Symposium EP12—Emerging Materials for Plasmonics, Metamaterials and Metasurfaces

An overview of the Electronic and Photonic Materials (EPM) program workshop, “Current Challenges and Future Opportunities in Electronic and Photonic Materials” published in the MRS Bulletin in December 2018 priorities will be presented. Various funding opportunities will be discussed.

The polarization of light is of interest in several technologically relevant fields ranging from chemical identification, where the differing handedness of a molecule can drastically alter its effects on the human body, to quantum computing, with the polarization of light serving as a qubit. Here, we report the polarization properties of L-shaped localized surface phonon polariton resonators etched into 6H-SiC, with modal Q-factors as high as 144. These high Q-factors allow us to identify three resonances and observe their evolution as a function of asymmetry, which would otherwise be muddled by the loss-induced broad linewidths in the comparable plasmonic structures. Full-wave electromagnetic simulations of the resonator reflection spectra and resonant charge distribution indicate that the low- and mid- energy modes are characterized by a longitudinal dipole and transverse dipole, respectively. This behavior is corroborated by polarization dependent reflectance measurements and simulations that reveal orthogonal excitation axes for these two modes for the symmetrical resonator. However, this orthogonality is broken as the relative leg length is increased. Interestingly, the simulated resonant charge distribution of the high-energy resonance suggests that it is associated with a higher order longitudinal mode. However, unlike the longitudinal dipole mode, the higher-order mode exhibits an anomalous polarization dependence that is characterized by a spectral weight that is only weakly dependent on the incident polarization. Furthermore, as this higher energy mode is tuned into resonance with the normally IR inactive zone folded longitudinal optical phonons, a coupling behavior is observed between the surface phonon polariton mode and the bulk phonons, with the resonant intensity of the phonon modes following the polarization dependence of the higher energy mode.

We discuss photonic material and design challenges facing future solar sail based interstellar missions. We discuss extreme conditions of solar radiation and corona that sails are exposed to during a solar gravity assist phase and outline designs that enable sustainable sail designs.
At present deep space exploration is limited by transit time and overall mission costs. Conventional space transportation systems typically employ chemical or solar electric propulsion, which have effectively reached the limit of their maximum performance potential; further development activities focus mainly on improving reliability and reducing cost. Recent studies show that solar sails with high area-to-mass ratoon and less than 0.1 AU perihelion approach (i.e., <20 solar radii) may enable missions with >30 AU/year velocities (i.e., more than 8 times faster than the Voyager – fastest space probe we’ve ever built). Achieving such velocities will pave the way to a new era of affordable and scalable deep space exploration. However, such solar sail based missions necessitate a very close pass near the sun, where the sail is exposed to extreme conditions of solar radiation and corona. Here, we study photonic properties of sail materials and discuss criteria needed for the design of such extreme solar sails.
First, we show that conventional polymer based sails cannot withstand high solar flux and justify the need for novel sail materials. We discuss necessary criteria and show that refractory dielectrics, such as diamond, boron nitride (BN), silicon dioxide (SiO₂) and titanium dioxide (TiO₂), owing to their low absorbance of solar spectrum and high melting temperatures are well suited as potential sail material candidates.
Next, we consider photonic designs that optimize reflectance and hence the radiation pressure force. We highlight the mass – reflectance tradeoff and show that sail materials with ~1 g/m² may be designed. We further outline conditions needed for maximizing excess velocity and, hence, minimizing mission time. Our estimates show that sails based missions with >20 AU/year are feasible.
Finally, with radiative cooling being the sole mechanism for thermal management, we study nanopatterning of the sail materials to increase their thermal emissivity in the infrared. We show that with a proper photonic design polar resonances of the aforementioned materials may harnessed to obtain relatively large emissivity and achieve sustainable sail temperatures, below the melting point of its constituent materials.

Recent years have witnessed the rapid development of flat optical elements, known as metasurfaces. With properly designed and arranged sub-wavelength structures over its plane, a metasurface can impart arbitrary, spatially variant amplitude, phase and polarization modulations on an incident electromagnetic wave. The highly customizable nature of a metasurface allows it to accomplish a variety of functions that have traditionally been fulfilled by a combination of different optical elements, such as gratings, lenses, polarizers, wave-plates, beam splitters, and hologram plates, with a significantly reduced physical thickness compared to traditional optical elements. So far, researchers have demonstrated various types of high-performance metasurfaces operating in the infrared (IR) and visible regimes, paving the way towards high-efficiency, multi-functional and compact photonic systems.

However, there has been a conspicuous lack of work in the ultraviolet (UV) region, which is the spectral range hosting an array of important applications such as photolithography, spectroscopy, sterilization, astronomy, medical therapy, and high-resolution imaging. High-performance metasurfaces working at IR and visible frequencies employ dielectric materials such as Si, TiO₂, and GaN. Direct scaling of operation frequencies of metasurfaces based on these material systems down into the UV is challenging because of the intrinsically lossy response of such materials at wavelengths below their bandgap, preventing high output efficiencies.

Here, we show how low-loss metasurface devices operating at UV wavelengths down to the deep-ultraviolet range can be implemented using Hafnium Oxide, an amorphous dielectric material most commonly exploited as a high static dielectric constant (high-k) material in integrated circuit fabrication, that is characterized by a wide-bandgap (5.8 eV). We demonstrate a variety of polarization-independent metasurface devices, including lenses, holograms and self-accelerating beam generators, operating at two near-UV wavelengths (364 and 325 nm) with efficiencies as high as 75%. Scaling down metasurface critical dimensions, we realize holograms operating with high efficiencies of 60% at a record-short, deep-UV wavelength of 266 nm. Finally, we demonstrate spin-multiplexed holograms and self-accelerating beams at 364 nm, and spin-multiplexed holograms at 266 nm.

Bulk metals are generally reflective and are therefore commonly overlooked as efficient absorbers; however, the fields of plasmonics and metasurfaces have given new insights into methods for turning reflective surfaces into absorbing ones. Another recent approach has been the use of subwavelength Fabry-Perot-like resonances in ultra-thin films, which has been used to achieve absorption above 70%, approaching the theoretical limit using traditional substrates. Here we take a different approach and show that near-perfect absorption is achievable provided that the ultra-thin metals are deposited on an index near zero (INZ) substrate. In this talk, I will present the various design considerations that allow for ultra-thin metal films on INZ substrates to obtain near-perfect absorption throughout the visible spectrum and into the near-infrared (NIR). We find that metals commonly used for plasmonics and hot carrier devices, such as Au and Ag, can obtain near-perfect absorption for near-ultraviolet and visible wavelengths, while metals such as Pd and Pt are efficient absorbers throughout the near-ultraviolet to near-infrared spectrum. Finally, I will present results where we use this mechanism to enhance the photocurrent of photodetectors based on hot carrier generation in thin metal films.

High contrast grating (HCG) metasurfaces are promising alternatives to Bragg filters as wavelength selective mirrors. HCG metasurfaces consist of a single material layer with a high index of refraction (n>3) patterned at a subwavelength thickness directly onto a lower index (n=1.5) substrate. Bragg filters are highly efficient in reflection, but suffer from a strong dependence on angle of incidence. Moreover, Bragg stacks use many layers of different indices to manipulate light, resulting in an expensive fabrication process. In HCG metasurfaces, the high contrast between the respective materials’ refractive indices enables scattering and mode interference that results in a sharp and tunable reflection peak.
We explore the design and fabrication of HCG metasurface mirrors using three different materials: aluminum antimonide, silicon, and silicon carbide. We optimize our design for application in a planar luminescent solar concentrator comprising mirrors cladding a planar waveguide with embedded quantum dot luminophores emitting at a peak wavelength of 635 nm design to reflect quantum dot luminescence emission that couples with the band edge of a III-V photovoltaic cell. We fabricated HCG mirrors on glass substrates via sputtering or PECVD and patterning via e-beam lithography and a dry etch. We find that the ideal HCG for a peak centered at 635 consists of a hexagonal array of cylindrical pillars with the radius and height approximately 100 nm and a pitch of roughly 495 nm. The reflectance peak of these materials ranges from 80% to 95% at 635 nm.

Atomically thin two-dimensional (2D) transition metal dichalcogenide (TMDC) materials present a remarkable excitonic systems with exciton binding energy usually one order of magnitude higher than those in typical semiconductor materials. As a result, these materials bear great potential to enable the development novel optical and optoelectronic devices. Here we demonstrate some exotic light-matter interaction properties of 2D TMDC materials that result from the extraordinary strong exciton binding energy. This include: 1) giant gating tunability in the optical refractive index of 2D TMDC materials, in which a tunability > 60% is achieved in the refractive index of 2D TMDC materials by electrical gating with a configuration compatible with CMOS devices; 2) room-temperature condensation, where the gas-like excitons may condense into a liquid-like state, electron-hole liquid (EHL). We will also demonstrate the magneto-optical properties enabled by magnetic TMDC materials.

The field of optical meta-surfaces is rapidly growing due to its great potential to enable thin optics implementation with relatively complex and flexible functions. In particular, high power laser systems could benefit from optical meta-surfaces that implement beam shaping, e.g., for wave-front aberration correction, but with the advantages of smaller accumulation of nonlinearity and lighter weight. Additionally, meta-surface technology could enhance laser optics with improved anti-reflective layer designs.
Current meta-surface technology is limited with respect to high power laser optics, which requires both scalability and laser intensity durability. The principal challenge arises from the necessity of patterning sub-wavelength features (to control the local optical properties by modifying geometrical properties) while being able to modify the structural parameters on the large optics scale used in high power laser systems (e.g., National Ignition Facility, Laser MegaJoule).The current patterning methods are either limited in scalability (e.g., FIB, e-beam lithography) or limited in robustness due to the usage of soft-materials (e.g., nanoimprint).
We are developing novel technology capable of generating robust and scalable all-dielectric based meta-surfaces. In this talk we will describe the method, show results of fabricated meta-surfaces, and discuss the various levels of control that we have with this process. This method extends the current application field of interest for meta-surfaces to high power lasers, with the potential to stimulate meta-surface utilization in additional fields requiring large and robust optics.
This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. LLNL-CONF-760584.

Plasmonic metasurfaces provide great flexibility to control the amplitude, phase and polarization response of light in broad wavelength ranges from UV to THz, thus hold promise to realize ultra-compact chip-integrated photonic and optoelectronic devices for various applications. Chiral plasmonic metamaterial as artificial counterpart for the rare chiral material in nature enables generation, manipulation and detection of circularly polarized (CP) light. Plenty of works have demonstrated circular dichroism spectroscopy and circularly polarization detection based on chiral plasmonic metasurfaces². Besides, full Stokes parameters detector have been proposed based on phase gradient arrays³, aperture antenna⁴ and in-line scatters⁵. Researchers are putting more efforts to develop new devices for high efficient and high performance polarization detection.
Here we present our experimental demonstration of highly efficient (>85% transmission efficiency) chiral metasurface structures as circular polarization (CP) filters with extinction ratio over 50 (defined as the ratio between transmission of CP light with the desired handedness and that of the other handedness). The proposed structures are composed of rationally designed plasmonic antennas and nanowires, which are vertically intergrated with a subwavelength-thick dielectric spacer layer. The total thickness of the device can be less than 1/10 of the operation wavelength. We investigated the design principle with anisotropic transfer matrix method and confirmed the design concepts with full wave simulation. According to our theoretical analysis, one can achieve CP filters with extinction ration over 1000 and transmission efficiency over 90%. In experiment, we have achieved over 85% efficiency and extinction ratio (r=T_LCP/T_RCP) over 50 at 4 µm. We have also integrated the CP polarization filters with nanowire grating linear polarization filters on the same chip for full stokes polarization detection. The measurement errors of our devices are 2%, 4% and 5% for S1, S2 and S3, respectively. And the errors for DOLP and DOCP are 5.4% and 6.6%, respectively, which to our knowledge are the best among the metasurfaces based polarization detection techniques presented in literature so far. The operation wavelength of the device can be engineered from NIR to FIR (1 µm to 30 µm) by simply changing the design parameters. Our designs can be directly integrated onto various semiconductor-based photodetectors and imaging arrays; thus enable on-chip polarization detection and imaging for various applications such as circular dichroism (CD) spectroscopy, polarimetric imaging and sensing, and molecular spectroscopy.

