrate directional emission from an atomically thin MoS₂ monolayer. Compared to the so-called Kerker condition for plane wave scattering based on the interference of electric and magnetic dipoles, we show that there are two possible mechanisms to direct the emission of a source dipole with a nanowire.
Additionally, we demonstrate that a nanostructured metallic mirror with high impedance can be used as an electrode with desirable optical properties. Metallic contacts normally show radiative losses as surface plasmon polaritons, which limit the efficiency of light-emitting devices. We demonstrate that our approach reduces these losses by studying the emission enhancement and photoluminescence lifetime for a dye emitter layer deposited on the electrodes. Our design behaves like a magnetic mirror with a maximum of electric field at the mirror surface, yielding enhanced light emission.

Reflected light exhibits complete phase reversal in traditional metallic mirrors, causing a standing wave with reduced electric field intensity directly above the reflective surface. This diminished field intensity leads to poor absorption near the interface. Metamaterial mirrors are increasingly being explored as a solution to this problem because they allow the phase of the reflected light to be tuned and can be used to eliminate phase reversal upon reflection. Here, we explore the incorporation of quantum dot (QD) films on dielectric metasurfaces, as the tunable QD band-gap makes them well-suited to take advantage of the wavelength-dependent electric field enhancements that metasurfaces exhibit. We are particularly interested in QDs with core/shell heterostructures. Thick QD shells help passivate the core surface and preserve their optical properties, but also lead to a lower optical density when in the solid-state, something that metasurfaces can help mitigate.

To avoid parasitic Joule losses exhibited by metal structures, we explored metasurfaces based on high-contrast dielectric materials. Using FDTD simulations and optical models measured via ellipsometry, we designed TiO₂ structures for phase tuning. Our set-up consists of an aluminum back reflector, a 20nm Al₂O₃ spacer layer, and a metasurface patterned using TiO₂ nanodisks. It was found that when using CdSe/CdS quantum dots, a nanodisk 80nm high and 175nm in diameter gave the desired 180° phase shift for 405nm incident light. For 445nm light, only a 65nm diameter was required for the desired phase shift. In both cases, simulations showed much higher electric field intensity and absorption in the quantum dot layer, compared to the case with no TiO₂ structures or with non-optimized TiO₂ diameters.

Experimentally, we patterned TiO₂ structures via both e-beam lithography and colloidal methods. For e-beam patterning, a top-down approach is used. PMMA is patterned on top of TiO₂ and a thin layer of chromium is deposited to serve as the TiO₂ mask. After lift-off, a dry-etching process is used to create TiO₂ pillars. Colloidally, TiO₂ nanospheres are synthesized through a sol-gel process. By varying the water content in our reaction mixtures, we have tuned the targeted diameter from 160 - 200nm with a size distribution of 11%. For the absorbing layer, CdSe/CdS QDs were synthesized using a non-hot injection method. Neat, thin-films of these QDs were spin-cast on top of our metasurface using a solid-state ligand exchange process. Once the structures are created, a customized optical setup was used to map the film’s optical characteristics over hundreds of square microns with nanometer control of the sample position, allowing us to track how different optical parameters change in the presence of the metasurface mirror and as a function of the local environment. The optical response of the colloidally-synthesized metasurface will be compared to the response of the pattern made via e-beam lithography.

Nanoplasmonics has enabled a new class of highly sensitive, on-chip biomarker recognition devices, especially relevant to point-of-care (POC) medical applications. However, the sensing properties have largely been studied and exploited at room temperature. In order to translate the significant benefits of on-chip nanoplasmonic sensing technology to more extreme environments, such as emerging exo-life detection applications, the basic transduction principle must be well-characterized in relevant environments. In this work, we study the optical response and sensing properties of nanoplasmonic structures over a wide range of temperatures, from ambient to cryogenic, relevant to exo-life detection. We explore both Au nanoantennas fabricated using electron beam lithography (EBL), and rough Au ‘nano-island’ surfaces fabricated via wafer-scale sputter deposition. The latter provides a facile means to fabricate surfaces with tailored optical resonances (by adjusting the sputtering rate and time), suitable for basic materials investigations as a function of temperature, while the former allows more control of the nanosensor geometry, to further elucidate material- and temperature-dependent effects. In order to measure the temperature-dependent sensing properties, we have designed and built a custom epi-fluorescence brightfield/darkfield microspectroscopy setup coupled to a cryogenic stage, capable of recording the spectra from an area less than 50 square microns and over a temperature range from ambient to 80 K. Through both experiment and simulation, we show how the plasmonic coupling and sensing properties (resonance quality factor, refractive index sensitivity, and sensor figure of merit) change with temperature and geometry, and discuss implications for cryogenic nanoplasmonic sensor application. Preliminary experimental results show that for the Au nano-island films, exposure to cryogenic temperature results in both a red-shift and reduced plasmon resonance quality factor, consistent with theoretical predictions for geometries supporting propagating surface plasmons. Isolation of individual nanoresonators is expected to reverse this trend, and improve sensing performance, as localized surface plasmon resonances exhibit opposite behavior at cryogenic temperature. This is, to our knowledge, the first time nanoplasmonic sensing properties have been studied at temperatures applicable to in-situ exo-life detection scenarios.

The major challenges for semiconductor fundamental research and technological development are confined to a variety of silicon-based nanostructured systems. The present talk aimed to show results and technology development in Si/Ge quantum dots (QDs) heterostructures, grown by molecular beam epitaxy. Hole transitions between the ground state confined in Ge dots to valence band continuum are fit to middle infrared (IR) range 3 - 5 μm. The disadvantage of SiGe-based QDIPs is the low absorption coefficient and hence small photoresponse in the mid-wavelength IR region. We have developed a few approaches allowing to improve performances of SiGe-based QDIPs. 1) A remarkable property of QDs originated from their discrete energy spectrum is a suppression of carrier relaxation rates due to the phonon bottleneck effect. In Ge/Si(001) QDs heterostructures we observed the general tendency: with decreasing the size of the dots, the dark current and hole capture probability are reduced, while the photoconductive gain and photoresponse are enhanced. It is attributed to a quenched electron-phonon scattering due to phonon bottleneck [Appl. Phys. Lett.,107, 213502 (2015)]. 2) The studies of the effect of quantum dot charging on the mid-infrared Ge/Si QDIPs operation have shown that doping to contain from about one to nine holes per dot induce an over 10 times gain enhancement and similar suppression of the hole capture probability with increased carrier population. The data are explained by quenching the capture process due to formation of the repulsive Coulomb potential of the extra holes inside the quantum dots [Mater. Res. Express 3, 105032 (2016)]. 3) The plasmonic sensing of Ge/Si QDIPs with wavelength optical response and polarization selectivity was found to show the most pronounced phenomenon. Ge/Si QDs heterostructures were monolithically integrated with periodic two-dimensional arrays of subwavelength holes perforated in gold films to convert the incident electromagnetic IR radiation into the surface plasmon polariton waves. The resonant responsivity of the plasmonic detector at a wavelength of 5.4 μm shows an enhancement of up to thirty times over a narrow spectral bandwidth (FWHM 0.3 um), demonstrating the potentiality of this approach for the realization of high-performance Ge/Si QDIPs [J. Appl. Phys., 122, 133101 (2017)]. 4) The growth of Ge/SiGe QDIPs on a virtual Si_(1-x)Ge_x (x=0.18) substrate show an over 100% photovoltaic response enhancement as compared to a conventional Ge/Si device due to smaller hole effective mass in the SiGe layers. A further enhancement in sensitivity is achieved by photodetector coupled with a plasmonic structure. The responsivity and detectivity values for the detector are 40 mA/W and 1,4 x 10¹¹ cm Hz^1/2/W at 90 K for zero bias operation, which are comparable or higher than n-type InAs/GaAs QDIPs [Optics Express, 25, No. 21, 16 Oct 2017]. The work was funded by Russian Scientific Foundation (grant 14-12-00931 Π).

