This PDF file contains the front matter associated with SPIE Proceedings Volume 11080, including the Title Page, Copyright information, Table of Contents, Author and Conference Committee lists.

Over the past decades, advances in metamaterial and metasurfaces inspired innovation in many imaging systems. Contrary to conventional optics where ray tracing and a Fourier transform exactly predict light propagation, those approaches are no longer valid in the case of metasurfaces and hybrid glass-metasurface coated lenses. We will discuss in this presentation our latest results and observations to develop new tools that can be incorporated within a lens design software. Analytical solution developed for this implementation will also be discussed. With these new tools, lens design can think of using metasurface within an optimized optical system

The preliminary study reported here investigates if unit-cell inclusion-symmetries may be broken in time-reversalinvariant topological insulator designs, while maintaining the desired global behaviour of pseudo-spin-dependent edge state based bi-directional, back-scattering robust, energy propagation. By allowing symmetries to be broken additional geometrical design freedom is attained, which may turn out to enable an improvement of various performance measures such as bandwidth and field confinement. The particular study considers a time-reversal-invariant acoustic topological insulator design, designed using a modified version of our recently proposed topology optimization based method for designing photonic and acoustic topological insulators.1 This method relies on a carefully constructed model system combined with the application of density based topology optimization to design two carefully interfaced crystal phases to maximize the flow of energy through the system. Through simple modifications of the method, we demonstrate that it is possible to design structures with different symmetry conditions from those that have previously been investigated using the method.

The optical properties of materials show extreme phenomena around the epsilon-near-zero (ENZ) point. For instance, the nonlinear index change (Δn) becomes large for vanishing permittivity (ε), since Δn~1/sqrt(ε). Here we show the precise fabrication control of ITO’s ENZ point to a targeted ±10nm spectral wavelength at NIR. Using this capability, we demonstrate a record-efficient plasmon MZI modulator with a FOM of just VpL=0.01-mm in Silicon photonics. Furthermore, we experimental show strong (10dB) all-optical loss-change to realize optical limiting mimicking Star Trek-like ‘shielding’ for future defense capabilities against directed laser weaponry. Lastly, we discuss a viable path towards realizing integrated metatronics showing a ΔS21=8dB transmission change for a serial vs. parallel nanostructured ITO metatronic switch.

The mid-infrared (mid-IR) spectral region contains the characteristic vibrational absorption bands of most molecules as well as two atmospheric transmission windows, and is therefore of critical importance to many biomedical, military, and industrial applications such as spectroscopic sensing, thermal imaging, free-space communications, and infrared countermeasures. Metasurface devices operating in the mid-IR potentially offer significantly reduced size, weight, and cost compared to traditional bulk optics, but they are also challenged with unique material and processing requirements. By combining high-index, broadband transparent dielectric materials with a Huygens metasurface design, we have experimentally realized high-performance metasurface devices with a low-profile, deep sub-wavelength thickness. Based on the platform, we demonstrated single-layer metalenses with focusing efficiencies up to 75% and diffraction-limited performance over a record field of view close to 180 degrees. These meta-optical devices can provide significantly enhanced design flexibility for future infrared optical systems.

We have already successfully employed the Generalized Dispersion Material (GDM) technique to include optical dispersion of different materials in the multiphysics time domain methods implementing the GDM model with various Auxiliary Differential Equation (ADE) and Recursive Convolution (RC) schemes. So far, we have demonstrated that the approach works efficiently to model the optical dispersion of metals, and to characterize the multivariate tunable dispersion of graphene. In this paper, we apply the GDM model to two emerging fields in the time-domain computational photonics. In the first part, we further extend the GDM model to the Bi-Anisotropic (BA) case, where a full BA material tensor comes from homogenization procedure in the frequency domain. Conventional BA homogenization is a powerful multiscale technique for rapid prototyping and optimization of metasurfaces. With a new extension, the BA-GDM model characterizes artificial dispersion obtained from the mathematical equivalence of physical effects and enables the multiscale modeling of metasurfaces in the time domain. Part 2 deals with new use of the GDM model in temperature-dependent time-domain simulations of phase change materials (PCMs). Optical PCMs, such as GST/GSST, are of critical utility in applications including, e.g., programmable metasurfaces, and nonvolatile memory. Typically, dispersions of amorphous and crystalline phases of PCMs are fitted separately in the frequency domain with a combination of the Tauc-Lorentz and Gauss terms, while Bruggeman’s mixing rule describes the transition states. Significantly advancing the-state-of-the-art, our GDM characterization describes dependency on temperature and crystallization levels explicitly and enables full wave modeling of PCMs in the time domain.