1. Hou-Tong, C.; Antoinette, J. T.; Nanfang, Y. Reports on Progress in Physics 2016, 79, (7), 076401.
2. Zheng, G.; Mühlenbernd, H.; Kenney, M.; Li, G.; Zentgraf, T.; Zhang, S. Nat Nanotechnol 2015, 10, (4), 308.
3. Khorasaninejad, M.; Chen, W. T.; Devlin, R. C.; Oh, J.; Zhu, A. Y.; Capasso, F. Science 2016, 352, (6290), 1190-1194.
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6. Martinez, N. J.; Gehl, M.; Derose, C. T.; Starbuck, A. L.; Pomerene, A. T.; Lentine, A. L.; Trotter, D. C.; Davids, P. S. Opt Express 2017, 25, (14), 16130-16139.
7. Garcia, N. M.; de Erausquin, I.; Edmiston, C.; Gruev, V. Opt Express 2015, 23, (11), 14391-14406.

Tunable metasurfaces enable active control of key constitutive properties of light at a sub-wavelength scale. So far, electrically-tunable active metasurfaces in the near-infrared wavelength regime have been typically achieved by incorporating active materials, such as tunable plasmonic structures or thermo-optically responsive media, into otherwise passive metasurface structures <!--[endif]---->[1-5]. To achieve a widely tunable response at low input power and high speeds in active metasurface structures, electro-optical modulation of resonantly excited quantum well structures can enable strong light-matter interactions by couple of an optical field enhancement in an active material that can also undergo a large DC field modulation.
Here, we report an all-dielectric active metasurface platform based on electro-optically tunable III-V multiple-quantum-well (MQW) Mie resonators. In this work, we design an epitaxial III-V compound heterostructure, comprising a distributed Bragg reflector (DBR) and a 1.23 μm-thick MQW layer grown on GaAs substrates. The DBR, which is used as a dielectric mirror, is comprised of 20 pairs of alternating layers of n-Al_0.9Ga_0.1As (76.5 nm) and n-GaAs (65 nm) with the n-Al_0.9Ga_0.1As as the topmost layer. We exploit the quantum-confined Stark effect in GaAs-based MQW Mie resonators to actively control the optical response of the metasurface phase and amplitude at near-infrared wavelengths. By applying a DC electric field across the Mie resonators, we dynamically modulate modes supported by the Mie-resonant metasurface elements. We experimentally observe a relative reflectance modulation of 270% accompanied with a continuous phase shift from 0° to 70°. In addition, we use our all-dielectric tunable metasurface to demonstrate a dynamically switchable diffraction grating. By selectively applying an electrical bias to metasurface elements, we can electronically modulate the metasurface period, enabling on-off switching of the first-order diffracted beam. We will also discuss how our tunable dielectric metasurface platform can be used for the realization of dynamically tunable ultrathin optical components, such as tunable metalenses, on-chip beam steering devices, active polarizers, and flat spatial light modulators.

References
1. G. Kafaie Shirmanesh, R. Sokhoyan, R. A. Pala, and H. A. Atwater, "Dual-gated active metasurface at 1550 nm with wide (>300°) phase tunability," Nano Lett. 18, 2957-2963 (2018).
2. J. Park, J.-H. Kang, S. J. Kim, X. Liu, and M. L. Brongersma, "Dynamic reflection phase and polarization control in metasurfaces," Nano Lett. 17, 407-413 (2017).
3. A. Howes, W. Wang, I. Kravchenko, and J. Valentine, "Dynamic transmission control based on all-dielectric Huygens metasurfaces," Optica 5, 787-792 (2018).
4. Y.-W. Huang, H. W. H. Lee, R. Sokhoyan, et. al., "Gate-tunable conducting oxide metasurfaces," Nano Lett. 16, 5319-5325 (2016).
5. J. Sautter, I. Staude, M. Decker, et. al., "Active tuning of all-dielectric metasurfaces," ACS Nano 9, 4308-4315 (2015).

Refractory nitride plasmonics offer the potential to realize enhanced light interactions for energy harvesting and photo-driven chemistry with materials systems that are thermally rugged, inexpensive, and potentially catalytic. Here, we have embedded commercial and in-house synthesized titanium nitride (TiN) nanoparticles into matrixes of titanium dioxide (TiO₂) and compared their ability to enhance electrochemical oxidation reactions to that of conventional gold (Au) nanoparticles. Although the photon-to-carrier conversion efficiencies were low (~10^-4 %), the reaction rates were enhanced by a factor of 4 in the visible and near-infrared for Au and TiN, respectively, compared to a pure TiO₂ control. The spectral dependence of reaction rate enhancement followed the nanoparticle extinction spectra and a linear power-dependence identifies a photo-excited carrier mechanism (i.e., decaying plasmons excite carriers which participate in chemistry rather than heating of the system). Lastly, photo-induced transients of the electrochemical signal are found to be consistent with the band structures of these heterosystems. Specifically, the TiN/TiO₂ system, which has little or no Schottky barrier, exhibits a bias-dependent photoelectrochemical response rate while the Au/TiO₂ system, which naturally forms a Schottky barrier that immediately separates charged carriers, exhibits a near-instantaneous response.

In this work, it was synthesized and characterized Lithium Borate Glasses doped with rare earths and containing Silver nanoparticles in different concentrations. The rare earths employed were Dy3+ and Yb3+. The Scanning Electron Microscope (SEM) show the formation of Silver nanoparticles, absorption spectra of the samples show the presence of bands in 420nm and 450nm associated with the SNP (Plasmon effect), and 750nm, 800nm, 875nm, 1098nm and 1278nm belonging to the Dy3+ and one large peak in 976nm belonging to the Yb3+. Emission spectra show two prominent bands in 480nm, 574nm, and one faint band in 665nm, all bands under 364nm pumping, and the fluorescence in the 550nm and 590nm spectral range enhanced two times. It was studied the Plasmon Effect due to the increment of the SNP in the samples.

The fascinating optical properties of noble metal nanocrystals are intriguingly offered by their localized surface plasmon resonances. Hybridized plasmon modes can be produced by placing two plasmonic nanocrystals close to each other. The electromagnetic interaction between the hybridized modes possesses unique plasmonic features, such as electromagnetically induced transparency, Fano resonance, magnetic plasmon resonance, and charge transfer plasmon (CTP). Plasmonic Fano resonance arises from the destructive interference between a broad, superradiant and a narrow, subradiant plasmon mode that overlap spectrally with each other. It features an asymmetric non-Lorentzian spectral profile with a narrow transparent window corresponding to the energy of the subradiant plasmon mode. The unique properties of Fano resonance help in revealing the underlying physics in many intriguing phenomena and make it promising for many plasmon-based applications, such as sensing, plasmonic switching, light slowing and stopping. However, most metal nanostructures can only support Fano resonances with relatively shallow dips in the visible and near-infrared ranges. Second, when the air or vacuum gap between two metal nanoparticles is smaller than ~0.3 nm, the classical electromagnetic framework breaks down and the quantum tunneling effect alters the plasmon coupling behavior substantially. A new CTP mode emerges as a characteristic signature of the interparticle electron tunneling across the junction. However, it has still been challenging to fabricate plasmonic nanocavities with ultrasmall gaps. In this regard, bridging molecular junctions have been introduced to allow electrons to tunnel across the gap with a moderate distance. Nonetheless, the observed CTP has been limited with dipolar plasmon modes.

We have fabricated gold nanoplate (NPL)–nanosphere (NS) heterodimers and observed Fano resonance with a deep dip (Nanoscale 2017, 9, 13222). When a gold NS is attached to one sharp vertex or one edge of a gold NPL on an indium tin oxide (ITO) substrate, a strong polarization-dependent Fano resonance with a deep dip is observed in the scattering spectrum. Under in-plane excitation that is polarized perpendicular to the connection axis between the NPL and NS, the Fano resonance can be “switched off”. In addition, by positioning a gold NS at the top facet of a gold NPL on a silicon substrate, the dipole-octupole interaction between the NS and NPL also gives rise to a deep Fano dip under out-of-plane excitation (Nanoscale 2016, 8, 17645). The dip depth and asymmetric line shape of the Fano resonance possessed in both structures can be well tailored by varying the sizes of the involved nanocrystals. When the gold NS and NPL are separated with a self-assembled monolayer of conductive molecules, electrons are allowed to transfer across the molecular junction. The electron tunneling effect largely modifies the Fano resonance feature in the scattering spectra, as well as the observation of a higher-order CTP.

Taken together, these results will be helpful for deepening the understanding of Fano resonance and electron tunneling in plasmon-coupled systems, picturing a promising future in developing various potential applications, including nonlinear and switchable metamaterials, displays, nanoantennas, optoelectronics, and new types of molecular-plasmonic devices.

We experimentally investigate the electrically tunable reflectance of an Ag/Al₂O₃/indium-tin-oxide (ITO) planar heterostructure, where optical modulation is facilitated by reflectance changes induced by metal ionic transport from the Ag electrode to the ITO layer under applied bias. The Ag/Al₂O₃/ITO planar heterostructure exhibits tunable optical response at ultralow bias voltages on the order of 1 mV. Remarkably, optical modulation caused by bias-induced migration Ag ions into Al₂O₃ and ITO layers can occur with a millivolt-scale applied bias [1]. In the present work, we investigate the key physical mechanisms that yield optical modulation at these record-low bias voltages. Our structure consists of an 80 nm-thick planar Ag electrode, a 5 nm-thick Al₂O₃ layer, and an ITO counter-electrode. The thickness of the ITO counter-electrode is found to affect the tunable optical response of Ag/Al₂O₃/ITO planar heterostructure, and to explore these phenomena we designed and characterized heterostructures with ITO thicknesses ranging from 10 nm to 110 nm. For a specific ITO layer thickness, we fabricated three different Ag/Al₂O₃/ITO heterostructures with different ITO carrier densities, which is expected to result in variation of the ITO work function thus modifying the built-in direct current (DC) electric field in the Al₂O₃ layer. Moreover, increasing (decreasing) the ITO carrier density would result in an increase (decrease) of the oxygen vacancy density in the ITO layer. To gain further insight into the details of the migration of Ag ions into ITO layers for several carrier densities, we perform atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) measurements under applied bias. Finally, we study the tunable optical response of a patch antenna-type plasmonic resonant cavity, which consists of an Ag/Al₂O₃/ITO planar heterostructure on which we lithographically define Ag patch antenna. Understanding the key defining factors behind the millivolt-scale optical modulation in Ag/Al₂O₃/ITO plasmonic heterostructure would enable realization of actively tunable metasurface based on bias-induced ion migration at ultralow applied electric fields.

[1] K. Thyagarajan, R. Sokhoyan, L. Zornberg, and H. A. Atwater, Advanced Materials 29, 1701044 (2017).