Controlling and understanding the assembly of colloidal nanoparticles remains a challenging issue for optimizing magnetic-plasmonic devices for various applications including sensors, displays, bio-imaging, and therapy. A magnetic field is successfully utilized to induce the fabrication of multidimensional structures composed of magnetoplasmonic (MagPlas) particles, which exhibit interesting optical properties. Notably, a magnetic-field assisted coating technique for fabrication of two-dimensional (2D) amorphous photonic crystal (APC) film of the MagPlas particles on a filter membrane is proposed. The MagPlas 2D APC exhibits strong dual reflected colors caused by structural scattering and plasmon resonance scattering. The water absorption ability of the membrane and the high refractive index sensitivity of plasmon resonance scattering are utilized to fabricate a simple colorimetric humidity sensor. Additionally, a mechanical colorimetric sensor that instantly exhibits responses to both bending and stretching forces is fabricated by embedding the 2D APC film into PDMS substrate. Because of unique features including dual-color characteristic, flexibility, and high plasmonic sensitivity, these kinds of the platform could be highly promising as wearable devices for physical, chemical and biological sensing with naked eye detection.

Optical sensors based on discrete plasmonic nanostructures are very attractive for probing biomolecular interactions at the single-molecule level and have been applied as single nanoparticles or plasmonic rulers or reconfigurable multi-nanoparticle assemblies. However, their adaptation as a versatile sensing platform is limited by the research-grade instrumentation required for single-nanostructure imaging and/or spectroscopy and complex data fitting and analysis. Additionally, the dynamic range is often too narrow for the quantitative analysis of targets of interest in biodiagnostics, food safety or environmental monitoring. Herein we present plasmonic assembly comprising a core nanoparticle surrounded by multiple layers of satellite nanoparticles through aptamer linker. The layer-by-layer assembly of the satellite nanoparticles yields uniform discrete nanoparticle clusters on a substrate with enhanced optical properties. Binding of the model target (adenosine 5’-triphosphate, ATP) induces disassembly and leads to a dramatic decrease in the scattering intensity that can be analyzed readily from darkfield images. The sensing performance of assemblies, such as detection limit, dynamic range and sensitivity, can be tuned by controlling the size of the assembly. Surprisingly, the substrate-anchored nanoparticle assemblies are selective to only ATP, and not other adenine-containing compounds such as AMP and ADP at concentration less than 50 mM. It presents an approach to increase the specificity of the aptamer, which otherwise binds to all adenine-containing compounds if in free-form in solution. By assembling the clusters on a flexible support, cellular ATP can be directly detected by lysing adherent cells in close contact with the sensor – a process that does not require any sample preparation or purification. Enhancing the optical detection signal via designing and engineering nanoparticle assemblies could enable their use with low-cost portable imaging systems and broaden their applicability beyond the study of biomolecular interaction.

Silicon nanostructures are good templates for the deposition of plasmonic metals to fabricate substrates showing activity in surface enhanced Raman scattering (SERS). Porous silicon formed by electrochemical anodic etching of monocrystalline silicon wafers is more commonly used while silicon nanowires grown by metal assisted chemical etching are also utilized. The main advantage of such substrates is that they provide deposition of polydisperse films consisting of densely arranged metal nanoparticles (NPs) from liquid solutions. The resulting films exhibit properties in SERS-spectroscopy typical for substrates made by other more expensive and complicated methods of nanoengineering. Majority of papers report on metal films consisted of quasispherical NPs, but some works demonstrates that it is also possible to form metal rods, dendrites and thorns of nanoscaled dimensions.
This work is aimed at studying how the type and level of doping of the original silicon affects the formation process and the morphology of the metal nanostructures, and hence their optical properties, which determine SERS-activity.
We deposited silver (the strongest plasmonic metal) on silicon nanostructures grown on n- and p-type wafers of varying doping level. It should be noted that, depending on the doping conditions, both photoluminescent and non-luminescent silicon nanostructures were obtained.
In addition, we studied the deposition of silver on silicon nanostructured particles obtained by thermal reduction of silicon dioxide extracted from the horsetail. Such nanoparticles are composed of only silicon atoms, i.e. are characterized by intrinsic conductivity.
It was found that the p-type of the initial substrate inhibits growth of metal nanostructures due to absence of free electrons. At the same time, there is a growing trend towards elongated and dendritic nanostructures.
The n-type silicon leads to the formation of quasispherical metal NPs and agglomerates. Remarkably, nanocrystals of intrinsic conductivity that have sizes of several nanometers are shown to have a chemical sustainability. As a result, they are not oxidized during immersion into the solution containing silver ions and silver is not deposited.
Influence of silver NPs on the photoluminescence of silicon nanostructures and the SERS-activity was studied. It is shown that the SERS-activity of quasispherical metal NPs on nanostructured n-type silicon rises as mass of the deposited silver increase until NPs coalesce into agglomerates.
On the other hand, the SERS-activity of single dendrites grown on p-type silicon nanostructures is higher than that in quasispherical NPs. However, as the amount of dendrites and the thickness of the dendritic layer increase, the activity decreases.
The application perspectives of the SERS-active substrates based on photoluminescent silicon nanostructures for simultaneous bioimaging and molecular identification are also discussed.

In this work, we report innovative concept, design, and fabrication of three-dimensional (3-D) nanoporous SERS nanosensors for dual-functionally ultrasensitive detection and tunably release of molecules. The nanosensor consists of a gold (Au) nanorod core and a silica shell with embedded superstructural nanopores, where large arrays of plasmonic silver (Ag) nanoparticles can be synthesized with controlled size and distributions both on the outer surfaces but also inside the nanopores. The rationally increased number of hot spots at the junctions of Ag nanoparticles, as well as the near-field electromagnetic coupling between the Ag nanoparticles on the surface and those embedded in the 3-D structure provide substantially enhanced Raman sensitivity for detection of biochemicals. The 3-D porous structures provides high surface areas for drug loading. Furthermore, when place in an external electric (E-) field, molecules on the nanosensors can be controllably released with tunable rates due to the induced E-field generated at the junctions of Ag nanoparticles, where the hotspots not only effectively enhance the Raman detection of molecules but also induce electrokintic effects to tune the release rate of molecules. Finally, these nanosensors are motorized, including both transport and rotation, owing to the electrically polarized metallic nanocores, which is highly potent for single-cell biological research and precision medicine.

With the remarkable light-matter interaction and in-situ surface plasmons tunability, 2D plasmonic materials are favored in various disciplines. At the current stage, however, most 2D plasmonic practices are restrained intrinsically at mid-infrared range which brings limitations to integration- and miniaturization-oriented practical applications. To address such challenge, we demonstrate here to employ highly doped atomically thin transition metal oxides (TMOs) as an alternative class of 2D plasmonic material. Molybdenum trioxide (MoO₃) is a representative TMO of which the layered crystalized structure enables the formation of 2D morphology. We synthesize and characterize few-layer α-MoO₃ nanoflakes highly doped with free electrons, which facilitate surface plasmons in near-infrared (NIR) region. After doping of abundant free electrons, the resultant sub-stoichiometric α-MoO_3-x nanoflakes possess quasi-metallic plasmonic behaviors. In our biosensing demonstration, we integrate the MoO_3-x nanoflakes with a flexible microfiber which is compliant with the commonly used and cost-effective optical system. The proposed highly integrated fiber-optic biosensor provides a detection limit of bovine serum albumin (BSA) as low as 1 pg/mL.