Recent years have seen an increase of interest in developing nano-scale sources of coherent radiation. Numerous schemes includes novel confinement techniques such as plasmons and micro resonators as well as novel materials, such as two-dimensional transition metal di-chalcogenides have been investigated. In the process a lot of confusion has been produced by introduction into consideration of numerous parameters such as Purcell factor and the beta (fraction of spontaneous emission going into a given mode). That confusion makes comparative analysis of miniature lasers difficult. In this talk we show that the only parameters defining the laser threshold are the modal loss, quantum efficiency and physical volume of gain medium. Using this method we compare the novel lasers with existing state of the art – DFB and VCSELs and draw conclusions.

A Hamiltonian H satisfies parity-time (PT) symmetry if it commutes with combination of parity operator P and time-reversal operator T, i.e., HPT=PTH. Currently, PT symmetric H is widely incarnated by symmetric refractive index profile n(x)=n(-x) and anti-symmetric gain and loss profile g(x)=-g(-x) in optical systems. Its eigenvalue can spontaneously transit from real to imaginary eigenvalues as |g(x)| increases, identified as PT symmetric or PT broken phase. This peculiarity has been utilized for inducing asymmetric mode coupling to achieve directional light transport [1], low-power optical diode [2], etc. However, all these structures can not be used for any nonreciprocal devices since their modes can always find pathways to scatter back to the input under symmetric scattering matrix [3]. Here, we introduce a magnetic-free dynamic modulation of gain and loss profile in an on-chip micro-ring resonator to break reciprocity. Applying a moving grating of alternating gain and loss along the azimuthal direction of the ring, the original degeneracy between clockwise whispering gallery mode (WGM) and the counter-clockwise one will be removed in the PT broken phase. With that, two laser emission of different frequencies can be simultaneously extracted by a bus waveguide to each end, respectively, where the frequency detuning is controlled by the modulation. Benefit from the PT symmetry of the system, the lasing threshold can be tailored by the strength of dynamic modulation. Our study pointed out a new viable way for on-chip integration of lasing sources which can generate and separate different emission frequencies simultaneously. [1]. Z. Lin, H. Ramezani, T. Eichelkraut, T. Kottos, H. Cao, and D. N. Christodoulides, “Unidirectional invisibility induced by p t-symmetric periodic structures,” Phys. Rev. Lett. 106, 213901 (2011). [2]. H. Ramezani, T. Kottos, R. El-Ganainy, and D. N. Christodoulides, “Unidirectional nonlinear pt-symmetric optical structures,” Phys. Rev. A 82, 043803 (2010). [3]. S. Fan, R. Baets, A. Petrov, Z. Yu, J. D. Joannopoulos, W. Freude, A. Melloni, M. Popovic, M. Vanwolleghem, D. Jalas et al., “Comment on ”nonreciprocal light propagation in a silicon photonic circuit”,” science 335, 38–38 (2012).

I will describe ra novel nonlinear material---an ultrathin time-varying semiconductor metasurface---that exhibits efficient blue-shifting of mid-IR photons [1]. The observed signature of such "photon acceleration" is third-harmonic generation with tunable frequencies and bandwidth. Several applications will be discussed, including resolving the bandwidth/efficiency paradigm in nonlinear optics using time-varying metasurfaces. [1] Maxim R. Shcherbakov,Kevin Werner, Zhiyuan Fan,Noah Talisa, Enam Chowdhury, and Gennady Shvets, "Nonlinear manifestations of photon acceleration in time-dependent metasurfaces: tunable broadband harmonics generation", arXiv:1710.06966v3 (2019).