Early and accurate diagnosis of cancer is extremely important to cancer treatment and improving patients’ survival rate. Liquid biopsy is an emerging approach that detects a panel of biomarkers that are available in bodily fluids, and potentially allows noninvasive diagnostics of a broad variety of cancers. Among the most studied biomarkers, microRNAs (miRNAs, single-stranded oligoribonucleotides) are viewed promising candidates, as they function as both oncogenes and tumor suppressors. The dysregulation of miRNAs has been shown to strongly correlate to the proliferation of cancer cells in various types of cancers. Current major approaches for miRNA detection rely heavily on enzymatic reactions that usually introduce biases, demand special instrument, and require long processing time. On the other hand, plasmonic nanoantennas have attracted considerable attentions due to its ultrahigh sensitivity, design flexibility, label-free detection and low instrument cost. However, miRNAs only have relatively small modulation of refractive index near the sensor surface due to their short sequence (~20 bp, or <7 nm in length), and hence usually results in small optical signals that is insufficient to accurate diagnosis.
Here we present a vertically coupled complementary structure, consisting of a nanobar antenna and a perforated nanoslit aperture antenna separated by SiO₂ nanopillar. This structure has shown a high sensitivity of 136 nm shift in detecting assembled 1-octadecanethiol in the mid-IR range (ACS Nano, 2017, 11 (8), pp 8034–8046). In our ongoing work, we design nanoantennas at visible and near-IR wavelength range, and optimize its sensitivity through vertical coupling between the nanobar and aperture antennas and horizontal coupling between adjacent nanobar antennas. Our full-wave simulation shows that the captured miRNA on the sensor surface can lead to ~23-30 nm shift upon miRNA hybridization to DNA probes. In our preliminary experiment, we assembled thiolated single-stranded DNA probes to the sensor surface, and demonstrated 13 nm resonance shift upon miR-10b hybridization at 100 pM concentration, which is comparable to the state-of-the-art research (~8.5 nm at same concentration). Currently we are evaluating the detection limit of the miR-10b on our plasmonic nanoantennas and studying the specificity using different probe designs. We expect to update our results at the conference. Our concept of plasmonic nanosensing can be widely extended to more complex structures and used for multiplexed detection, which have broad applications in early, low-cost, and portable cancer diagnosis.

With the recent development of precision lithography such as EBL, FIB, and EUV technique, it now became possible to overcome the stringent requirements of nano-scale offset, high step-coverage and sub-wavelength resolution [1], in the fabrication of nano-photonic structures. These efforts, especially toward visible regimes, have also enabled promising applications such as display [2], antenna [3], color filter [4-5] and information encoding [6-7] over recent years.
Nonetheless, in view of the costly, non-scalable, equipment dependent and time-consuming processes involved in the precision lithography techniques, the realization of cost effective, litho-free, and large-area fabrication of nano-photonic structures still remains as a challenge.
In this work, we present a colloidal lithography which uses metallic spheres as a patterning mask, to achieve hybrid metal-dielectric-metal (MDM) structures of controlled geometry in a large-scale. By adjusting the etching conditions between the metal- or dielectric preferential settings, the eccentricity of metal spheroid and height of dielectric pillar are adjusted for the fine tuning of coupling strength between the residing hybridized modes, and therefore to deliver significant modulation of the reflection spectra in the visible regime (400nm ~ 800nm). Control of the full width at half-maximum (FWHM) bandwidth from 50nm to 250 nm, and near-perfect absorption (> 95%) at the resonance wavelength are demonstrated, for future applications in colored light harvestings, anti-counterfeitings and anti-reflecting coatings. As a simplest demonstration, we will present a large-area (> 4 inch) and full gamut color filter, with iridescent response. Our result of large-area, litho-free colloidal metal-particle mask technique will offer a new path in the fabrication of cost-effective and fabrication-friendly, precision nano-photonic devices.

REFERENCE
[1] Zhu, X., et al. Nat. nanotech. 11, 325-329 (2016)
[2] Tasi, Y., et al. ACS Appl. Mater. Interfaces 9(41), 36045–36052 (2017)
[3] Lin, F. H., et al. IEEE. 65(4), 1706-1713 (2017)
[4] Miyata, M., et al. Nano Lett. 11, 4419-4427 (2016)
[5] Yang, C., et al. Adv. Opt. Mater. 4, 1981–1986 (2016)
[6] Park, K., et al. Adv. Funct. Mater. 1800369 (2018)
[7] Nagasaki, Y., et al. ACS Photon. 5, 1460−1466 (2018)

Metal-assisted chemical imprinting (MAC-imprinting) is a combination of metal-assisted chemical etching (MACE) and nanoimprint lithography (NIL) that can selectively etch arbitrary and complex 3D microscale features in silicon wafers such as parabolas, parabolic cylinders [1], inverted pyramids [2], rectangular grooves and holes [3]. The patterning resolution of this process can be controlled by varying etching parameters such as solution composition or external bias [4] down to the sub-5 nm range. Thus, this process can deliver mirror-finish surfaces suitable for manufacturing of optical elements such as lens and gratings in which low surface roughness is critical. In this paper, we demonstrate MAC-imprinting of diffraction gratings onto Si wafers and examine the pattern fidelity and roughness across stamp and substrate. MAC-imprinting stamps consisting of patterned SU-8 with periodic linear grooves were prepared using UV-NIL. Patterned SU-8 was then coated with a Cr adhesive layer and an Au catalytic layer by magnetron sputtering, and used in MAC-imprinting of p-type low-doped (100) Si wafer. Visual examination of imprinted Si chips revealed surfaces exhibiting diffraction pattern similar to its corresponding stamps. Analysis of SEM data determined a width of 504 nm and period of 902 nm of the imprinted groove corresponding to the same values of stamp features. According to AFM-analysis surface roughness of the grooves was approximately 18 nm (RMS) matching the surface roughness of the MAC-imprinting stamp features. Thus, the origin of the roughness was attributed to the grain size of the sputtered Au layer rather than to the MAC-Imprint process itself. As a result, MAC-imprinting of diffraction gratings onto Si surface was successfully performed and the imprinted groove’s width, period and surface roughness was found to match the corresponding values of the imprinting stamp. This result serves to validate the use of MAC-imprinting to fabricate optical elements with superior quality.
Bibliography
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Possibility to control spontaneous emission via modified optical environment provides new opportunities for magnetic dipole transitions, which are commonly very weak in comparison with electric transitions.
We discuss various approaches for enhancing magnetic transitions. In particular, we concentrate on effect of propagating surface plasmon polaritons on emitters located in very close vicinity of plasmonic metal.
In comparison with electric dipoles, magnetic emitters have lower quenching, and practically the same efficiency of SPP excitation. This prediction is confirmed by experiment using spontaneous emission of Eu ³⁺ ions at magnetic dipole and electric dipole transitions.

With the increased use of electronics over the broad frequency spectrum from microwaves to millimeter waves, high-performance absorbing and shielding materials are needed to ensure electromagnetic wave control and compatibility. Acquisition of wide bandwidth absorption with a planar layer and a small layer thickness has been a major challenge for commercial and military applications. Unfortunately, it is a difficult task to achieve both bandwidth enhancement and thickness reduction simultaneously because of an inverse proportional relationship among the bandwidth, thickness, and reflectance of the absorbers. In this study, the design of a thin and ultra wide-bandwidth microwave absorber has been conducted by layering two frequency selective surfaces (FSS) with different patterns (square loop, patch) and geometries on the grounded dielectric substrate. The circuit parameters of inductance and capacitance of the FSS are retrieved using the equivalent circuit model and utilized in the design of the wide-bandwidth absorber. Optimal design for the surface resistance of the FSS and spacer thickness of the double-layer absorber provides very large absorption bandwidth and small total thickness (for FSS of square-loop geometry, for instance, 10 dB bandwidth is 5.2 – 38.3 GHz and total thickness of the absorber is 6.3 mm, which is very close to the theoretical limit). Admittance analysis for substrate (layer thickness) and FSS (patterns, geometries) is made for the design scheme of the ultrawide bandwidth absorber. The free space measurement result with a test sample prepared by the screen printing method was in good agreement with the simulation result and strongly verified the validity of proposed design method. For these periodic array structures, however, the grating lobes or high frequency harmonics were observed in the high frequency region above which the unit cell periodicity is larger than the wavelength, and as the incidence angle increased, the grating lobe occurred at a lower frequency for both TE and TM polarization. The unit cell periodicity must be controlled in the periodic array structure, particularly for large oblique incidence angles.

One of the goals of nanotechnology is to develop complex molecular machines that can be operated in a solid-state environment. This talk will present molecular motors and molecular linear transport devices operating in the quantum regime on materials surfaces. Fundamental operations of these machines are investigated in an atomically clean environment using low temperature scanning tunneling microscopy, and molecular manipulations. These investigations reveal how charge and energy transfer are taken place within single molecule machines and molecular networks [1,2]. Moreover by introducing dipole active components in the rotor arms, communication among the molecular motors can be introduced [1]. Synchronization of the motors can be achieved depending on the symmetry of the molecular assemblies and the strength of the electric field. For the linear transport, we will present the development of molecular cars for a control transport at the nanoscale.
1. Y. Zhang, H. Kersell, R. Stefak, J. Echeverria, V. Iancu, U.G.E. Perera. Y.Li, A. Deshpande, K.-F. Braun, G. Rapenne, C. Joachim, and S.-W. Hla. Nature Nanotechnology, 2016, 11, 706.
2. U.G.E. Perera. F. Ample, H. Kersell, Y. Zhang, G. Vives, J. Echeverria, M. Grisolia, G. Rapenne, C. Joachim, and S.-W. Hla. Nature Nanotechnology, 2013, 8, 46.

A revolution in materials discovery has yielded a diverse portfolio of new classical and quantum photonic materials. In particular, a variety of two-dimensional layered architectures can be crafted with structural precision approaching the atomic scale. In parallel, advances in nanophotonics, plasmonics, and metasurfaces have enabled precise control of light-matter interactions down to the nanoscale. By combining concepts from both of these young and rapidly developing fields, there is now an opportunity to create photonic matter where optical properties are tailored to attain thermodynamic
limits. Here, we present our research exploring ultrathin nanostructured materials and metasurfaces for bright quantum emission and nonreciprocal optical transmission. First, we correlate the local atomic structure of color centers in van der Waals materials with their optical emission at a single defect level. We use transmission electron microscopy and cathodoluminescence spectroscopy to investigate color centers in two-dimensional hexagonal boron nitride. This wide bandgap material is capable of room-temperature, high-brightness visible quantum emission, though the atomic structure of color centers remains unknown. Through high-resolution spatial maps, we find that multiple quantum emitters reside within a ~100nm region, each contributing slightly distinct spectral signatures. Defects are found in locations usually associated with regions of high strain and fork dislocations. Through correlation of cathodoluminescence to photoluminescence, we show that two classes of emitters exist, including charge neutral and charged emitters. Importantly, all are stable upon multiple exposures to the electron beam.

Next, we introduce nanophotonic designs that enable nonreciprocal transmission of visible and near infrared light within subwavelength optical paths. We show that the Kerr effect, in which the refractive index depends on the local light intensity, can produce passive directional transmission. In this case, slight structural asymmetries lead to a directionally dependent field enhancement and, consequently, a directional Kerr shift of the resonant dip. We achieve nonreciprocal transmission for silicon films as thin as 100nm with incident powers of a few kW/cm². We additionally show a nonreciprocal Kerr shift in a phase gradient metasurface, making nonreciprocal beemsteering and lensing possible. This platform enables time-reversal-symmetry breaking for arbitrary free-space and modal optical inputs in a simple, robust materials platform, with potential for creating nanoscale optical diodes and photovoltaic devices that violate Kirchhoff’s law.