The layered MoO₃ nanoflakes are synthesized by liquid phase exfoliation. Observed under TEM and HRTEM, the as-prepared MoO₃ samples are flake-like in shape and preserve the same single crystal nature as their bulk counterparts. AFM reveals the average thickness of MoO₃ nanoflakes is ~2.8 nm, which is exactly the thickness of two planar units of α-MoO₃. Abundant free electrons are introduced into MoO₃ nanoflakes via H⁺ intercalation, and the color of nanoflakes suspension gradually turns from colorless to dark blue. Meanwhile, a strong absorption peak of nanoflakes suspension appears at ~735 nm. From XPS analysis, we verify that the free electron density increases, and two oxidation states Mo⁶⁺ and Mo⁵⁺ coexist in MoO_3-x after H⁺ intercalation and account for 71.7% and 28.3%, respectively. Since MoO_3-x is positively charged, we immobilize the nanoflakes via electrostatic interaction onto a microfiber evenly functionalized with negative charges. Benefited from the compactness of microfiber, 50 µL MoO_3-x nanoflakes suspension is adequate for inducing strong plasmon resonance in the transmission spectrum. The positively charged MoO_3-x nanoflakes stabilized on microfiber surface show good affinity to negatively charged BSA molecules. The plasmon resonance performs linear response as BSA concentration increases from 1pg/mL to 100 ng/mL. Based on the surface plasmon attenuation band, we deduce the Drude model of MoO_3-x and numerically prove the enhanced plasmon-matter interaction induced by MoO_3-x. This study reveals unprecedented potentials of employing 2D TMOs in highly sensitive plasmonic devices with access to frequently used optical windows, high degree of integration and flexibility, and simple fabrication procedures.

Microsphere photolithography, owing to microsphere lenses’ excellent focusing properties and adjusting light scattering, has proved to be a good candidate for fabrication of large-area tunable surface nanopattern arrays^[1,2]. Here, close-packed and non-close-packed Mie resonance microsphere photolithography are studied. Different patterns on photoresist surface are obtained theoretically by adjusting optical coupling among neighboring spheres with different gap sizes. The effect of light reflection from the substrate on the pattern produced on the photoresist with a thin thickness is also discussed. Sub-micron hexagonal star-shaped and ring-shaped patterns arrays are achieved with close-packed spheres arrays and spheres arrays with big gaps, respectively. Changing of star-shaped vertices is induced by different polarization of illumination. Experimental results agree well with the simulation. By using smaller resonance spheres, sub-400nm star-shaped and ring-shaped patterns can be realized. These tunable patterns are different from results of previous reports of microsphere photolithography, and the suggested methods are much easy than microsphere assisted lithography to obtain star-shaped and ring-shaped patterns arrays^[3], which can find application in biosensor and optic devices.
References:
[1] Wu, W.; Katsnelson, A.; Memis, O. G.; Mohseni, H. A deep sub-wavelength process for the formation of highly uniform arrays of nanoholes and nanopillars. Nanotechnology 2007, 18, 485302.
[2] Chang, Y. C.; Lu, S. C.; Chung, H. C.; Wang, S. M.; Tsai, T. D.; Guo, T. F. High-throughput nanofabrication of infra-red and chiral metamaterials using nanospherical-lens lithography. Scientific Reports 2013, 3, 3339.
[3] Sun, Z. Q.; Yang, L.; Zhang, J. H.; Li, Y. F.; Zhao, Z. H.; Zhang, K.; Zhang, G.; Guo, J. R.; Yang, B. A Universal Approach to Fabricate Various Nanoring Arrays Based on a Colloidal-Crystal-Assisted-Lithography Strategy. Advanced Functional Materials 2008, 18, 4036-4042.

Biogenic amines (BAs) are a family of organic bases that may be found in foods such as fish and meat. The presence of BAs is the result of the decarboxylation of certain amino acids by microorganisms in these foods. The presence of microorganisms in food products not only poses a potential harm to human health, research has also found specific BAs (e.g. histamine) to cause headaches, diarrhea, edema, and other adversities. Thus, the simple, sensitive, and low-cost detection of BAs is of great importance in food safety. Methods such as HPLC and GC-MS have been employed in the sensing of BAs. However, tedious pretreatments, long reaction times, and high equipment costs limit the practicability and applicability of these techniques. Recently, localized surface plasmon resonance (LSPR) of metal nanoparticles (NPs) has been utilized for chemical and biological sensing. In particular, the collective oscillation of electrons within the NPs upon irradiation of incident light is sensitive to changes in environmental refractive index, making NPs suitable for developing plasmonic sensors.
Herein, we have developed paper-based plasmonic refractometric sensors through the embedding of metal NPs onto flexible papers using reversal nanoimprint lithography. The NP-embedded papers can serve as gas sensors for the detection of volatile BAs released from spoiled food. Commercial inkjet papers were employed as sensor substrates—their high reflectance (>80%) and smooth surfaces (roughness: ca. 4.9 nm) providing significant optical signals for reflection-mode plasmonic refractometric sensing and high particle transfer efficiency, respectively; in addition, because inkjet papers have light weight and are burnable and flexible, they are especially suitable for developing portable, disposable, cost-effective, eco-friendly sensing platforms. Solid silver NPs (SNPs), solid gold NPs (GNPs), and hollow Au–Ag alloyed NPs (HGNs) were immobilized on a solid mold and then transferred directly onto the softened paper surfaces. The particle number density and exposure height of the embedded NPs were dependent on two imprinting parameters: applied pressure and temperature. The optimal samples exhibited high particle transfer efficiency (ca. 85%), a sufficient exposure surface area (ca. 50% of particle surface area) presented to the target molecules, and a strong resonance reflectance dip for detection. Moreover, the HGN-embedded paper displayed a significant wavelength dip shift upon the spontaneous adsorption of BA vapors (e.g., Δλ = 33 nm for putrescine; Δλ = 24 nm for spermidine), indicating high refractometric sensitivity; in contrast, no visible spectroscopic responses were observed with respect to other possibly co-existing gases (e.g., air, N₂, CO₂, water vapor) during the food storage process, indicating high selectivity. Further in situ analyses for real samples (e.g., salmon) are in progress and will be presented at the conference.

The incorporation of direct-write printing toward realization of optically active structures has been a limiting factor toward wide-spread incorporation of the technology. Specifically, compatibility and integration of the printing process with spatially constrained microfluidic devices is essential for point-of-care (POC) sensing and diagnostics. However, the current approaches are bound by resolution, accessibility and multi-step fabrication constraints. In this work, we develop a plasmonic bubble based approach, wherein arrays of Ag rings are fabricated from a precursor Ag ink in a single-step. A 532nm continuous-wave laser is focused on the gold nanoisland (AuNI) substrate with diamminesilver (I) acetate precursor ink covering the substrate. The array pattering is achieved via intermittent laser exposure (<200 ms) and stage translation. Using the above “point-and-shoot” approach, we fabricate optically active Ag rings with tunable diameters between 1-2 µm and a lattice spacing of 3mm. The thermally reduced Ag from the precursor is immobilized along the bubble/water interface to yield instantaneous ring morphology. Analytical modelling of the fabrication process substantiates the realization of the ring geometry. The hybrid Ag ring/AuNI substrate exhibits plasmonic resonances in the mid-IR and visible regime. Finite-difference time-domain (FDTD) simulated electric-field distribution at a single Ag ring establishes that the resonance in the mid-IR regime arises from the dipolar plasmon mode of the Ag ring. The visible component of the hybrid substrate arises from the AuNI particles. We show simultaneous surface-enhanced infrared spectroscopy (SEIRS) of 2, 4, 6 – trinitrotoluene (TNT) and surface-enhanced Raman spectroscopy (SERS) of rhodamine 6G (R6G) to demonstrate the dual-mode applicability. The aromatic (3.43 µm) and aliphatic (3.51 µm) C-H stretch bands of TNT on the Ag ring-AuNI substrate are enhanced via SEIRS. Further, we measured the SERS spectra of R6G molecules drop-casted on the Ag ring-AuNI substrate and also within a microfluidic chamber. The SERS signal is enhanced due to the high E-field enhancement at the Ag-ring/AuNI interface, which arises from the intense hot spots at the multiple Ag-Au junctions with the sub-20 nm gaps. Similar fabrication and sensing characteristics along with high stability under various flow conditions are realized within a spatially-constrained microfluidic channel. With simplicity, high efficiency and integrability in fabrication of micro/nanostructures, “point-and-shoot” approach enables realization of device miniaturization, portability and multi-functionality.