A strong optical field, ~0.1-1 V/Å, changes solids on the attosecond time scale, i.e., within an optical cycle. Such fields drive ampere-scale currents in dielectrics and adiabatically controls their properties, including optical absorption and reflection, extreme UV absorption, and generation of high harmonics in a non-perturbative manner [1-5]. We will concentrate on ultrafast phenomena defined by nontrivial topological properties of solids in the reciprocal space, which are described by non-zero Berry (topological) curvature and Berry flux, which to a significant degree define their behavior in strong optical fields. In particular, these are graphene [6, 7], silicene [8], and surfaces of topological insulators (TI’s) (semimetals) [9], monolayer transitional metal dichalcogenides (TMDC’s) [10], black phosphorus and phosphorene (direct bandgap semiconductors), and hexagonal boron nitride (h-BN) (dielectric). For two-dimensional semiconductors such a TMDC’s, we predict a new attosecond phenomenon in a strong chiral optical fields – a topological resonance [11]. This manifests itself in the establishment of a strong valley polarization during just a single optical cycle, i.e., in the fundamentally fastest way possible. It structures the reciprocal space into topologically distinct areas. This phenomenon is promising for ultrafast recording of both classical bits and cubits for quantum information processing. Another distinct class of two-dimensional systems in a strong pulse field that we consider are surfaces of TI’s. These are crystals characterized a non-zero topological invariant Z2=1 where bulk is semiconducting but surfaces are Dirac semimetals. In the surface reciprocal space, they contain a single Dirac point with a Berry phase of ±π at the Г-point. Subjected to circularly-polarized ultrashort strong pulses they exhibit chiral textures in the reciprocal space and topologically-protected currents [9]. Finally, we will present our latest results on Weyl semimetals in ultrafast strong chiral fields. Such fields induce topological resonances and ultrafast bulk currents on femtosecond time scales. References 1. A. Schiffrin, T. Paasch-Colberg, N. Karpowicz, V. Apalkov, D. Gerster, S. Muhlbrandt, M. Korbman, J. Reichert, M. Schultze, S. Holzner, J. V. Barth, R. Kienberger, R. Ernstorfer, V. S. Yakovlev, M. I. Stockman, and F. Krausz, Optical-Field-Induced Current in Dielectrics, Nature 493, 70-74 (2013). 2. M. Schultze, E. M. Bothschafter, A. Sommer, S. Holzner, W. Schweinberger, M. Fiess, M. Hofstetter, R. Kienberger, V. Apalkov, V. S. Yakovlev, M. I. Stockman, and F. Krausz, Controlling Dielectrics with the Electric Field of Light, Nature 493, 75-78 (2013). 3. F. Krausz and M. I. Stockman, Attosecond Metrology: From Electron Capture to Future Signal Processing, Nat. Phot. 8, 205-213 (2014). 4. B. Förg, J. Schötz, F. Süßmann, M. Förster, M. Krüger, B. Ahn, W. Okell, K. Wintersperger, S. Zherebtsov, A. Guggenmos, V. Pervak, A. Kessel, S. Trushin, A. Azzeer, M. Stockman, D. E. Kim, F. Krausz, P. Hommelhoff, and M. Kling, Attosecond Nanoscale near-Field Sampling, Nat. Commun. 7, 11717-1-7 (2016). 5. S. Ghimire, G. Ndabashimiye, A. D. DiChiara, E. Sistrunk, M. I. Stockman, P. Agostini, L. F. DiMauro, and D. A. Reis, Strong-Field and Attosecond Physics in Solids, Journal of Physics B: Atomic, Molecular and Optical Physics 47, 204030-1-10 (2014). 6. H. K. Kelardeh, V. Apalkov, and M. I. Stockman, Wannier-Stark States of Graphene in Strong Electric Field, Phys. Rev. B 90, 085313-1-11 (2014). 7. H. K. Kelardeh, V. Apalkov, and M. I. Stockman, Graphene in Ultrafast and Superstrong Laser Fields, Phys. Rev. B 91, 045439-1-8 (2015). 8. H. K. Kelardeh, V. Apalkov, and M. I. Stockman, Ultrafast Field Control of Symmetry, Reciprocity, and Reversibility in Buckled Graphene-Like Materials, Phys. Rev. B 92, 045413-1-9 (2015). 9. S. A. Oliaei Motlagh, J.-S. Wu, V. Apalkov, and M. I. Stockman, Fundamentally Fastest Optical Processes at the Surface of a Topological Insulator, Phys. Rev. B 98, 125410-1-11 (2018). 10. S. A. Oliaei Motlagh, V. Apalkov, and M. I. Stockman, Interaction of Crystalline Topological Insulator with an Ultrashort Laser Pulse, Phys. Rev. B 95, 085438-1-8 (2017). 11. S. A. Oliaei Motlagh, J.-S. Wu, V. Apalkov, and M. I. Stockman, Femtosecond Valley Polarization and Topological Resonances in Transition Metal Dichalcogenides, Phys. Rev. B 98, 081406(R)-1-6 (2018).