Despite their nanometer thickness, metasurfaces comprised of resonant metallic or dielectric nanostructures offer comprehensive control over light fields and allow for the creation, detection and transformation of light. Often, their optical functionality is only obtained at a fixed wavelength, determined by the geometric design and the material properties. For optimal functionality, they need to be freely programmable and have low optical losses.
Phase-change materials (PCMs) provide a switchable dielectric environment for resonant nanostructures, altering their resonance frequencies in a non-volatile, reversible way. PCMs have a high optical contrast between their amorphous and crystalline phases, while only exhibiting low optical losses in the infrared [1,2]. Using PCMs for active metasurfaces offers adjustable, designed functionality for manipulation of light. Among recent examples are polarization filters [3], beam steerers and lenses [4]. During crystallization of the PCM on a hot plate, intermediate states of the PCM have also been exploited to obtain gradual resonance shifts [5,6]. These shifts due to partial crystallization are uniform over the whole metasurface and usually described by effective medium theory.
We now show how we can simultaneously control the size, position and crystallization depth of the switched phase-change volume within each meta-atom to selectively tune its optical properties. By using visible light for switching an infrared metasurface, we could modify the PCM phase on length scales which are smaller than the size of a meta-atom. When combining this with present designs for infrared metasurfaces, it will become possible to program the reflection amplitude and phase of each individual meta-atom.
This goes beyond effective medium concepts, especially when accounting for the spatially localized electric near-fields of nanoantennas in the meta-atoms. With the proposed concept, it should be possible to actively impart multiple complex functionalities onto the same metasurfaces or to make small adaptations to correct external aberrations and fabrication errors.

References
[1] A.-K. U. Michel et al., Nano Lett. 13, 3470 (2013).
[2] M. Wuttig et al., Nat. Photon. 11, 465 (2017).
[3] X. Yin et al., Nano Lett. 15, 4255 (2015)
[4] X. Yin et al., Light Sci. Appl. 6, e17016 (2017)
[5] Chen, Y. et al., Sci. Rep. 5, 8660 (2015).
[6] Chen, Y. G. et al. Opt. Express 21, 13691 (2013).

Chalcogenide Phase-Change materials (PCMs) exhibit high electro-optical contrast between their amorphous to crystalline states. The switching between such states can be quickly and reversibly controlled on demand, by making use of different heat stimuli such as electrical, thermal or optical pulses [1]. For the last couple of decades, PCM applications have been therefore focused on the development of non-volatile electrical and optical memories [2]. More recently, however, PCMs have attracted much interest as a tuneable element in plasmonic optical metasurfaces [3, 4], due to the fact that PCMs have very different refractive indices when in amorphous and crystalline states, but can also provide intermediate crystallization levels having optical properties lying between those of the two structural phases. In this work, therefore, the potential of phase-change “meta-devices” is illustrated via the introduction of various device prototype concepts in which the state (fully-crystalline, partially crystalline or fully amorphous) of the PCM layer can be used to control device functionality.
As a first example, by appropriately combining plasmonic metasurfaces with the well-known phase-change alloy Ge2Sb2Te5, we have developed a dynamic beam steering meta-device working in the near infrared (specifically at l = 1550 nm). As a second proof-of-concept, we demonstrate that thin films made of GeTe (another widely employed PCM), can be used to create broadband amplitude-only modulators in the visible regime. In such devices, the amount of light reflected can be controlled via changing the fraction of crystallization of the GeTe active layer, without introducing any optical phase shifts during the process (i.e. we can create an amplitude-only modulator). Finally, we show that optical phase manipulation is also possible over a 2π range, by combining novel low-loss PCMs (here Sb2S3) [5] with all-dielectric asymmetric Fabry-Perot cavity inspired metasurfaces [6]. Our approach here consists of arrays of geometrically identical elements (or meta-atoms) where the optical phase can be locally tuned between 0 and 2π at a wavelength of l = 850 nm (commonly used in biomedical diagnostics) by changing the level of crystallization Sb2S3 in a pixelated device.
We believe that PCM-based metasurfaces and meta-films, such as the ones presented here, could be well-suited for many exciting applications, such as optical telecommunications, LIDAR scanning systems, autonomous vehicles, reflective displays, holography or wavefront shaping in biological tissues.

References:
[1] D. Loke et al., Science 2012, 336, 1566
[2] M. Wuttig, N. Yamada, Nat. Mater. 2007, 6, 824.
[3] B. Gholipour et al., Adv. Mater. 2013, 25, 2050.
[4] C. Ruiz de Galarreta et al., Adv. Funct. Mat. 2018, 28, 10.
[5] W. Dong et al., arXiv:1808.06459 [physics.optics], 2018.
[6] C. Ruiz de Galarreta et al., ISCAS 2018 (Florence, Italy)

Optical devices applied in research and industry environments utilize wavelength filtering to achieve optical responses necessary for proper function. This applies to lasers, sensory equipment, and other optical signal generators and detectors. Most optical signal filters require power during usage and/or modification, and so the development of a power-free reconfigurable photonic alternative is desired. We realize Mg-based nanophotonic color pixels with transient functionality, where triple-stack Mg/MgO/Mg thin films filter visible light based on nanocavity interference. We achieve transient color pixels, which can be fabricated to transmit any hue within the sRGB gamut. These pixels maintain their ability to transmit color up to an angular range maximum of 80 degrees with respect to an incident beam independent of sample rotation. We determine the transient optical response of the Mg-based pixels during their dissolution in water under pH neutral, room temperature conditions. The functionality of these transient pixels extends into the realms of encryption and anti-counterfeiting measures by allowing for the complete negation of an optical response in under 10 minutes as a result of exposure to water. Our spectroscopic ellipsometry experiments are corroborated by 3D full-field computation using finite-difference time-domain (FDTD) simulations. The application of Mg as a material for transient photonic devices allows for the rapid, inexpensive adjustment of optical responses for wavelength filtering. The low cost of Mg compared with other traditionally used materials for photonic devices such as Au and Ag along with its unique property of transience make it a prime candidate for future research in photonics.

Modernization and advancement in technology has substantially raised the need of electrically tunable photonic devices such as electro-optic (EO) modulators. Ferroelectric Strontium Barium Niobate exhibits tetragonal tungsten bronze (TTB) structure with highest electro-optic coefficient. While there have been many studies on bulk SBN crystals, there is very limited knowledge of SBN thin film properties, which could depart substantially from the bulk properties. The SBN thin films have been extensively exploited for device applications such as pyro-electric infrared detector, phase-conjugated mirrors, memory devices, photorefractive optics etc. Nevertheless, these are of interest for many of the possible applications of this material, which would require its use in thin film form. Pulsed Laser Deposition (PLD) has recently been proved very successful in growing thin films of multicomponent material. Majority of the reports available in literature on SBN thin films focus on the study of variation of effective EO coefficient with the applied dc electric field. However, it is very important to study the dispersion of EO coefficient with frequency in order to modulate the light over a wide range of frequencies. The effective EO coefficient measured contains both primary and secondary components of ionic and electronic polarization which are frequency dependent. Thus, it becomes an important aspect to know the variation of effective EO coefficient with frequency for the exploitation of a material as electro-optic modulators.
The present work reports the growth of SBN75 thin films using PLD technique with a targeted composition of x =0.75 (SBN:75) under the optimized parameters. The growth pressure and the laser fluence was varied in accordance to obtain the TTB structure exhibited by SBN75 thin films. For exploring the dynamic response, a simplistic approach of Senarmont Compensator technique has been utilized. The waveguide structure in SBN75 thin film guides the EM light radiation of wavelength, across the substrate. A bright streak of light was obtained by coupling the SBN75 sample with the high indexed rutile prism (n = 2.84). The variation of effective electro-optic coefficient r_c with the frequency of the applied electric field was obtained. The electro-optic coefficient was found to roll off at higher frequencies.

The quest for alternative materials that can replace gold and silver and extend the application potential of plasmonics has been extended during the last decade to nitrides, oxides, silicides and 2D materials, among others. Among metals, copper and aluminum have been usually overlooked as they are prone to oxidize, limiting the long-term stability of devices. However, the optical response of both Cu and Al can be well described with the Drude model. Cu shows similar damping and plasma frequency values to Au and Ag and there is an on-going research to prevent oxidization in Cu nanostructures. On the other hand, the large plasma frequency of Al makes it the best candidate for UV plasmonics.
In this work the plasmonic performance of Cu and Al is studied and compared to that of Ag and Au. Numerical simulations are employed to quantify the tuning degree and quality factor of different nanostructures (cubes, rods, hollow and coupled particles) and taking into account the presence of oxidization. In these cases, it appears that geometry can play a dominant role over material properties and the performance of Cu and Al becomes comparable to that of Ag and Au for systems of non-spherical particles and strong electromagnetic coupling among particles. This fact is confirmed by the fabrication and characterization of Cu and Al metal island films using electron beam evaporation. Determination of the effective optical response of the samples by means of spectroscopic ellipsometry confirms similar performance to that obtained for Ag and Au and suggests that Cu and Al metal island films can represent an efficient low-cost platform for certain plasmonic applications such as solar energy harvesting.

This talk will review our recent work on exploring functional plasmonics (i.e., switching, plasmonic Coulomb blockade) achieved in subnanometer gaps at the transition point towards tunneling. This is an ultra-sensitive plasmonic platform, akin to the metal-insulator transition achieved bulk materials like vanadium dioxide; however, subnanometer gaps provide a designer-based approach to switching.

I will also discuss our recent work on dual-wavelength upconversion and upconversion from single upconverting nanocrystals trapped optically within an aperture in a metal film. We demonstrate that tuning the aperture dimensions can have a massive impact on the upconversion detected.

Plasmonic color generation have attracted tremendous interest in recent years as a solution for ink-free color printing. The prominent advantages of plasmonic coloration are better robustness compared with organic dyes while also providing high chromaticity and brightness. However, use of costly nanofabrication techniques to make reflective displays restricts potential applications and production of functional devices. Here, our aim is to generate plasmonic structural colors at high resolution using inkjet printing, which is scalable to large areas and avoids complicated fabrication steps. Our plasmonic metasurfaces contain three solid films on flexible plastic substrates. A silver film (~500 nm) was first inkjet-printed on the substrate to provide a high base reflection. Then an organic material was printed as a dielectric spacer layer with varying thicknesses to tune the reflective color by Fabry–Perot interference. A semi-transparent plasmonic top mirror completes the cavity and also improves colour generation via wavelength-dependent plasmon absorption. Here, I will present our work towards facile fabrication of structural colors and discuss our results based on reflectivity measurements of red, green and blue pixels using micro-spectroscopy. We believe this inkjet printing process has the potential to generate reflective displays with high resolution, low cost, flexibility and compatible with large-scale production.