In this study, we report the crystallinity effects of submicrometer titanium dioxide (TiO₂) nanotube (TNT) incorporated with silver (Ag) nanoparticles (NPs) on surface-enhanced Raman scattering (SERS) sensitivity. Furthermore, we demonstrate the SERS behaviors dependent on the plasmonic−photonic interference coupling (P-PIC) in the TNT-AgNP nanoarchitectures. Amorphous TNTs (A-TNTs) are synthesized through a two-step anodization on titanium (Ti) substrate, and crystalline TNTs (C-TNTs) are then prepared by using thermal annealing process at 500 °C in air. After thermally evaporating 20 nm thick Ag on TNTs, we investigate SERS signals according to the crystallinity and P-PIC on our TNT-AgNP nanostructures. (A-TNTs)-AgNP substrates show dramatically enhanced SERS performance as compared to (C-TNTs)-AgNP substrates. We attribute the high enhancement on (A-TNTs)-AgNP substrates with electron confinement at the interface between A-TNTs and AgNPs as due to the high interfacial barrier resistance caused by band edge positions. Moreover, the TNT length variation in (A-TNTs)-AgNP nanostructures results in different constructive or destructive interference patterns, which in turn affects the P-PIC. Finally, we could understand the significant dependency of SERS intensity on P-PIC in (A-TNTs)-AgNP nanostructures. Our results thus might provide a suitable design for a myriad of applications of enhanced EM on plasmonic-integrated devices.

Atomic-thin two-dimensional (2D) materials exhibit many unique and extraordinary properties such as excellent mechanical flexibility, good optical transparency, high thermal conductivity, and diverse electronic properties. Owing to all these characteristics, 2D materials have been extensively investigated as emerging materials for various fields, including electronics, nanophotonics, biology, and energy harvesting. Among most of these applications, it is essential to obtain desired micro/nano-scale patterns. The current lithographical methods such as electron beam lithography demonstrate the capability of high-resolution patterning, however, they usually require high cost and multi-step processing with complex capital instruments. Recently, researchers have developed direct laser ablation to achieve 2D materials patterning. Yet, the use of high-power ultrafast lasers is needed.
Herein, we report plasmon-enhanced optothermal nanoscissors (OTNS) to directly pattern 2D materials with a low-cost continuous-wave (CW) laser on a plasmonic substrate. The plasmonic substrate consists of a thin layer of quasi-continuous gold nanoislands (AuNIs). The plasmon-enhanced photothermal effect generates abundant highly localized heat, enabling rapid and precise ablation of 2D materials at any specific locations. By translating the sample stage or scanning the laser beam, desired patterns of 2D materials can be created.
OTNS can fabricate complex and arbitrary patterns on 2D materials with low power and high resolution down to ~300 nm. The feature size can be precisely controlled within a wide range from 300 nm to micrometer scale. Various periodic nanostructures were successfully patterned on graphene and MoS₂, such as nanoribbons, nanodisk array, and nanohole array, proving the great reproducibility of OTNS. We further show OTNS can be used for fabricating complex patterns with arbitrary sizes and shapes. The results also demonstrate the defining strength of OTNS, where small feature < 1 μm can be realized. With the low-power operation, tunable feature size, and versatile patterning capabilities, OTNS offers the possibility to scale up the fabrication of 2D-material nanostructures for many applications such as biosensing and infrared photonic systems.

Numerous studies have been reported on integration of plasmonic nanomaterials and nanostructures into wearable and flexible devices. For their widespread and practical utilization, it is beneficial to realize two crucial features: From a wearable technology standpoint, it is critical to realize large-area/flexible/biocompatible clothing with specific functionalities (e.g., biosensing, communication, and hazard protection). From a manufacturing standpoint, it is important to eco-friendly produce nanomaterials on a large scale. In this respect, there is always a combined need for both flexible/scalable/biosafe functionalities and green manufacturing/production for plasmonic nanomaterials and nanostructures. In this presentation, we report that native silk produced by silkworms appears to be an alternative plasmonic photonic platform for hybridizing metal nanoparticles and natural biomaterials via green chemistry. This approach is inspired from ‘silk weighting’ which is the old method in the 19th century for increasing the weight of raw silk for high price. Silk has a strong affinity for several metallic salts, which results in the formation of metal nanoparticles with finite sizes inside the interfibrillar nanostructures of silk. Moreover, this process can be enhanced by plant-derived polyphenolic chemistry. The reported wholly integrated plasmonic native fibers are distinct from other nanomaterial hybridizations that are focused on attaching metal nanoparticles on the fiber surface. We further demonstrate plasmon-enhanced photoluminescence of far-red fluorescent protein (mKate2) in silk produced by genetically engineered silkworms (i.e., silkworm transgenesis). Our results provide the groundwork for exploiting natural silk as a photonic nanomaterial hybridization platform to implement embedded functionalities in a fiber geometry, which can be constructed into large-area fabrics. Additionally, this insect factory along with green chemistry could potentially open an alternative nanomanufacturing strategy in an eco-friendly, scalable, and sustainable manner, minimizing the use of complex nanofabrication methods.

Directional solidification of eutectics presents a unique opportunity to control self-assembly of multiple-phase materials and to organize matter over large length scales. Eutectics can possess simple microstructures like lamellar, rod or even complex motifs like split-ring resonator like geometries, wherein these phase-separated materials can be selected from a diverse set of metals, ceramics, polymers, organics or salts.^[1,2] This vast choice of materials chemistry renders these self-assembled microstructures with metal-metal, metal-dielectric or all-dielectric features. By integrating these material systems within templates, we can achieve both long and short-range eutectic microstructures that have sufficient order for photonic effects. Templated-eutectics provide with interesting morphologies not present in the native eutectic or the starting template.^[3,4] The initial attempts with silica opal template and cage-like structures have provided us with novel geometries and helped develop design tools to produce metamaterials. By simplifying the template design new microstructures have been obtained which upon post-synthetic transformations can potentially be used as photonic-plasmonic devices. Also the challenges and opportunities in this approach for designing novel microstructures for photonic metamaterials using template-guided directionally solidified eutectics are addressed.

References:
1. K. Jackson, J. Hunt, Transactions of the Metallurgical Society of AIME 1966, 236, 1129.
2. D. A. Pawlak et. al, Advanced Functional Materials 2010, 20, 1116.
3. J. Kim et. al, Advanced Materials 2015, 27, 4551.
4. Z. Yan et. al, Proceedings of the National Academy of Sciences 2017, 1713805114.

Recently, metasurfaces have emerged as a powerful and versatile paradigm for making many optical devices especially optical lenses. However, the approach usually used to engineer metasurface devices assumes that neighboring elements are identical, by extracting the phase information from simulations with periodic boundaries, or that near-field coupling between particles is negligible, by extracting the phase from single particle simulations. This is not the case in general and the approach thus prevents the optimization of devices that operate away from their optimum. We propose a versatile numerical method that we call local phase method (LPM) to obtain the phase of each element within the metasurface while accounting for near-field coupling with different neighbors. The proposed local phase method paves the way to highly efficient metasurface devices including, but not limited to, deflectors, high numerical aperture concentrators, lenses, cloaks, and modulators.

We propose a novel class of tunable nanowires by incorporating transparent conducting oxides and active semiconductors such as Indium tin oxide (ITO) and 4H-silicon carbide (4H-SiC) into multimaterial configurations in which a continuous tunability of optical properties can be obtained via field-effect modulation by applying an external bias. These building blocks offer the opportunity to develop a new generation of metamaterials, in which the optical properties can be dynamically changed in real-time. In particular, the resonant characteristic of an ITO-integrated multimaterial nanowire is exploited to obtain a continuous tunability of transmission phase over 260 degrees which enables beam-shaping to the desire through a geometrically fixed metasurface by controlling the external bias for each element. Moreover, it is demonstrated that deeply subwavelength elements of such multimaterial configurations can yield strong variations in the effective permittivity by changing the bias voltage which can add tunability into metamaterial platforms. The field-effect modulation can also be exploited to tune the effective permeability of epsilon-near-zero nanowires which enables tuning the impedance of zero-index metamaterials. The performance of these electro-optical platforms is characterized rigorously by linking the charge transport and electromagnetic models via dispersion models. The charge transport model is governed by drift-diffusion equations while the electromagnetic modeling is carried out using a robust semi-analytical method based on transition matrix formulation which enables accurate and efficient analysis of multimaterial nanowires. The proposed tunable nanowire antennas can enable on-demand controllable meta-systems.