High-harmonic generation (HHG) has been used to generate extreme ultra-violet (EUV) light sources to probe fast electron dynamics in the attosecond time scale. While traditionally observed in rare-gas atoms, HHG has also recently been reported in solids, with reduced threshold pump field and the additional advantage of producing stable EUV waveforms in a compact setup. Unfortunately, above-band-gap absorption restricts the HHG process to a very thin layer of the solid-state material (typically tens of nanometers in thickness), significantly limiting the generation efficiency. Here, we use a material operating in its epsilon-near-zero (ENZ) region, where the real part of its permittivity vanishes, to greatly boost the efficiency of the HHG process at the microscopic level. In experiments, we report high-harmonic emission up to the 9th order directly from a low-loss, solid-state ENZ medium: indium-doped cadmium oxide, with an excitation intensity at the GW cm-2 level. Furthermore, the observed HHG signal exhibits a pronounced spectral red-shift as well as linewidth broadening, resulting from the photo-induced electron heating and the consequent time-dependent resonant frequency of the ENZ film. Our results provide a novel nanophotonic platform for strong field physics, reveal new degrees of freedom for spectral and temporal control of HHG, and open up possibilities of compact solid-state attosecond light sources

We present beam deflection measurements to study the nondegenerate nonlinear refraction of highly doped semiconductors at epsilon-near-zero (ENZ) for several different pump and probe polarizations. Beam deflection is sensitive to induced optical path length as small as 1/20,000 of a wavelength, which enables us to resolve NLR in the presence of large nonlinear absorption backgrounds. The optically induced index changes in these materials can be both very large (on the order of unity) and fast (on the order of 100 fs). Our results show that carrier redistribution effects dominate the nonlinear refraction, and by independently tuning the pump and probe wavelengths, we find that the strong wavelength dependence of nonlinearities around the ENZ point is different for pump and probe waves. These nonlinear optical properties, where the ultrafast index change can be larger than the linear index, offer new paradigms for dynamically switchable diffractive elements that respond to structured light, allowing manipulation of optical beams in transmission and reflection not only along the two spatial dimensions but also in time. This is a revolutionary change in the field of nonlinear optics allowing a myriad of potential applications, ranging from rapid all-optical beam steering and switching, to spectral scanning, spatial mode conversion, as well as pulse shaping and suppression, all on sub-picosecond time scales.

Vacuum ultraviolet (VUV) light is electromagnetic radiation in the wavelength regime between 100nm and 200nm. Due to its high photon energy, VUV light is very important in many advanced applications, such as photochemistry, nanolithography, and novel spectroscopy. While several methods to generate coherent VUV light have been developed, a multifunctional nanophotonic device for efficient VUV light generation and control still remains an outstanding challenge. In this work, we demonstrate an all-dielectric metasurface device (metadevice) which can be used for nonlinearly generating coherent VUV light. Metasurfaces are artificial nanostructures with resonance properties that can be designed by carefully tailoring their geometric parameters. The metadevice demonstrated in this work consists of an array ZnO nano-resonators. To fabricate the metadevice, a 150-nm multicrystalline ZnO was nanopatterned using a focus ion beam system. In the linear transmission measurement, the metadevice shows a resonance dip at around 400nm. This resonance was found to be associated to a magnetic dipole resonance of the nano-resonators. When a visible femtosecond laser beam (wavelength: 394nm) as an excitation source was loosely focused on the metadevice, VUV light originating from second harmonic generation of the metadevice was produced. The dependence of the VUV signal on the excitation power as well as on the incident angle were carefully measured and analyzed. Furthermore, manipulation of the output VUV wavefront was realized by carefully controlling the nano-resonator arrangement. This work paves a novel route toward high-efficiency multifunctional nanophotonic devices for VUV applications [Nano Lett. 18, 5738 (2018)].

Metasurfaces and metalenses have shown remarkable progress and performance in laboratory results in the past years. For metasurfaces to make the transition from laboratory to large scale adaptation, several challenges need to be overcome: high manufacturing yields must be shown to reach cost-effective fabrication, the robustness of metalenses to various imperfections should be demonstrated and integrated systems with metalenses need to be made with higher performance than conventional systems. Controlling the various types of unavoidable fabrication imperfections is critical for achieving these. In this paper we present Monte-Carlo simulations of metalenses with random fabrication defects as a tool to quantify the performance loss associated with different types of errors. We model changes in sidewall steepness, feature size and missing structures. We study these effects in a reference metalens consisting of SiliconNitride pillars on a glass substrate. The methods presented in this paper can be readily applied to other metalens and metasurface designs and are implemented in the PlanOpSim software package. We find that when the deviations in sidewall steepness are within a distribution with root mean square of 1.5° and changes in pillar shape have a root mean square no larger than 10nm the focusing efficiency of the metalens remains within 90% of its nominal value.