Structural colors, originated from interaction of visible light with subwavelength nanostructures, offer high spatial resolution color printing, beating fundamental resolution limit of a light field optical microscope. Such structural colors, arising from resonant scattering of light from subwavelength nanostructures, can be printed at resolutions as high as ~100,000 dots per inch (DPI). A variety of design, methods have evolved over the years for structural color generation and color printing at optical diffraction limit of light¹. A novel approach, utilizing focused ion beam (FIB) direct fabrication of high-index silicon (Si) 3D photonic nanostructures for multicolor generation with enhanced color purity and high resolution color printing applications was recently developed². Although successful, the systematic variation of geometrical parameters for tuning of color performance can be quite cumbersome sometime and require significant time and experiments, and does not necessarily allow tailoring the chromatic response in an optimal way prior fabrication.
In this work, a general framework for optimization of photonic nanostructures for multicolor generation is presented. The design of photonic nanostructures for desired color printing is optimized numerically through the formulation of a nonnegative quadratic programming problem. This problem is solved by an iterative approach based on multiplicative updates algorithm³ for an optimal solution of ion beam dwell time maintaining high accuracy. The optimization algorithm is aimed at minimizing the deviation of a simulated geometry by FIB nanofabrication from the desired geometry of photonic nanostructures for color printing. The optimization algorithm for Si photonic nanostructures to develop structural color printing is further coupled with electromagnetic simulations based on finite-difference time-domain (FDTD) calculations. The chromatic response of periodic subwavelength nanostructures is calculated using FDTD simulations. The numerical results show that this hybrid approach is a versatile tool for the design of photonic structures for multicolour generation, which can be extended for different design and application perspectives. In summary, this work provides a general framework for design and optimization of photonic nanostructures for multicolor generation. This will enable automatic design of photonic nanostructures for high resolution structural color printing applications with ion beams. The possible ways to expand the color gamut and improve the color performance of such photonic color filters on CIE 1931 (Commission Internationale de l'Eclairage) color space will be further discussed. Finally, such optimization of photonic nanostructures is not just limited to structural color printing and has tremendous potential for future optics offering unique opportunities for optical security, polarimetry, spectral imaging applications etc.



(1) Kumar, K.; Duan, H.; Hegde, R. S.; Koh, S. C. W.; Wei, J. N.; Yang, J. K. W. Printing Colour at the Optical Diffraction Limit. Nat Nanotechnol 2012, 7 (9), 557–561.
(2) Garg, V.; Mote, R. G.; Fu, J. Focused Ion Beam Direct Fabrication of Subwavelength Nanostructures on Silicon for Multicolor Generation. Advanced Materials Technologies 2018, 3 (8), 1800100.
(3) Sha, F.; Lin, Y.; Saul, L. K.; Lee, D. D. Multiplicative Updates for Nonnegative Quadratic Programming. Neural Computation 2007, 19 (8), 2004–2031.

An idea “to marry” silicon as a semiconductor material with noble metals to fabricate a highly ordered plasmonic nanostructures has been an obsessive dream of specialists in metamaterials for photonics, surface enhanced vibrational spectroscopy, optoelectronics, etc. Porous silicon is one of the most favorable materials to achieve this objective as it can be used as a tool for integration with silicon technology. Metamaterials must have an ordered periodical structure. However porous silicon, which is traditionally formed by an electrochemical etching of monocrystalline silicon, is typically characterized by a significant deviation of geometrical parameters of pores up to 50%. This leads to non-reproducibility of morphology of plasmonic nanostructures depositing on such porous material. In present paper, we developed a novel metal-assisted electrochemical method to obtain extremely ordered structure of mesoporous silicon with pore diameter deviation less than 10%. Following deposition of thin films of coinage metals was shown to provide a formation of so-called plasmonic nets composed of periodically crossing nanowires. Simulation of an electric field in the cavities of these nets showed its localization in the pore mouths due to strong rim component. It is proposed to use the profit of this component in the surface enhanced Raman scattering (SERS) spectroscopy to detect and study analytes at submolar concentration down to the single molecule level. To place analyte species in the pore mouth we used sandwich film of graphene-test molecule-graphene covering the plasmonic net. Graphene was found to play a role of support for analyte molecules and a protection for heat sink induced by laser excitation. In addition to the improved plasmonic properties of the coinage metals organized into net on modified mesoporous silicon, the paper also presents a description of changes in magnetic and catalytic properties of other metals deposited on the developed template.

Polaritons – hybrid collective light-matter excitations – play an important role in both fundamental and applied sciences, since they enable the confinement and manipulation of light in deep sub-wavelength scale structures. Particularly large polariton confinements and long lifetimes (at room temperature) can be found in van der Waals (vdW) materials supporting phonon-polaritons (e.g. h-BN or MoO₃). In some frequency ranges these phonon-polaritons can have anisotropic dispersion: the isofrequency curves can be either ellipses or hyperbolas. Here we report on both theoretical and experimental (near-field micrioscopy) studies of anisotropic polaritons propagating along natural biaxial vdW materials as well as along artificially-structured vdW metasurfaces.

Plasmonic nanostructures can significantly enhance electric and magnetic fields at nanoscale dimensions. Fields localized in the narrow gaps of plasmonic nanostructures can be several orders of magnitude higher than the incident light field, and this field enhancement strongly affects light-matter interaction processes in plasmonic nanostructures. Here, we study field enhancement in nanoparticle dimers and analyze surface plasmon amplification by stimulated emission (spaser) in the dimer gap. We consider both sphere and disk silver nanoparticles and employ finite-difference time-domain simulations for modeling field enhancement in the dimer.

Our in-house finite-difference time-domain (FDTD) method includes non-uniform, geometry-specific, semi-conformal meshing of the nanostructures and enables accurate resolution of electromagnetic ‘hot spots’ in the nanoparticle gap without parasitic staircasing. Our technique allows for FDTD simulation of fully-anisotropic permittivity and permeability material tensors. Semi-conformal meshing is enabled by placing grid boundaries along the surfaces of devices in critical areas and Maxwell’s equations encapsulating a coordinate transformation into the material tensors.

Using this approach, we numerically observe gap modes with even and odd field distributions within the 10-nm dimer gap and identify spatial regions with nodes and anti-nodes of the field enhancement. We use quantum density matrix (optical Bloch) equations for the spaser, calculate the plasmon amplification in the gap, and show its correspondence to the total mode characteristics. Because of the strong field enhancement in the gap, amplification of plasmons in a nanoparticle dimer is more efficient than ones in the proximity to a single nanoparticle and can be used an ultra-compact light source in optical devices and medical applications.

Acknowledgment. This material is based upon work supported by the Air Force Office of Scientific Research under Grant No. FA9550-19-1-0032.

We report a novel fabrication method that exploits the differential chemical and mechanical properties of colloidal nanocrystals (NCs) and bulk materials to transform 2D bilayer heterostructures into 3D architectures by design. Colloidal NCs are hybrid systems, composed of inorganic cores and organic ligand shells, which make their surfaces chemically addressable and their assemblies mechanically soft. Using ligand exchange methods, bulky ligands that originally stabilize as-synthesized NCs are replaced by more compact ligands within a few minutes. Ligand exchange triggers a large volume shrinkage in NC assemblies, and in bilayer NC/bulk heterostructures generates a large misfit strain that drives bending. Following this strategy, we establish a set of design rules to create various 3D, cell-sized superstructures. These 3D structures may be harvested from surfaces and suspended in solvents or they can remain bound to surfaces. By choosing different colloidal NC building blocks, we demonstrate the versatility of this technique to create 3D structures with unique optical and magnetic physical properties. For example, we apply the design rules to construct 3D metasurfaces with giant chiroptical properties.

Wavelength-selective light absorbers and emitters, or spectroscopic energy transducers for light, are expected to provide wide variety of applications in energy harvesting, remote sensing, and label free bio-sensing. By utilizing the strong near field of optical antennas and metamaterials, strong signal enhancement of molecules becomes operative to realize high sensitivity vibrational sensing. Recent developments in infrared plasmonic materials, nanostructure fabrication techniques, as well as their surface functionalization techniques have enabled us to propose and fabricate various types of advanced nano-sensing devices. In this talk, we exemplify the detection of tiny amount of molecules by surface-enhanced infrared absorption spectroscopy, using Al infrared antennas and metamaterials. We also introduce our new approach utilizing infrared surface plasmons in optical antenna array made of phosphonic acid-functionalized ITO nanorods to detect proteins with high specificity and sensitivity. In the latter half of the talk, we introduce some of our recently developed uncooled infrared sensors combined with spectroscopic perfect absorbers for wavelength-selective infrared ray detection. The MIM metamaterial IR sensors with pyroelectric detection as well as thermoelectric detection exhibit resolutions better than 1um with wide acceptance angles. By adopting Gires-Turnois structure with plasmonic metals, the wavelength resolution goes lower than 50 nm at resonance wavelength of 3-4 um. Such structures also shows sharp thermal emission when it is fabricated with low-loss materials and heated at high temperatures. These devices will open a new avenue for potential applications in multiband temperature sensing, infrared-color imaging for material sensing, and NDIR gas sesnors for security gas sensors and combustion analysis.

References:
[1] T.D. Dao, S. Ishii, T. Yokoyama, T. Sawada, S. Ramu Pasupathi, K. Chen, Y. Wada, T. Nabatame, T. Nagao,? ACS Photonics?3, 1271-1278 (2016).
[2] T. Yokoyama, T. D. Dao, K. Chen, S. Ishii, R. P. Sugavaneshwar, M. Kitajima, T. Nagao, Adv. Opt. Mater. 4 (12), 1987-1992 (2016).
[3] Kai Chen, Peijun Guo, Thang Duy Dao, Shi-Qiang Li, Satoshi Ishiii,?Tadaaki Nagao, Robert P. H. Chang,? Advanced Optical Materials?5?[17], 1700091-1?(2017).

The noble metals, particularly Au and Ag, are widely used as plasmonic materials. Although Au offers terrific chemical stability and several options for nanofabrication, it suffers from relatively high plasmon damping losses. In contrast, Ag has very strong plasmon resonances but corrodes under ambient conditions, limiting device lifetimes. The limitations of Au and Ag has fuelled the search for alternative plasmonic materials.

The sodium tungsten bronzes (Na_xWO₃) are sub-stoichiometric metal oxides with variable Na content, 0 ≤ x ≤ 1. Above x > 0.3, the intercalated Na donates its 3s electron to the WO₃ conduction band, giving Na_xWO₃ metallic electrical and optical properties which vary with the Na content. From electron energy-loss spectroscopy (EELS) studies, Na_xWO₃ has been shown to support strong bulk plasmon resonances which blueshift with increasing x, from near-infrared to visible frequencies. However, characterisation of localised surface plasmon resonances (LSPRs) has been limited due to the lack of fabrication techniques for Na-rich (x > 0.5) nanoparticles.

Here, we demonstrate the synthesis of Na_xWO₃ nanoparticles across 0.41 ≤ x ≤ 0.83 using a high-temperature method. Results from ex-situ and in-situ diffraction techniques provide insight into optimising the synthesis procedure to minimise the particle size of the products and to maximise phase-purity. Spectrophotometry and spatially-resolved EELS are used to investigate the plasmon responses. Strong LSPRs are observed which increase in frequency with x, similar to the variation in the bulk plasmon frequency. Experimental results are supported by simulated responses, which are calculated using the boundary element method (BEM).

Based on experimental and calculated responses, potential applications in solar control filtering, plasmon-assisted photovoltaics and plasmonic photocatalysis will be discussed. Given their high chemical stability, simple nanoparticle fabrication and tunable optical properties, we show that the sodium tungsten bronzes are promising alternative materials for plasmonics.