Part I: Today’s communication systems use electrons, photons or light or more general electromagnetic waves as carriers for data transfer in integrated circuits. As the research and development focus in electronics is on reaching higher frequencies, researchers presented already communication systems in the THz-range, others try to integrate photonic systems on a chip. There are several challenges to overcome on both sides: the lasing in crystalline silicon material is still a problem as the band-gap is not appropriated for direct band transition and the efficiencies in emitting light in silicon is rather low; on the other side, the electronic transistors working on high transition frequencies have a high heat dissipation or are difficult to fabricate. The main target of this work is to show an adequate alternative to the standard approaches in communications. The author was motivated by the relatively new achievements in the field of heat and sound control in non-conduction and semi-conducting materials and their properties at the dimension in nanoscale. A general overview will be given by the author on the materials choice and the structures to control elastic vibrations and make them appropriate for information transmission.
Part II: The German Research Foundation (Deutsche Forschungsgemeinschaft, DFG) is a self-governing organization under private law and focuses its funding exclusively on fundamental science with a annual budget of 3.07 billion Euros. This means that the funding is invested into risky and basic understanding of research questions and not in applied research. Researchers with a PhD degree at German universities are invited to submit their proposals mostly without any kind of deadlines. Among the broad spectrum of research fields between engineering, natural science, life science and humanities you can find the detailed spectral range of electrical engineering and especially the active vibrations on material fabrication, device technology, circuits and communication. With our funding programs we want to push the current research in engineering far beyond the state of the art and support innovative ideas as well as collaborations internationally wide.
In my presentation I will focus on the German funding system of research and bring it into the resonance with the field of electronic, photonic devices and circuits.

Metasurfaces have shown great abilities on controlling light properties on demand at a subwavelength resolution [1]. They therefore are very promising for the development of flat optical components. However, the building blocks of metasurfaces usual exhibit strong dispersion effect, which results in chromatic aberration apparently [2-4]. For addressing this issue, we come out a design principle by incorporating geometric phase with integrated-resonant unit elements for realizing specific phase compensation at corresponding spatial position [5]. The basic building blocks of the metasurface are gold nano-rods and their assemblies. The metal-dielectric-metal structural configuration is implemented to access a cavity-like resonance for the improvement of working efficiency. As a proof of concept, two reflective achromatic metasurface devices (focusing metalens and beam deflector) are demonstrated for circularly-polarized incident light. They are capable of eliminating the chromatic effect over an ultra-broad continuous bandwidth in near-infrared (from 1200 nm to 1680 nm). To our best knowledge, this is the state-of-the-art demonstration for realizing truly achromatic devices with flat optical metasurfaces. The working wavelength can be further pushed to the visible spectrum through replacing the metallic structures by high-index dielectric unit-elements [6]. Our design principle paves a way to flexibly engineer the phase dispersion, benefitting the development of feasible application for full-color imaging systems and detections, just named a few.

References
1. H.-H. Hsiao, C. H. Chu, and D. P. Tsai, "Fundamentals and applications of metasurfaces," Small Methods 1, 1600064 (2017).
2. P. Wang, N. Mohammad, and R. Menon, "Chromatic-aberration-corrected diffractive lenses for ultra-broadband focusing," Sci. Rep. 6, 21545 (2016).
3. E. Arbabi, A. Arbabi, S. M. Kamali, Y. Horie, and A. Faraon, "Controlling the sign of chromatic dispersion in diffractive optics with dielectric metasurfaces," Optica 4, 625-632 (2017).
4. P. C. Wu, W.-Y. Tsai, W. T. Chen, Y.-W. Huang, T.-Y. Chen, J.-W. Chen, C. Y. Liao, C. H. Chu, G. Sun, and D. P. Tsai, "Versatile polarization generation with an aluminum plasmonic metasurface," Nano Lett. 17, 445-452 (2017).
5. S. Wang, P. C. Wu, V.-C. Su, Y.-C. Lai, C. Hung Chu, J.-W. Chen, S.-H. Lu, J. Chen, B. Xu, C.-H. Kuan, T. Li, S. Zhu, and D. P. Tsai, "Broadband achromatic optical metasurface devices," Nat. Commun. 8, 187 (2017).
6. B. H. Chen, P. C. Wu, V.-C. Su, Y.-C. Lai, C. H. Chu, I. C. Lee, J.-W. Chen, Y. H. Chen, Y.-C. Lan, C.-H. Kuan, and D. P. Tsai, "GaN metalens for pixel-level full-color routing at visible light," Nano Lett. 17, 6345-6352 (2017).

We propose a novel architecture to achieve strong electric field enhancement by combining two different mechanisms: the plasmonic resonances of metallic oligomers and coherent scattering from a periodic structure (i.e. Rayleigh anomaly or modes). Metallic oligomers are well-known for their ability in providing giant field localization at subwavelength scale thanks to the role of surface plasmon polaritons. However, due to intrinsic nonlocality of the dielectric response of the metals along with their inherent loss, the field enhancement that can be achieved has an ultimate constraint. Consequently, it is of crucial importance to make use of another mechanism along with the plasmonic resonance to further enhance the field. One promising mechanism is to employ a periodic superstructure along with the plasmonic oligomers. We call our synergistic design a two-scale structure since the two are fabricated using complementary and very different methods. We carry out a thorough study of this structure for different polarizations of the incident light and verify the simulations experimentally. Depending on the periodicity and the incident field polarization, the periodic structure may support a mode that can be coupled to the localized surface plasmon resonance mode of plasmonic oligomers for further field enhancement. Moreover, according to Rayleigh anomaly, when the period matches to the wavelength of the normal incident light, multiple scatterings from the periodic structure interact constructively. In this case, if the period matches the resonance wavelength of the plasmonic oligomers, the local field at the oligomers hot spot would be further enhanced. Such a giant field enhancement can be used in surface enhanced Raman spectroscopy (SERS) to detect trace amounts of analytes. We experimentally verify the ability of our structure for field enhancement by exploiting the two-scale structure which consists of gold nanorods and oligomers of gold nanospheres for the SERS enhancement. Our experimental results confirm our theoretical results that the SERS for this case is boosted several times.

The Starshot Breakthrough Initiative established in 2016 sets an audacious goal of sending a spacecraft beyond our solar system to a neighboring star. Its aim is to launch a ‘lightsail’ with a nanocraft payload that reaches a relativistic speed of ~60 000 km/s (20% the speed of light) using radiation pressure from a high-powered phased array of lasers on Earth (~10GW/m² of net laser intensity). It is expected that the Starshot lightsail would have an area of ~10m² and be kept to a mass of under ~1 gram implying that ultrathin (~10-100 nm) materials must be used. Such stringent criteria require a comprehensive multiphysics modelling of the light-sail interaction. In this work, we explore lightsail photonic designs accounting for both nanophotonic phenomena as well as large-scale photonic sail, that address requirements for the sail reflectivity and radiation pressure, and outline criteria for the sail shape design to achieve stable spacecraft propulsion.
In our analysis of photonic lightsail spacecraft propulsion to near-relativistic speeds, we reveal a tradeoff between the high sail reflectivity needed for efficient photon momentum transfer, large bandwidth, accounting for the Doppler shift, and the low mass requirements. We consider several typical geometries where desired figures of merit may be achieved, including 1D photonic crystals, two-dimensional photonic crystal slabs and metasurfaces.
Lastly, we discuss the requirements for lightsail/nanocraft stability along its trajectory during the acceleration phase. We derive a theoretical formalism for the radiation force on the sail, and show an intimate relation between the sail shape, beam profile, and local sail reflectivity. We outline possible instability mechanisms and key challenges. We further discuss design scenarios that may provide avenues for stable spacecraft propulsion.