Higher-order topological (HOT) states are topological states localized in more than one dimension of a D-dimensional system. In the recent years, HOT states have been shown to exist in classical wave-systems such as photonics and acoustics and have been used to explore a host of topological phenomena that have typically been associated with condensed matter systems. In our work, we construct the 3D acoustic metamaterial with HOT states through a rapid prototyping process and manufacture the individual metaatoms and metamolecules, which can then be snapped together to form 3D metamaterials with complex geometries. The assembled 3D topological metamaterial represents the acoustic analogue of the pyrochlore lattice with acoustic modes strongly bound to the individual resonant cavities and a design that only allows for nearest neighbor coupling. This provides us with the framework to explore the topological nature of the structure in a semi-analytical way (tight-binding model) while comparing it to the first-principles finite element method (FEM) model, and then comparing both theoretical results to the experiment. Consistent with the models, we observe the third-order (0-D) topological corner states along with second-order (1D) edge states and first-order (2D) surface states within the same topological bandgap, thus establishing a full hierarchy of HOT states in three dimensions. Additionally, we experimentally measure the field profile of each topological mode, which are in excellent agreement with the numerically calculated profiles of the HOT states.

Topological insulators are generally characterized by means of integral invariants defined using cocycles over the Bloch bundle (e.g.: Chern or Stiefel characteristic classes). In the case of photonic topological insulators made of continuous media, I will show that the invariants result from the consideration of a projective algebraic curve (the spectral curve) and of the bundle of eigenvectors over it. The main result is that photonic Chern insulators do not exist in continuous media

The strong optical chirality arising from certain synthetic metamaterials has important and widespread applications in polarization optics, stereochemistry and spintronics. Recently we have shown that strong intrinsic optical chirality can arise in planar high-index dielectric nanostructures whose thickness is greater than an optical wavelength, due to the excitation of magnetic dipoles that lie in the same plane as, but are orthogonal to, their electric counterparts. However these structures were comprised of a lossless dielectric, and incident light of the undesired helicity was diffracted instead of transmitted in the zeroth order. Here we explore the possibility of designing absorptive, subwavelength chiral metasurfaces with a desired transmission and absorption spectrum. We find that while the usual design process using discrete, well-known geometries can lead to structures with efficient contrast in transmission, it is extremely challenging to simultaneously engineer the reflection or absorption spectrum. We use topological optimization techniques to realize chiral metasurfaces comprised of freeform geometries, and show that they can exhibit a large intensity contrast in both transmission and absorption, depending on the helicity of incident circularly polarized light. These metasurfaces could be useful particularly for display technologies, and potentially overcome the inherent 50% transmission limit set by a regular circular polarization analyzer comprised of an absorptive linear polarizer and half-waveplate.

The discovery of topological condensed-matter systems has promoted extensive research on analogous classical photonic systems, motivated by the prospect of backscattering-immune wave propagation. So far, photonic topological insulators have mainly relied on engineering bulk modes in photonic crystals and waveguide arrays in two-dimensional systems. However, these realizations suffer from bulky structures, intricate design/material requirements, or limited operational bandwidth. Here, we present symmetry-protected topological states akin to quantum spin-Hall and valley-Hall effects by engineering surface modes over open-boundary metallic metasurfaces of infinitesimal thickness. As a result, the proposed structures support robust gapless edge states, which are confined and guided along a one-dimensional line rather than a surface interface. To emulate the spin degree of freedom, we exploit the electromagnetic-duality symmetry by stacking two complementary metasurfaces. Straightforwardly, the modal degeneracies are formed at high-symmetry K/K′ points due to the use of hexagonal unit cells, while the strong effective magneto-electric coupling inherent to the overlapped metasurfaces opens a wide non-trivial bandgap. To emulate the valley degree of freedom, on the other hand, we exploit the mirror symmetry of the structure by reducing the lattice symmetry of the hexagonal cell-based metasurface, which has either inductive or capacitive response, into C3υ point symmetry. Consequently, the degeneracy between the two valleys in reciprocal space is lifted. Owing to the simplicity, compactness, tunability, and openboundary nature of the proposed system, it constitutes an attractive tabletop platform for the study of classical topological phases, as well as for practical applications advancing the potential of photonic topological insulators.