Nanoscale particles are a promising class of materials for optical nonlinear conversion because of the strong field localization and the phase-matching-free conditions. While their utilities are envisioned in a spectrum of applications, a general layout to achieve and enhance nonlinear conversions is barren. The Mie and plasmonic resonances have been utilized in nano-scale particles to enhance the nonlinear response, especially the second-harmonic generation (SHG). While showing great promises, geometrical and bandwidth constraints in resonant enhancement, as well as loss and synthetic complexities in embedding plasmonic constituents impose challenges. In this study, we experimentally demonstrate an approach to achieve and enhance optical nonlinear conversion through a non-resonant dielectric particulate platform. The platform is based on a dielectric microsphere with a nonlinear nano-structured shell. The shell operates as a metamaterial-based impedance matching interface between the input light and a photonic jet and, at the same time, forms the material basis for the nonlinear wavelength conversion. We have adopted variants of the ‘hedgehog’ particles, which comprise an array of high-aspect ratio ZnO nano-spikes arranged orthogonally onto a silica microsphere of 1 mm in diameter. The sub-wavelength arrangement of these nano-spikes forms a spherical metamaterial shell exhibiting a radially graded effective index. Such a configuration effectively confines the incident light into a strong photonic jet within the shell, which is made of a well-known c⁽²⁾ material, thereby significantly enhancing the SHG signal compared to a flat array of similar nano-spikes. In addition, due to the non-resonant nature of the nonlinear conversion, following advantages are further conceived: 1) relaxation of the geometrical constraints in the construction of the metamaterial shell, 2) broadband wavelength tuning, 3) lower sensitivity to angle of incidence and polarization and 4) directional scattering of the SHG signal. We have experimentally observed strong enhancements of SHG in a variety of ‘hedgehog’ particles. Through the FDTD simulations, we observed strong enhancement of the SHG conversion efficiency in ‘hedgehog’ particles having 300 ZnO nanospikes whose geometrical features do not coincide within the resonant conditions. The simulations revealed 6×10³-fold enhancement in the SHG efficiency compared to a single ZnO spike. The demonstrated concept of enhanced SHG in a dielectric particle with a nonlinear metamaterial shell may open up pathways for efficient nonlinear conversion in nanoscale particles.

In the past two decades, plasmonic nanostructures have gained tremendous interest as both diagnostic and therapeutic agents for cancer detection and treatment. In this talk I will show the utility of gold nanostar probes designed in my lab for rapid and noninvasive detection of multiple immunomarkers of cancer to enable patient selection for immunotherapies, as well as response to treatment after immunotherapy. We combined a clinical and pre-clinical imaging technique, positron emission tomography with surface-enhanced Raman spectroscopy (ImmunoPET-SERS) in vivo by labeling gold nanostars with radiolabels, Raman reporter molecules, and targeting antibodies. Multimodal ImmunoPET-SERS seamlessly integrates depth-resolved whole-body imaging and high sensitivity of PET with high spatiotemporal resolution and multiplexing of SERS providing dynamic immunomarker profiling in vivo. Further, we determined the immunomarker status of mice treated with combinatorial immunotherapy with ImmunoPET-SERS and demonstrated real-time feedback of CD8+ infiltration in tumors which was confirmed with IHC ex vivo. High resolution SERS maps of tumor sections provide quantitative measure of multiple receptor expression in the same tumor section which correlate well with histological analysis of tissue. I will also demonstrate how plasmonic gold nanostars efficiently convert light to heat and enable highly specific drug delivery combining photothermal therapy with chemo and immunotherapy for enhanced treatment outcome in highly aggressive breast cancer.

Excitonic semiconductor thin films are emerging as next-generation active layer materials in optoelectronic devices used for display, solid-state lighting, laser and energy harvesting applications. However, active layer thicknesses in these devices are typically limited to 200 nm or less due to limited charge carrier diffusion lengths in excitonic semiconductor materials. Therefore, light management and localization in excitonic semiconductor thin films requires manipulation of light below the diffraction limit. As a result, to improve energy conversion efficiency in excitonic semiconductors, light manipulation using plasmonic structures integrated with the thin film excitonic material is of interest.

Here, our work on improving light trapping, light extraction and stimulated emission in excitonic organic semiconductor thin films using a variety of plasmonic metasurfaces will be presented. Numerous optical phenomena, such as absorption induced scattering, out-of-plane waveguiding and morphology-dependent surface plasmon outcoupling, are identified due to exciton-plasmon coupling between the organic semiconductor and the plasmonic metasurface. Interactions between localized and propagating surface plasmon polaritons and the excitonic transitions of a variety of organic conjugated polymer materials will be discussed and ways in which these interactions may be optimized for particular optoelectronic applications will be presented. Furthermore, our recent studies on the impact of exciton-plasmon coupling on stimulated emission from thin-film spasers will be presented.

The nitrides of the group IVb, Vb and VIb metals (TiN, ZrN, HfN, VN, NbN, TaN, MoN, WN) have emerged as important alternative plasmonic materials due to the unique combination of substantial electric conductivity and plasmonic features in the NIR-Vis-UV ranges with their refractory character and chemical inertness, which provide durability of nanostructures upon exposure to high-power laser beams. However, their refractory character can turn from a blessing to a curse due to the excessive density of structural defects when grown at relatively low temperatures. The extended defects, i.e. the grain boundaries, scatter the conduction electrons resulting to enhanced electron losses compared to conventional metals (Au, Ag, Al, etc). Unlike the conventional plasmonic metals, the conductive nitrides may also incorporate point defects, which introduce states close to Fermi level that trap electrons and enhance the dielectric losses, as shown by previous ab initio calculations. For some conductive nitrides, like TaN, MoN and WN, point defects were reported to form spontaneously as a mechanism of stabilization of epitaxial films. In order to identify the effects of point and extended defects we provide a detailed study of the optical properties and plasmonic behavior of a wide range of polycrystalline and heteroepitaxial TiN films as measured by ellipsometry in the broadband range of 190-40000 nm. These films have varying crystallographic features and chemical composition and were grown by reactive magnetron sputtering at various temperatures and irradiation conditions. The crystallographic features are studied by X-Ray diffraction (XRD) at various geometries (Bragg-Brentano, Phi-scans, Rocking Curves), the chemical composition by X-Ray Photoelectron Spectroscopy (XPS) core level spectra, the point defects by Raman spectroscopy, and the electron density of states (EDOS) by valence band XPS spectra. We show that relying exclusively on XRD for the evaluation of crystalline quality of TiN may be misleading. Indeed, the combined XRD-XPS-Raman results clearly show that the point defects have more crucial consequences to the plasmonic performance of TiN than the extended defects; as a result, an effective strategy to grow efficient plasmonic TiN is to use intense irradiation conditions that anneal the point defects out of the grains, albeit in expense of grain size. In a final effort to generalize these results we extend this study to polycrystalline VN, NbN and TaN films and their plasmonic performance.

The U.S. Army Research Laboratory (ARL) is exploring using metamaterials and metasurfaces for polarization-independent, wavelength-selecting notch filters in the short-wave infrared (SWIR), mid-wave infrared (MWIR), and long-wave infrared (LWIR) spectral regions. The filters are being designed, fabricated, and characterized both in-house and in collaboration with Academic and Industrial partners. We will show some of our recent designs, simulations, fabricated devices, and spectroscopic results. There will also be a short discussion of the various funding opportunities and mechanisms available with ARL, at the request of the symposium organizers.

Plasmonic nanoantenna offers the ultimate spatial control over electromagnetic waves by localizing the optical energy in the nanoscale and has significant potential for the nanophotonic applications utilizing enhanced light matter interactions. A nanogap structure in particular have an extremely small mode volume and a large field enhancement and a resultant large Purcell factor. However, formation of a sub-10 nm gap with an accuracy of ~1 nm is still challenging even by the state-of-the-art lithography techniques. Recently, plasmonic nanoparticle on mirror antennas in which a metal nanoparticle (NP) is placed on a metal film via dielectric spacer have shown the great potential in nanophotonic applications.[1,2] The structure can be fabricated by a bottom-up process using colloidal gold and silver NPs with a sub-10 nm gap. However, in spherical NP on mirror structure being studied experimentally, the degree of freedom of the antennas in terms of spectral and polarization control is limited.
In this work, we report spectral shaping and polarization control of Purcell-enhanced fluorescence by the gap plasmon modes of an anisotropic gold (Au) nanorod on a mirror (NRoM) antenna.[3] We first simulate the property of the resonant modes under different excitation conditions. Systematic calculations demonstrate the richer resonance behaviors of a NRoM antenna than a spherical NPoM antenna due to the hybridization of the bright and dark modes driven by the structural anisotropy. We fabricate a NRoM antenna by placing a Au NR on an ultra-flat Au film via a mono- and multi-layers of light emitting quantum dots (QDs) (~3 nm in diameter). The experimentally measured resonance spectra of single NRoM antennas agree quite well with those of the numerical simulations. We demonstrate large enhancement (>900-fold) and shaping of the luminescence spectrum from QDs in the gap due to the coupling with the hybridized mode of a NRoM antenna. We also show that the polarization property of the emission is controlled by that of the mode coupled. These findings provide novel insights for the design of spectroscopy-based sensing and imaging at the single nanometer scale using the nanoantenna-enhanced emission enabled by hybridized plasmonic modes.
[1] R. Chikkaraddy, et al., Nature, 535, 127 (2016). [2] S. Yashima, H. Sugimoto, et al., J. Phys. Chem. C, 120, 28795 (2016). [3] H. Sugimoto et al., ACS Photonics, 5, 3421 (2018).

Since the development of diffractive optical elements in the 1970s research has focused on replacing bulky optical elements such as lenses and grating by thin counterparts. Over the last decade, nanophotonic metasurfaces rapidly advanced the development of flat optical elements based on the realization that resonant optical antenna elements enable local phase control. Present applications of metasurface flat optical elements include lenses, polarization control, and beam steering.

Next-generation applications of flat optics require dynamic control over optical functionalities, e.g. the focal position or efficiency of optical elements. However, most nanophotonic structures are static after design and fabrication. Current approaches for dynamic control like electrical gating exhibit limited tunability due to the finite few-nm extend of the depletion and accumulation layers as result of coulombic screening.

Here, we demonstrate actively-tunable and atomically-thin optical lenses by carving them directly out of monolayer transition-metal dichalcogenides (TMDs) like WS₂ with a strong excitonic resonance in the visible spectral range. This turns the 2D material into the antenna or metamaterial and incorporation of active materials into larger antenna structures will no longer be needed. Due to their sub-nm thickness, these materials are highly tunable through external control. We demonstrate dynamic electrical tuning of the focusing efficiency through manipulating of the excitonic material resonance properties as opposed to tuning of antenna resonances.

In bioanalysis, especially in single cell analysis, label-free detections and recognitions for molecular analysis are highly demanded. IR spectroscopy is one of label-free methods and it gives us chemical specificity and molecular information, yet its application in bioanalysis is limited due to its low sensitivity. Recently, plasmonic nanostructures were intensively studied to improve the sensitivity of IR spectroscopy by several orders of magnitude, however positioning analytes exactly at the hot-spots is still challenging [1]. We propose a device that utilizes nanofluidics to manipulate analytes into the hot-spots of metamaterials, consequently an ultra-high sensitivity of IR absorption detection can be achieved.
The structure consists of metal square-disks array and metal mirror separated by a nano fluidic channel. The interaction between top square nanostructure and bottom mirror forms the quadrupole resonance, and it suppresses the light reflection from the device. When the molecule whose absorption is overlapped with this mode is introduced, strong interaction between molecules and metamterials is excited and it creates the reflection light within the absorption band of the metamaterial. The sensitivity was achieved at molecule density of ~10^-4 molecules/Ų, which is improved by 2 orders compared to reported plasmonic induced IR detection methods [2]. We also succeeded in the quantitative determination of absolute number of molecules by precise fluidic operation.
Moreover, the device allows the confinement of both molecules and plasmonic energy inside the nanocavities. We confirmed the presence of a strong H-bond network and the scaling behavior of water confined in 10-100 nm regime. Our method can provide the capability for in-situ probing molecules and chemical reactions under nanoconfinement [3].
When we make the size of unit cell of metamaterial absorber down to several hundreds nanometers, the absorption bands move to the visible light region. We present a simple yet efficient approach for ink-free color printing employing sub-micrometer scale plasmonic pixels of single constituent metal structure that supports near unity broadband light absorption at two distinct wavelengths, facilitating the creation of saturated colors [4]. The dependence of these resonances on two different parameters of the same pixel enables controllable color attributes such as hue, brightness and saturation across the visible spectrum. Here we present an up scalable color printing scheme using plasmonic pixels of single constituent metal structure, enabling the design of full colors with controllable color attributes.