In this talk, I will address a few aspects in metamaterial properties, related to their design, which pose a perceivably substantial theoretical and practical importance, and yet are often underestimated in our community.

One example is an exceptionally strong effect of metamaterial boundaries and shape of metamaterial samples on their observable properties. The effect owes to extraordinarily strong mutual interaction between closely positioned elements, which enhances the boundary role and results in remarkable deviations between the effective medium predictions and realistic properties if finite samples. The eventual convergence towards a homogenisable response is quite slow, which poses direct implications for any conceivable practical designs.

Another example is related to opto-acoustic metamaterial designs, which offer artificial electrostriction and enhanced stimulated Brillouin scattering (SBS). On this track, a simple and non-resonant composite medium, such as an array of spheres embedded in a matrix of a different material, attains an artificial term in the electrostriction coefficient so the resulting photoelasticity can exceed that of the individual components, potentially offering more than a threefold enhancement in the electrostriction. We have further extended these approaches to inverse opal structures, paving a way towards all-Si-based CMOS-compatible SBS.

Yet another example is the largely unexplored potential of artificial diamagnetics, where a robust and easily scalable design, based, once again, on non-resonant elements, demonstrates outstanding performance compared to traditional weak diamagnetism. The great role of mutual interaction and strong coupling in a specially tailored lattice of meta-atoms, is once again a key feature here, easily leading to a magnitude of effective permeabilities below 0.1, further armed with efficient reconfigurability, and playing a step on the way to the near-zero range.

I greatly acknowledge the contribution of R. Marques, L. Jelinek, M. J. A. Smith, C. Wolff, B. T. Kuhlmey, C. Martijn de Sterke, P. A. Belov, Y. S. Kivshar, R. C. McPhedran, and C. G. Poulton to various aspects of the reported research.

Specific design of nanostructured surfaces allows enhanced and directional in- and out-coupling of light for targeted wavelengths [1], [2]. Moreover, the ability to control both the polarization-dependent response of an antenna and the polarization state of the outgoing light, offers an effective strategy to structure electromagnetic fields [3], [4]. Thanks to these properties, optical nanoantennas represent a powerful tool in a wide range of applications including optical imaging, light harvesting, and sensing [1].
One of the most successful examples of directional plasmonic antennas are the so-called bull’s eye structures [5-7]. Using concentric circular grooves, these single-resonant antennas provide spectrally selective and directional transmission of light [5,6,8]. Here, inspired by these structures, we introduce a new class of bull’s eye antennas, consisting of concentric polygons [9]. In contrast to the traditional circular bull’s eyes, our polygonal bull’s eyes can accommodate multiple resonances by introducing variations in the periodicity along the different axes of the structure. As such, this structure can provide independent control over emission directionality for multiple wavelengths. Moreover, since each resonant wavelength is directly mapped to a specific polarization orientation, spectral information can be encoded in the polarization state of the out-scattered beam. To demonstrate the potential of such structures in enabling simplified detection schemes and additional functionalities in sensing and imaging applications, we use the central subwavelength aperture as a built-in nano-cuvette and manipulate the fluorescent response of colloidal quantum dot emitters coupled to the multiresonant antenna.
Extending the concept of our multiresonant bull’s eyes beyond the plasmonic antenna structure, we translate our design to structures made entirely out of colloidal quantum-dots [8]. Efficient Bragg scattering of the quantum dot fluorescence yields a high degree of directional and polarization control. Our results on plasmonic and quantum dot structures will be discussed in the context of polarimetric applications that may generate new forms of structured light and enable advanced concepts in spectroscopy and display applications.

References
[1] A. E. Krasnoket al., Uspekhi Fiz. Nauk 183, 561–589 (2013)
[2] L. Novotny et al., Nature Photonics 5, 83 (2011)
[3] C. Osorio et al., Scientific Reports 5, 9966 (2015)
[4] D. Van Damet al., Nano Lett. 15, 4557–4563 (2015)
[5] H. J. Lezec et al., Science 297, 820–822 (2002)
[6] J. Schuller et al., Nature Materials 9, 193 (2010)
[7] S. Han et al., Physical Review Letters 104, 043901 (2010)
[8] F. Prins et al., Nano Lett. 17, 1319–1325 (2017)
[9] E. De Leo et al., in preparation (2017)

Abstract: We experimentally demonstrate efficient broadband THz generation, ranging from 0.1 - 4 THz, from a thin layer of SRRs with few tens of nanometers thickness by pumping at tele-communications wavelength of 1.5 microns (200 THz).

The terahertz spectral range of the electromagnetic spectrum—from about 100 GHz to 15 THz—has long been a challenging region in between the successful realms of electronics and photonics, because of the lack of efficient and compact sources and detectors for terahertz radiation. In the past few decades, however, the development of technologies like quantum-cascade lasers, terahertz wave generation through nonlinear crystals and terahertz time-domain spectroscopy has enabled the exploration of terahertz science and the rapid rise of terahertz imaging and spectroscopy for, amongst others, biomedical and security applications. Likewise, electronic devices are pushing their boundaries to the THz regime from below, where many potential applications exist in different disciplines. The strategic emphasis of our research has been to merge two prominent research themes of current focus, namely femtosecond nonlinear technology and metamaterials. One key challenge is to develop ultrafast few-cycle THz pulses with extraordinary stability and a gapless spectrum covering the entire THz region.

In summary, we have shown that a single nanometer-scale layer of SRRs merges nonlinear metamaterials and THz science/technology, representing a new platform for exploring artificial magnetism induced nonlinear THz generation. This leads to broadband THz emission from deep-subwavelength meta-atoms.

Microstructuring of materials on the scale of the wavelength or below is a well established approach to realize new optical properties not present in the bulk material itself. In the first part, we report on direct laser written photonic topological insulators supporting a topologically protected edge mode. While transport along such an edge mode is known to be stable against static defects, we examine the influence of time-dependent defects on the transport properties [1]. We find that in contrast to static defects time dependent defects are not excluded from transport, eventually leading to a splitting of the edge mode.
In the second part, we study the influence of intensity patterns on the magnetic properties of spin-wave guides made from Yttrium-Iron-Garnet (YIG) and find that such materials can easily be reconfigured, allowing for the formation of so called magnonic crystals [2] and for mode conversion between modes not adressable due to energy and momentum conservation. [3]

[1] "Dynamic defects in photonic Floquet topological insulators", C. Jörg, F. Letscher, M. Fleischhauer, and G. von Freymann, New J. Phys. 19, 083003 (2017)
[2] "Optically reconfigurable magnetic materials", M. Vogel et al., Nature Physics 11, 487 (2015)
[3] "Adiabatic Control of Spin-Wave Propagation using Magnetisation Gradients", M. Vogel et al., submitted (2017)

Simple metallic mirrors flip the spin of a circularly polarized wave upon normal incidence by inverting the direction of the propagation vector. Altering or maintaining the spin and polarization of waves carrying data is a critical need to be met at the brink of photonic information processing. In this work, we report a chiral metamaterial mirror that strongly absorbs a circularly polarized wave of one spin state and reflects that of the opposite spin in a manner conserving the circular polarization. A circular dichroic response in reflection as large as ~0.5 is experimentally observed in a near infrared wavelength band. By imaging a fabricated pattern composed of the enantiomeric unit cells, we directly visualize the two key features of our engineered meta-mirrors, namely the chiral-selective absorption and the polarization preservation upon reflection. Beyond the linear regime, the chiral resonances enhance light-matter interaction under circularly polarized excitation, greatly boosting the ability of the metamaterial to perform chiral-selective signal generation and optical imaging in the nonlinear regime. Chiral meta-mirrors, exhibiting giant chiroptical responses and spin-selective near field enhancement, hold great promise for applications in polarization sensitive electro-optical information processing and biosensing.

