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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Li Lu acknowledges his scholarship from Singapore Ministry of Education.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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


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

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

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

The recent approach to utilize metasurfaces made from resonant nanostructures has been revolutionizing our perception of nonlinear optical processes. Due to the relaxed phase-matching requirements, simultaneous generation of various nonlinear optical processes can be expected from them. In this work, we show that seven nonlinear processes, including fourth-harmonic generation, four-wave mixing (FWM), and six-wave mixing (SWM) processes can occur simultaneously in GaAs dielectric metasurfaces.
The dielectric metasurfaces used in this work consist of a square array of GaAs nanocylinder resonators that are spatially separated from a GaAs substrate by AlGaO. The nanocylinders have diameters of ~420 nm and support magnetic and electric dipole resonances at ~1520 nm and ~1250 nm, respectively. To explore frequency mixing processes, the GaAs metasurface sample was pumped by two near-infrared femtosecond beams. The optimization of the frequency-mixing signal was achieved by overlapping the pumps’ wavelengths with the two dipole resonances of the nanoresonators. When the two pump pulses spatially and temporally overlap, eleven spectral peaks are observed spanning from UV to near-infrared wavelengths. We divide the newly generated frequencies into two groups: those relying on only one of the two pump beams such as second-, third- and fourth-harmonic generation and two-photon absorption induced photoluminescence; and those relying on both pump beams such as sum-frequency generation, three types of FWM processes, and SWM process. We identify these mechanisms by measuring the power dependence, as well as by matching the photon energy. For example, the observed fifth-order nonlinear effect, SWM, was verified by tunning the wavelengths of the two pumps. To confirm the resonantly enhanced behavior, we also measured the generated nonlinear spectra on an unpatterned sample and observed at least two orders of magnitude smaller signal intensities for most the spectral peaks.
Our demonstration of frequency-mixing in dielectric metasurfaces combines strong material nonlinearities of GaAs, enhanced electromagnetic fields in resonators, and relaxed phase-matching conditions in nanostructures, to allow simultaneously generate of eleven new frequencies. Use of III-V metasurfaces paves a road for realizing ultra-compact optical frequency-mixer for various applications, such as telecommunication technologies.
This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering and performed, in part, at the Center for Integrated Nanotechnologies, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science. Sandia National Laboratories is a multi mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.

A conventional thermal emitter exhibits a broad emission spectrum with a peak wavelength depending upon the operation temperature. Recently, narrowband thermal emission was realized with periodic gratings or single microstructures of polar crystals such as SiC [1, 2]. These polar crystals support Surface Phonon-Polaritons (SPhPs) [3], which offer lower losses and higher resonance quality factors due to longer lifetime than the commonly used Surface Plasmon Polaritons (SPPs). Due to the strong confinement of SPhPs, subwavelength resonators can host different, spectrally narrow modes depending on geometry and period.
Here, we go one step further and investigate the coupling of adjacent phonon-polaritonic nanostructures, specifically deeply sub-diffractional bowtie-shaped silicon carbide nanoantennas. We experimentally demonstrate that the nanometer-scale-gaps can control the thermal emission frequency while retaining emission linewidths as narrow as 10 cm^-1[4].
To prove that the thermal emission originates from of nanoantenna structures and for an unambiguous assignment of the strongly localized SPhP resonant modes, we employ infrared far-field reflectance spectrosco