[1] A. Ishikawa and T. Tanaka, Scientific Reports 5, 12570 (2015).
[2] T. H. H Le and T. Tanaka, ACS Nano 11, 9780-9788 (2017).
[3] T. Le, A. Morita, K. Mawatari, T. Kitamori, and T. Tanaka, ACS Photonics 5, 3179-3188 (2018).
[4] R. Mudachathi and T. Tanaka, Scientific Reports 7, 1199 (2017).

Over several decades, eye tracking has been providing indispensable analysis tool in wide range of research fields including Ophthalmology, Psychology, and Neurology. Recently, augmented reality is pushing its demands on more compact, transparent optical eye trackers, which are compatible to head-mounted display or heads-up display. The requirement augmented reality imposes on the optical eye trackers is tracking human eyes while it does not perturb human vision. The optical eye trackers should have acceptable power efficiency as well as maintain optical transparency. The two requirements- power efficiency and high-transparency to human vision- are typically conflict to each other. Many of proposed optical eye trackers such as oblique half-mirror and holographic waveguide are suffering from either severe human vision interference or poor operation efficiency. In addition, these are bulky, poorly transparent, and producing rainbow images due to non-zero diffraction in the visible spectrum.
Here, we demonstrate ultra-flat, transparent, and rainbow-free diffractive eye tracking glass based on guided mode resonance which supports near-unity transmission. It is based on a 200-nm-thick Si₃N₄ slab dielectric optical waveguide between a quartz substrate and a 100-nm-thick SiO₂ capping layer. The vertical feature is designed for high transmission (>90%) over the whole visible spectrum. We insert 3-nm-thick Si grating layer between the Si₃N₄ slab dielectric waveguide and SiO₂ capping layer which launches high-quality (Q~2,000) guided mode resonances in the slab waveguide. We rigorously characterize the resonantly diffractive, dispersive properties of the fabricated structures through angle-resolved confocal spectroscopy. The guided mode resonance is excited when light of resonant wavelength (870 nm) in near-infrared spectrum is normally incident on the structure. The scattered light components at individual the Si gratings constructively interfere to be funneled into quasi-guided waves in the Si₃N₄ slab dielectric waveguide. During long (2,000 optical cycles) travelling time inside the Si₃N₄ slab before re-radiation into the free space, the light is building up its intensity and coming out with high diffraction efficiency (13%) into the desired direction which is useful for eye tracking. When the incident light is in the visible spectrum, on the other hand, the guided mode resonance becomes weak due to Si absorption, resulting in suppressed rainbow-producing diffractions with negligible (0.1%) efficiency. We also demonstrate the imaging of artificial eye by a single webcam located at 60 degree from the surface-normal direction of fabricated 2 cm by 2 cm sample. The full anterior images of an artificial eyes are obtained at the oblique direction and we hope that this opens a promising route toward ultra-compact, transparent, and non-obtrusive imaging for displays and optical switching applications.

The infrared spectra of many polar semiconductors are dominated by highly reflective reststhralen bands that occur between the transverse and longitudinal optical phonons. Through the LOPC effect, free carriers shift the reststrahlen band to higher frequencies. We have previously shown that photoinjected carriers transiently and reversibly modify the infrared reflectivity of bulk SiC. Within the reststrahlen band, SiC and InP nanostructures can exhibit surface-phonon polariton resonances. Here we report, for the first time, active tuning of SiC and InP surface-phonon polariton resonances via carrier photoinjection, achieving better modulation depths than active tuning in plasmonic systems. In SiC, ultraviolet excitation with femtosecond laser pulses induces >10 cm^-1 shifts in the transverse dipole resonance (width = 7 cm^-1). Time-resolved infrared reflection spectroscopy reveals that the photoinduced shifts decay in tens of ps, depending on the initial carrier density. Our results suggest that spatial redistribution of photoexcited carriers dominates the time dependence of the active tuning. We also report, for the first time, direct time-resolved infrared spectroscopy of the LO mode of GaN, made experimentally accessible through the Berreman effect.

Extraction of the properties of optical wavefields, such as wavelength, polarization and phase, is widely employed in a range of applications including optical communications, sensing and imaging. These attributes of light are, however, not directly sensed by photodetectors and additional optical components and/or computational post-processing of intensity images is required to extract this information. Similarly, optical information processing is widely used for enhancing images and the visualization of transparent objects such as live unstained cells. Various well-known computational and all-optical strategies have also been developed to perform image manipulation. To sense properties of light other than its amplitude or to selectively modify optical images, therefore, either real-time approaches requiring bulky and expensive optical components, or methods utilizing potentially time-consuming computational strategies are employed. Here we discuss the use of plasmonic thin films and metasurfaces to directly convert amplitude and phase gradient information in an optical field into enhanced intensity images and the incorporation of plasmonic antennas into photodetectors to sense polarization, phase gradients and color. These devices provide an avenue for the development of ultracompact systems that will perform on-chip, real-time, single-shot conversion of wavefield information to readily measured and enhanced intensity distributions or a measurable photocurrent.

There have been several recent demonstrations of the use of metasurfaces and plasmonic thin films to perform mathematical operations. For example, a metasurface can be used as a tailorable spatial filter in a conventional optical spatial filtering system where optical differentiation or integration can be performed using independent tuning of the amplitude and phase of scattered light at different spatial locations on the filter. Alternatively, and the method to be discussed here, the angular sensitivity of certain plasmonic resonances permits direct access to the Fourier (or angular) spectrum of a reflected or transmitted field establishing the possibility of ultra-compact approaches to optical information processing. The angular dependence of prism coupling to surface plasmons on a thin metal has been previously used to demonstrate differentiation of images both amplitude and phase objects. Here the use of 'Salisbury' screen absorbers and subradiant modes of metasurfaces to perform processing of optical information will be discussed.

Metasurfaces consisting of optically thin arrays of nanoscale antennas exhibiting characteristic resonances also have the capacity to convert ‘invisible’ properties of light to readily sensed intensity information that can be used to generate a photocurrent. Furthermore, their planar geometry facilitates their integration into metal-semiconductor-metal (MSM) photodetectors using CMOS compatible fabrication processes to produce new optoelectronic devices that can, for example, significantly reduce the effects of cross-talk in colour pixels. Here we present results demonstrating polarization and angle-sensitive MSM photodetectors integrating nanoantennas as well as metasurface-integrated colour sensors.

Strong modification of local environment associated with plasmonic nanostructures provides possibilities to control various processes, including charge transfer processes and chemical reactions. In this work, we explore opportunities to enhance electrochromic polymer performance using plasmonic metasurfaces, and study the origin of this enhancement. Electrochromic polyaniline (PANI) films deposited onto gold metasurfaces demonstrate non-monotonnous coloration behavior at low voltages, step-like color-change and much faster saturation in color with the increase in voltage in comparison with polyaniline films deposited on flat gold. The additional small voltage peak in the cyclic voltammogram in nanomesh/PANI cell and the asymmetric and nonlinear I-V characteristics of the sandwich nanomesh/PANI/flat gold structure indicate a possible formation of Schottky-like interface between polyaniline and nanostructured gold, whereas the Ohmic contact is observed for the flat gold-polyaniline system. The results are discussed in terms of the modified work-function of nanostructured gold, interface charging and threshold-like charge transport. Possibility to engineer optical and charge transport properties of electrochromic materials via nanostructured interfaces can bring various optoelectronic applications.

Many metamaterial devices are designed to operate at a wide range of angles of incidence. However, the ability to control reflection at an angle is useful for 3-D imaging and detection, as well as controlling light for photonic circuits. We have demonstrated a metal-dielectric-metal metamaterial perfect absorber that can be either angle-dependent or angle-independent based on the dielectric material chosen for the spacer layer. Use of platinum and silicon nitride produced increased selectivity of the near infrared perfect absorber at wide angles of incidence under p-polarized light. This enhanced selectivity came from increased reflection at wavelengths smaller than the target wavelength for absorption.
Variable angle spectroscopic ellipsometry was used to explore the enhanced reflection. Based on the Mueller matrix, the sample exhibited a small amount of anisotropy, though to what degree depended on the angle of incidence and orientation of the sample. When the long elliptical axis was perpendicular to the plane of incidence, the sample appeared more anisotropic, particularly at wider angles of incidence. Depolarization and scatterometry measurements showed that the anomalous behavior is not a function of scattering or cross-polarization, indicating that a resonant mode within the metamaterial is being induced. FEM simulations confirmed that anti-parallel electric field curls are only present in the cases where enhanced reflection occurs, generating a magnetic dipole that contributes to the metamaterial’s response. To determine if this behavior is specific to this combination of materials and device dimensions, additional simulations were conducting with varying material parameters and elliptical axes.
To explore the design space where the enhanced reflection occurs, the permittivity of the dielectric layer and the long elliptical axis were varied. From these simulations, three regions appeared. The first, with high permittivities and long elliptical axes nearing the size of the unit cell, showed angle of incidence independent behavior with no enhanced reflection, which was consistent with devices found in literature. The second region, where the nanostructure approached a circle as the elliptical axes became equal and the permittivities were low, showed extreme increasing in reflection, though with losses to the designed absorption peak. In this region, many designs did not exhibit a metamaterial response at all as the permittivity of the dielectric layer was too low to support the resonant modes. Finally, in region three, a range of permittivity values and elliptical column sizes resulted in the reported enhanced reflection but with high levels of absorption at the target wavelength. Therefore, this behavior is more common than previously thought. Establishing a thorough understanding of the design space like this will lead to better informed device decisions for other applications.

Ordered arrays of plasmonic nanovoids (antinanoparticles) provide unconventional plasmonic platforms for detection and study of organic molecules at ultralow concentrations by surface enhanced Raman scattering (SERS) spectroscopy. They are especially attractive for analysis of large molecules that cannot be objectively studied with traditional plasmonic nanoparticles. This is caused by the small areas of localization of electromagnetic field (hot spots) that is induced by surface plasmon resonance in nanoparticles. As a result, Raman signal from particular bonds of large molecules, which are located beyond hot spots, is not enhanced. Electromagnetic field of nanovoids is mostly localized inside cavities due to multiple bounces of excitation light. If an analyte molecule appears in nanovoid, Raman signal of all bonds will be increased nearly equally. To promote significant enhancement plasmonic nanovoids should have depth and diameter more than 0.5 microns. Such structures are usually fabricated by nanosphere lithography, which main drawbacks are (i) limited area of plasmonic sample and (ii) high consumption of noble metals. Here we propose to fabricate plasmonic nanovoids by sculpturing the silicon template and following electrochemical/electroless deposition of silver film. Arrays of voids with proper dimensions in silicon can be fabricated by electrochemical anodic etching [1] or direct electrochemical imprinting with dotted stamp [2]. Using the first approach, we were able to form arrays of macropores in lightly doped p-type silicon, which have sizes deviating from 0.5 to 1.5 microns and slightly variable profile causing partial non-repeatability of the substrate’s optical and electromagnetic properties. On the other hand, the second method leads to etching the voids with well-reproducible periodical structure and related properties. Effectiveness of nanovoids was estimated by simulation of electric field distribution under different excitation wavelengths for the cavity in silicon coated with continuous silver film with 100 nm thickness. The distance between nanovoids was 10 nm. The depth and diameter of simulated nanovoids were 0.5 microns. We solved a 3D frequency-domain wave equation for electric field with a finite element method (FEM). To simulate the excitation light we used a plane wave with a normal incidence. Simulations showed electric field enhancement in the void volume due to internal reflections. It was found that the greatest electric field in nanovoid is 2×10³ V/m. The maximal electric field shifts from the bottom to the mouth of nanovoid according to increase of excitation wavelength from 473 to 980 nm. Localization of electric field maximum in the center of nanovoid was typical for 785 nm. Experimental SERS measurements of peptides and proteins at micromolar concentration showed that the most number (80%) of full-view spectra were obtained at 785 nm. Considering simulation results this can be explained by uniform overlapping the analyte molecules with the centered electric field. In future work, fabrication of films of other metals and optimization of their deposition process on the template formed by direct electrochemical imprinting with stamp is needed. This would enable to analyze large molecules at lower concentrations and achieve reproducibility of their SERS-spectra for 90 – 95% of nanovoids.