The study of the optical response of high refractive index nano-particles has revealed that these resonant structures are capable of controlling different degrees of freedom of light fields with unprecedented versatility [1]. The ability of these particles to control the intensity, phase and polarization of light has unveiled a plethora of new physical effects and applications. To mention a few, these particles have allowed controlling the directionality of optical antennas in an unprecedented manner [2], they have shown promise in enhancing molecular circular dichroism spectroscopy [3] and all- optical chiral separation methods based on enantioselective photo-excitation of chiral molecules [4].

In this letter, we unveil a new phenomenon that to the best of our knowledge was not reported up to date; the natural emergence of an optical vortex in the back scattering of cylindrically symmetric high index resonators when illuminated at their first Kerker condition of anomalous scattering. Firstly, based on singular optics arguments [5], we deduce the emergence of the vortex for a high index nano-particle Illuminated by circularly polarized light at the first Kerker condition. Secondly, using the recently developed helicity and angular momentum conservation framework, we prove that the modulus of the topological charge of the vortex has to be equal to 2. Lastly, we verify our predictions through analytic and numerical calculations.

Moreover, we analize the emergence of polarization singularities (C lines and L surfaces) in the scattering of optical resonators excited by linearly polarized light. We demonstrate both analytically and numerically that high refractive index spherical resonators present such topologically protected features and calculating the polarization structure of light around the generated C lines, we unveil a Möbius strip structure in the main axis of the polarization ellipse when calculated on a closed path around the generated lines of circular polarization.

References
[1] García-Etxarri, A., et al. "Strong magnetic response of submicron silicon particles in the infrared" Optics express 19, 4815-4826 (2011)
[2] Geffrin, Jean-Michel, et al. "Magnetic and electric coherence in forward-and back-scattered electromagnetic waves by a single dielectric subwavelength sphere." Nature communications 3, 1171 (2012)
[3] García-Etxarri, A., and Dionne J. "Surface-enhanced circular dichroism spectroscopy mediated by nonchiral nanoantennas." Physical Review B 87.23, 235409 (2013)
[4] Ho C., Garcia-Etxarri A., Zhao Y., Dionne J. "Enhancing enantioselective absorption using dielectric nanospheres", ACS Photonics, 4 (2), pp 197–203 (2017)
[5] Garcia-Etxarri A. "Optical Polarization Möbius Strips on All-Dielectric Optical Scatterers", ACS Photonics 4 (5), 1159-1164 (2017)

Active control of polarization states of electromagnetic waves is important in applications of information processing, telecommunications, and spectroscopy. However, despite the recent advances using artificial materials [1,2], most active polarization control schemes require optical stimuli requiring complex optical setups. Recently, we experimentally demonstrated that the polarization state of terahertz waves can be tuned electrically [3]. Combining a chiral metamaterial with a gated single-layer sheet of graphene, we showed that transmission of a terahertz wave with one circular polarization can be electrically controlled without affecting that of the other circular polarization, achieving large intensity modulation depths (>99%) with a low gate voltage. This effective control of polarization is made possible by the full accessibility of three coupling regimes, that is, underdamped, critically damped, and overdamped regimes by electrical control of the graphene properties.

In this work, to quantitatively understand the control of coupling, we derive an analytical expression of transmission coefficients by taking account of the two resonances in the graphene chiral metamaterial and investigate how damping coefficients play a role in varying the coupling regimes. In particular, we employed the coupled mode theory to derive the transmission coefficients [4]. Because there are three coupling channels between the incident and transmitted waves, that is, two via the resonance modes and one direct coupling, the complex transmission amplitude coefficient for right-handed circular polarization can be written as a sum of two Lorentzian functions and one constant transmission. The analytical expression provides us with a clear explanation on how to access critical coupling by changing the gate voltage in graphene chiral metamaterials.

References
[1] S. Zhang, J. Zhou, Y.-S. Park, J. Rho, R. Singh, S. Nam, A. K. Azad, H.-T. Chen, X. Yin, A. J. Taylor, X. Zhang, Nat. Commun. 3, 942 (2012)
[2] T.-T. Kim, S. S. Oh, H.-S. Park, R. Zhao, S.-H. Kim, W. Choi, B. Min, O. Hess, Sci. Rep. 4, 5864 (2014)
[3] T.-T. Kim, S. S. Oh, H. Kim, H. S. Park, O. Hess, B. Min, and S. Zhang. Science Advances 3, e1701377 (2017)
[4] S. Fan, W. Suh, J. D. Joannopoulos. J. Opt. Soc. Am. A 20, 569–572 (2003)

Current development of nanofabrication techniques makes it possible to produce plasmonic films of precisely controlled thickness down to a few monolayers for a variety of applications in optoelectronics and to probe the fundamentals of the light-matter interactions at the nanoscale [1–3]. As the film thickness decreases, the strong vertical electron confinement can lead to new confinement related and dimensionality related effects [4–6], which require theory development to understand their role in the light-matter interactions and optical response of thin and ultrathin plasmonic films. We develop a quasiclassical theory for the electron confinement effects and their manifestation in the optical response of ultrathin plasmonic films of finite variable thickness [5]. We start with the Coulomb interaction potential in the confined planar geometry to obtain the equations of motion and the conditions for the in-plane collective electron (plasma) motion. The plasma frequency thus obtained, while being constant for relatively thick films, acquires spatial dispersion typical of 2D materials and gradually shifts to the red as the film thickness decreases. As a consequence, the complex-valued dynamical dielectric response function shows the gradual red shift of its epsilon-near-zero point with the dissipative loss decreasing at any fixed frequency and gradually going up at the plasma frequency as it shifts to the red with the film thickness reduction. We argue that these are the universal properties peculiar to all ultrathin plasmonic films and metasurfaces. Our theory explains recent plasma frequency measurements done on ultrathin TiN films of controlled variable thickness [3], offering ways to tune spatial dispersion (and so magnetic permeability [7]) and magneto-optical properties of plasmonic films and metasurfaces — not only by varying their material composition but also by precisely controlling their thickness and by choosing deposition substrates and coating layers appropriately.

Acknowledgements: NSF-ECCS-1306871 (I.B.), NSF-DMR-1506775 (V.S.)

References:
[1] J.-S.Huang, et al., Nature Commun. 1, 150 (2010)
[2] H.Reddy, et al., Opt. Mater. Express 6, 2776 (2016)
[3] D.Shah, et al., Adv. Opt. Mater. 1700065 (2017)
[4] A.Manjavacas and F.J.García de Abajo, Nature Commun. 5, 3548 (2014)
[5] I.V.Bondarev and V.M.Shalaev, Opt. Mater. Express 7, 3731 (2017)
[6] T.Stauber, G.Gómez-Santos, and L.Brey, ACS Photon., DOI:10.1021/acsphotonics.7b00524
[7] L.D.Landau and E.M.Lifshitz, Electrodynamics of Continuous Media, Pergamon, NY, 1984

Titanium nitride (TiN) nanoparticles represent interesting materials with surface plasmon resonances in the visible to near infrared spectrum. TiN is biocompatible and chemically inert, making TiN nanoparticles attractive as biomedical photoacoustic imaging probes or for optical and photonic components. Today, the synthesis of TiN nanoparticles is typically performed at high temperatures between 700-1200°C or requires sophisticated vacuum technologies (CVD, plasma arc). The morphology of the particles is often ill defined, with round particles to cuboidal shapes and the syntheses lead to polydisperse sets of nanoparticles.
We will present a low temperature (~270°C) solution-based synthesis of TiN nanoparticles, leading to monodisperse cuboidal particles with large crystal domains. The size can be tuned between 5 and 100 nm and we will investigate the optical characteristics of these TiN nanoparticles. The synthesis will allow for better quality and availability and of TiN nanoparticles and in situ surface modification will enable more widespread application leading to facile generation of hybrid TiN core/organic shell structures also for the area of biomedical imaging.

Localized surface plasmon resonance (LSPR) has garnered interest in a variety of fields recently, such as photocatalysis, photovoltaics, biophotonics, spectroscopy, sensing, and wave-guiding. LSPR is correlated with the density of free charge carriers in nanoparticle materials, so metals tend to have the highest LSPR frequencies, with some absorbing within the visible spectrum. Cost and production concerns motivate the search for alternative plasmonic materials, like Group IV transition metal-nitrides.¹ We present a novel technique for the synthesis of plasmonic zirconium nitride (ZrN) nanoparticles using a scalable non-thermal plasma process. The system employs a solid zirconium tetrachloride precursor, which is heated to increase its vapor pressure. It is then mixed in the vapor phase with ammonia and passed through a non-thermal plasma reactor to dissociate the precursors and form ZrN. The synthesized particles exhibit a plasmonic absorption peak well within the visible spectrum, tunable from ~530 nm to ~650 nm. From XRD and TEM we infer the production of crystalline ZrN particles with a cubic rock salt structure and a tunable size distribution below 10 nm. Due to the tendency of this material to oxidize, we developed a modular non-thermal plasma system that coats the particles with amorphous silicon nitride in flight. This coating acts as an oxygen diffusion barrier when the material is exposed to atmosphere and yields blue-shifted and increased-intensity absorption. We observe different oxidation behavior than plasmonic titanium nitride (TiN) nanoparticles made using a variation of the same non-thermal plasma technique.² In TiN nanoparticles, oxidation causes a red-shift and intensity reduction in the plasmonic absorption spectrum.² In ZrN, oxidation does not result in a significant red-shifting of the absorption peak, though the intensity is reduced. We present additional data from density-functional theory calculations of the effect of oxidation on plasmonic resonance in each material, providing theoretical support for our observation of the variant effects of oxidation in the two similar materials.
¹ Boltasseva, A. & Atwater, H.A. Science 331, (2011), 290-291.
² Alvarez Barragan, A., Ilawe, N.V., Zhong, L., Wong, B.M., & Mangolini, L. J. Phys. Chem. C 121(4), (2017), 2316-2322.

Once the chromophore of green fluorescent protein (GFP) is optically pumped (~395 nm) at its protonated form (A to A*), it undergoes a series of spontaneous steps, downhill in energy. In several picoseconds, it deprotonates to an excited intermediate state, I*, which subsequently de-excites in few nanoseconds to a ground intermediate state, I, by fluorescence (509 nm). Finally, the chromophore protonates and converts back to its A-state in less than a nanosecond. The cycle, A to A* to I* to I and then back to A, is known as the Förster cycle. So far, the I-state has only been identified on the basis of time-resolved fluorescence spectroscopy. The optical absorption spectrum of GFP at its I-state is difficult to acquire due to its short lifetime in the Förster cycle. Here, by Resonant Energy Transfer (RET) from a single Ag nanoparticle (AgNP) to adsorbed GFP molecules, we report capturing the optical absorption band of the I-state (~490 nm) under 405 nm optical pumping. RET is observed in the form of quenching dips on the Mie scattering spectrum of a single AgNP. However, we acquire the correct absorption spectrum by dividing RET spectrum by plasmon scattering lineshape and multiplying it with the fourth power of the photon energy, following the basic RET physics. GFP is conjugated to AgNPs through histidine-Ag binding. Without 405 nm optical pumping, we acquire two absorption bands, at 476 nm and 443 nm, which are precisely indicative of the B-state of GFP (stable deprotonated form) and denatured GFP, respectively. Apparently, a fraction of the GFP denatures during the 10-min scattering acquisition under intense broadband excitation. Although the major GFP population is expected to be at the A-state, the A-state marker band (395 nm) is out range of the measurement. Here, we rely on multiple attributes of a plasmonic AgNP to observe the I-state of GFP chromophore. First, the intense Mie scattering is measurable from a single particle. Second, oscillator strength of RET is much stronger than that of optical absorption. Third, by plasmon-enhanced (surface-enhanced) optical pumping, the dynamic GFP population can be significantly shifted from the A-state toward its excited states, A*, I* and I, in the Förster cycle. Thereby, RET from a single AgNP to the I-state GFP adsorbates can be resolved from Mie scattering.

Dynamic quantitative analysis of biological molecules is indispensable to understand complex cellular processes and gain insights into disease mechanisms resulting from abnormalities of the organelles. Nanowire optical endoscopes, which utilize a nanowire waveguide to shuttle light into a target subcellular region without perturbing the outer cell membranes, hold great potentials to meet such ironclad requirements. The nanometer-sized internal light source from the waveguide to locally probe chemical environments can significantly suppress the background fluorescence, minimize photodamages and directly access subcellular regions with pL~fL sensing volumes. Combined with high-sensitivity molecular-specific SERS (Surface Enhanced Raman Spectroscopy) markers, the NW endoscope may lead to label-free, single-molecule, multiplexed sensing of target bio-species with high spatial and temporal resolution and provide a unique means of probing cellular dynamics. In this work, we report a proton-sensing SERS live-cell endoscope for the first time. By coupling excitation laser from a tapered optical fiber to a plasmonic nanowire with high crystallinity and atomically smooth surface, the laser can be delivered into the cell with a high efficiency leading to lower fluorescent background and photodamages imposed upon the cell by reducing the illumination area. In addition, the guidance of light as propagating surface plasmon polariton is diffraction-free and therefore, the reduction of the insertion volume of the probe can be fulfilled. Using an interfacial photothermal assembly method, we were able to assemble silver nanocubes, which are functionalized with a pH sensitive Raman marker to act as proton-specific, field- enhancing nano-cavity antennas, were integrated on the very tip of the nanowire waveguide. This highly integrated SERS endoscope can report intracellular pH accurately without damaging the cell membranes or perturbing normal cellular functions.

Colloidal particles exhibit intriguing collective behavior beyond their individual properties when they are organized into colloidal matter. Treating colloidal particles as individual atoms, researchers are exploring new strategies to build complex colloidal structures for new functional devices. In particular, the organization of colloidal particles with sizes comparable to the optical wavelength (also known as optical matter) will enable diverse novel optical performances, including structured color imaging, Fano resonator, and chiral metamaterials. However, the optical properties of colloidal matter are highly sensitive to the geometric configurations, which requires rigorous and precise structural control and gives rise to the challenge in the development of assembly approach.
Herein, we propose a light-directed particle-to-particle assembly technique - opto-thermophoretic assembly (OTA) - to build colloidal matter at a low optical power. To enable the optical manipulation and assembly, we add an ionic surfactant (i.e., cetyltrimethylammonium chloride) into the particle suspension, which serves as a surface charge source, a macro ion, and a micellar depletant above its critical micelle concentration. Taking advantage of the thermophoretic migration of different ionic species in the fluidics and the plasmon-enhanced photon-phonon conversion, we create a thermoelectric field to manipulate colloidal atoms at single-particle resolution. Specifically, the depletion of the ionic micelles provides a strong attractive force to achieve tunable inter-particle bonding. The OTA strategy is applicable to colloidal particles in a wide range of sizes, shapes, and materials.
The OTA strategy can release the rigorous design rules required in the existing assembly techniques and enriches the structural complexity in colloidal matter, which will open a new window for rational design of functional colloidal devices. As an example, we demonstrate the application of the OTA approach in the reconfigurable assembly of colloidal chiral metamolecules. Selecting plasmonic metallic nanoparticle and dielectric silicon nanoparticles as the building block, we succeeded in the assembly of colloidal metamolecules with optical chirality. Beyond the geometry chirality well known in traditional chiral materials, we demonstrate the composition chirality using colloidal particles with different materials, but with the same size. Specifically, the colloidal metamolecules can be switched between their left-handed and right-handed configurations through controlling the light field. In addition, we study the origin of chirality in the colloidal metamolecules by rationally changing their configurations, and thus controlling the inter-particle interaction.