1. K.V. Girel, A.Yu. Panarin, H.V. Bandarenka, G. Isic, V.P. Bondarenko, S.N. Terekhov, Plasmonic silvered nanostructures on macroporous silicon decorated with graphene oxide for SERS-spectroscopy, Nanotechnology, 2018, 29, P. 395708, https://doi.org/10.1088/1361-6528/aad250.
2. B.P. Azeredo, Y.-W. Lin, A. Avagyan, M. Sivaguru, K. Hsu, P. Ferreira, Direct Imprinting of Porous Silicon via Metal-Assisted Chemical Etching, Adv. Funct. Mater. 2016, 26 (17), P. 2929-2939, https://doi.org/10.1002/adfm.201505153.

Sub-wavelength periodic structures have potential in many various applications due to their unique spectral characteristics. These arrays exhibit narrow near-unity reflectivity resonances that arise from the coupling of an incident wave into a leaky waveguide mode via a grating vector that is subsequently reradiated, a phenomenon commonly known as a guided mode resonance (GMR). Such spectral characteristics are well-suited for multi- and hyper- spectral filtering applications in the infrared. We designed an all-dielectric platform consisting of amorphous silicon (a-Si) (n ≈ 3.5) nanopillar arrays embedded in SiO₂ (n ≈ 1.4) with a ratio of radius, r, and a center-to center distance, a, of r/a ≈ 0.2 in simulation and experiment for application as narrow stopband filters. These filters are ultra-thin (<0.1λ), polarization-independent, relatively straightforward to fabricate compared to conventional Bragg stack reflectors, and possess greater efficiencies relative to their plasmonic-based counterparts due to low material absorption. The choice of a-Si as the nanopillar material stems from its low cost, high index of refraction, and a band gap of 1.55 eV near the edge of the visible.

We present the tunability of the spectral characteristics of the GMR in these arrays through variation of array geometric parameters in simulation and experiment and validate the GMR formulation with photonic crystal slab band diagram calculations. The GMRs are observed experimentally in fabricated arrays with amplitude >0.85 with FWHM values ranging from 20-50 nm. We further extend our analysis to periodic arrays of finite size, which are required for high resolution snapshot imaging. GMR designs often consider only the case of an infinite array, where the leaky waveguide mode can propagate laterally for hundreds of periods, allowing for this mode to eventually scatter out of the array giving rise to the characteristic narrow near-unity rapid spectral variations of a GMR. With an insufficient number of periods, the quality factor and thus the optical filtering performance is greatly diminished. We present arrays that operate under finite size limitations optimized for spectral imaging applications in the near-infrared.

To generate Cherenkov radiation (CR) in natural medium, the electron energy threshold is higher than hundreds of keV. Even though various approaches were adopted, the high-energy electrons as high as tens of keV is still required in experiment. Here we proposed to eliminate the threshold of electron energy to generate CR with the help of hyperbolic metamaterial (HMM). The analytical and simulation results indicate that, even though electron energy is lower than 0.1keV, the CR could be obtained in HMM in a visible and near-infrared frequency region. Further, the on-chip integrated threshold-less CR source has been realized. It is demonstrated that the electron energy generating CR experimentally is two~three orders of magnitude lower than that in natural media and the first on-chip integrated free electron light source is realized.
Moreover, having free electrons interact with surface plasmon polariton (SPP) cavity mode, the surface plasmon amplification by stimulated emission of radiation (SPASER) is discovered and used for realizing lasers at nanometer scale. The conventional gain media applied in SPASER are solid materials such as organic dye or semiconductor, which limits the frequency range of SPASER. Actually, the free electrons could be considered as a kind of gain medium for emitting radiation. Here, we investigate theoretically the SPASER excited by free electrons and demonstrate numerically the tunable, deep-ultraviolet, and ultracompact laser by having free electrons interact with surface plasmon polariton mode supported on metal surface. The wavelength in deep ultraviolet could be widely tuned by varying the electron energy.
Our work opens up the possibility of exploring high performance on-chip integrated free electron light source and optoelectronic devices, and provides a way for realizing integrated free electron laser in ultraviolet frequency region.

When plasmonic metal nanoparticles are arranged in extended, one- or two-dimensional periodic arrays, the localized surface plasmon resonances (LSPRs) of the individual particles will couple radiatively to form a collective, propagating photonic-plasmonic mode known as a surface lattice resonance (SLR). Currently, SLRs and their potential applications in photonic devices, such as solar cells and light-emitting diodes, are growing topics of interest in the literature. In particular, the interaction of propagating, delocalized SLRs with the highly localized excitons of organic dye molecules is being investigated, and exotic phenomena such as the Bose-Einstein condensation of polaritons composed of dye excitons coupled to SLRs was recently reported(1). We report on the fabrication of SLR-supporting arrays of aluminum nanodisks on glass, and the coupling of these SLRs to dye excitons via coating of the arrays with dye-doped poly(methyl methacrylate). In particular, the interaction of the SLRs with boron dipyrromethene (BODIPY) dyes is examined, and the resulting effects, including angle-dependent fluorescent emission and enhancement of energy transfer between two different BODIPY dyes, are reported.

(1) Hakala, T. K.; Moilanen, A. J.; Väkeväinen, A. I.; Guo, R.; Martikainen, J.-P.; Daskalakis, K. S.; Rekola, H. T.; Julku, A.; Törmä, P. Bose–Einstein Condensation in a Plasmonic Lattice. Nature Physics 2018, 14 (7), 739.

Chiral electromagnetic metamaterials that have chiroptical activity, have attracted attention due to their suitability for many potential photonic and biological applications including lasers, molecular sensing, structural studies of organic molecules such as proteins and DNA. Despite the recent advances in chiral metamaterials, to date, very little attention has focused upon artificially large optical activity in the UV range, where molecular circular dichroism (CD) bands are most frequently encountered. In this work, a systematic investigation is made into CD behavior in the UV range with respect to geometry and critical dimensions with a semiconducting metamaterial. The glancing angle deposition (GLAD) method was used as it provides a means of experimentally investigating the relationship between morphology and optical activity, which is critical to optimizing design. We also explore more complex, multi-material and multi-functional hybrid metamaterials that are active across the UV and visible spectrum. This technique allows us to tailor optical activity making it possible to achieve chiroptical activity over a wider range of wavelengths.

The nanophotonics field has long sought to identify mechanisms to realize dynamical control of optical modes. In most approaches, the magnitude of tuning is dependent upon the degree to which the optical permittivity is malleable upon some parametric change, such as carrier concentration. Here, through a multi-wavelength Raman spectroscopic examination of 4H-SiC nanopillars, momentum is identified as a means to enhance spectral tunability of nanophotonic modes owing to the spatial dispersion implicit in the infrared (IR) optical permittivity of polar semiconductors. Experimentally, beyond the Raman features observed in bulk 4H-SiC films, a ``forbidden" Raman mode at ~780 and the surface-optic phonon at ~950 cm^-1 emerged with intensities dependent upon nanopillar diameter and the wavelength of incident light. The evolution of these modes is accompanied by a red-shift and spectral narrowing of the longitudinal-optical plasmon coupled (LOPC) mode exhibiting a similar wavelength and diameter dependence. Mie resonances, identified using ultraviolet-visible (UV-VIS) spectroscopy and excited by the visible light of the Raman experiment, acted to vary the momentum sampled during the Raman experiment leading to these spectral dependencies. The deduced permittivity possesses a momentum-dependent sensitivity to carrier concentration that highlights the potential of leveraging spatial dispersion to accentuate the performance of tunable IR phonon-polaritonics. Altogether, momentum not only provides a path by which to excite polaritons but also one to enhance their utility.

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.

Plasmonic nanostructures have gained prominence thanks to their ability to confine electromagnetic fields far below the wavelength. By virtue of the collective resonant oscillations of free electrons in metallic structures, extremely high field enhancements can be achieved [1]. These resonances are strongly related to the shape, size as well to the material properties of the nanostructures. While till date, in most cases, the spectral properties have been determined by the shape and size, the ability to tune the material properties, provides additional advantages and offers a new degree of freedom for designing plasmonic nanostructures. Alloying is an attractive way of tuning the properties of metals since it alters the permittivity and, consequently, changes the resonance wavelength [2]. In our work, we employed gold and silver alloys (Au/Ag alloy) of different composition to fabricate nanostructures of different shapes and sizes including complex structures exhibiting Fano resonances. To this end, a very original low-temperature process has been developed, which allows the nanostructures to retain their shape.
Our work aims to fabricate alloyed plasmonic nanostructures using standard techniques of e-beam lithography and lift-off. We deposit consecutive metal layers, where the thickness, hence the volume, of each layer determines the desired composition, using e-beam evaporation technique, in contrast, to often used co-sputtering [4]. Here, we demonstrate the performance of a newly developed low-temperature annealing process using three different Au/Ag alloys, namely, Au_0.2Ag_0.8, Au_0.5Ag_0.5, and Au_0.8Ag_0.2. A uniform alloy is achieved by annealing the previously deposited metal layers at temperatures of 300°C for 8 h and 450°C for 30 mins under N₂ atmosphere. The experiments reveal that using such low temperatures, far below the Tammann temperature, at which the mobility of the molecules in a solid sets in, the shape of the nanostructures are perfectly conserved! EDX and XPS measurements carried out on 150 nm thin films of the Au/Ag alloys confirm a homogeneous alloy. Ellipsometric studies carried out on thin films confirm further that the permittivity of alloys can be tuned. After successful fabrication of alloyed thin films, 50 nm high equilateral nanotriangles of 300 nm side length and nanorods of 400 nm and 50 nm length and width, respectively, and periodicity of 4 µm were fabricated by e-beam lithography. Shape and size of the structures are perfectly retained after annealing and EDX measurements confirm fully homogeneous alloys.
We also fabricated more complex Au/Ag Fano-resonant plasmonic nanostructures using this technique. The optical response of a 4 rod Fano resonant structure excited with light polarised along its long axis [4] has been simulated for five different compositions, namely, pure Ag, Au_0.2Ag_0.8, Au_0.5Ag_0.5, and Au_0.8Ag_0.2 and pure Au. The simulations clearly show a variation of the spectrum by altering the composition. These nanostructures were successfully fabricated, and experimental spectra confirmed the variation of resonances by varying the composition.
To conclude, we successfully demonstrated a novel fabrication technique where plasmonic nanostructures of different shapes and compositions retain their shape after low-temperature annealing of consecutively deposited Au and Ag layers. This result opens up the opportunity of fabricating plasmonic nanostructures of different material compositions, which offers a new degree of freedom for tailoring the optical properties of plasmonic nanostructures.

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[2] O. Peña-Rodríguez et al. Opt. Mater. Express vol. 4, 403 (2014).
[3] Guang Yang et al., J. Alloys and Compounds, vol 551, 352 (2013).
[4] X. Wang et al., Small vol. 13, 1700044 (2017).