Since the development of diffractive optical elements in the 1970s, major research efforts have focused on replacing bulky optical components by thinner, planar counterparts. The more recent advent of nanophotonic metasurfaces has further accelerated the development of flat optical elements through the realization that resonant optical antenna elements can be utilized to facilitate local amplitude and phase control In this presentation, I will show how metasurfaces can start to impact Augmented and Virtual Reality applications. I will first discuss the creation of high-efficiency metasurface-based optical combiners for use in augmented reality devices and near-eye displays. I will also illustrate how guided mode resonance waveguides can be used effectively for tracking the motion of the human eye. The proposed optical elements can be fabricated by scalable fabrication technologies, such as nanoimprint lithography, rolling optical lithography, and direct write optical lithography.

The sensing of chiral molecules is important for chemical, pharmaceutical, and medical applications. The fast and accurate testing of small quantities of analyte by optical means is desirable, in particular for lab-on-a-chip technologies. Different nanostructures have been proposed for enhancing the molecular circular dichroism (CD) in their near fields, yet not all structures and near fields serve this purpose equally well. We will present the design guidelines that lead to nanostructures that enhance the CD signal of molecular solutions, use these guidelines for optimizing arrays of silicon disks, and demonstrate a more than tenfold increase in sensitivity.

We explore light-driven manipulation, levitation, and propulsion of ultralight weight macroscopic objects (size >> ) whose properties are tailored by nanophotonic design. Our analysis expands the regime of self-stabilizing optical manipulation from the regime of microscopic (i.e., wavelength-scale) objects such as nanoparticles to the macroscopic regime of many m, mm, cm, or even meter-scale objects, which can be achieved by tailoring the radiation pressure forces by controlling the anisotropy and spatial distribution of light scattering along the object surface. From this has emerged a general, scale-independent theory for the light-induced manipulation of macroscopic objects with patterned nanoscale components that impart optical anisotropy. From the theory, we can develop specific examples, including a scalable design that features silicon resonators on a silica substrate where these nanophotonic structures serve to self-stabilize the body dynamics.

Insensitivity of random systems to the polarization of incident light even for anisotropic and asymmetric particles, larger information capacity and higher level of information transport security as a result of larger degrees of freedom and the absence of spurious diffraction orders observed in periodic structures with large periodicity are among unique features making disordered structures a promising candidate to address challenges in the optical wave manipulations. Most of the metasurfaces are arranged in a periodic grid and the required phase profile for a desired performance is achieved by engineering elements via extracted information from periodic/unit cell simulation definitely not addressing the near field coupling between randomly positioned elements and so not helpful for the design of disordered metasurfaces. In this numerical study, we show how random arrangement of particles affect their phase shift compared to the periodic ones. We propose a new phase-map to design random metasurfaces benefiting from the statistical nature of random media and addressing the near field coupling between resonant elements. This phase-map provides us with the information on the geometry of particles located at random positions for a specific phase shift. Design of random metasurfaces by the proposed random phase-map reveals efficiency improvement compared to those designed based on periodic phase-map. We hope this new phase-map can pave the way towards random optical system outperforming the periodic counterpart in secure optical information processing.

The emergence of metasurface technology and its accompanying design principles are enabling the development of optical components with multiple functionalities, e.g., polarization discrimination and focusing. In this report, we highlight our experimental results associated with the characterization of a reflective mid-wave infrared (mid-IR) metasurface designed for both imaging and polarization-specific beam splitting in the 4.2 through 4.8 m region of the electromagnetic (EM) spectrum. A large area metasurface (1 cm diameter), fabricated using nano-lithography, was observed to exhibit a high degree of discrimination between transverse electric (TE) and magnetic (TM) polarized light with near diffraction limited focusing.

Transmissive-type metasurfaces represent an ultrathin alternative to traditional optical elements, e.g., lenses and waveplates. However, transmissive-type plasmonic metasurfaces (PMs) have significantly low efficiency compared to dielectric metasurfaces and reflective type PMs particularly in the visible range. For example, the state-of-the-art geometric PMs transmission efficiency is ≤10% with extinction ratios ~ 0 dB. The low transmission efficiency is mainly due to three loss channels (i) absorption losses in metals, (ii) diffraction to undesired high-orders, and most importantly (iii) symmetric forward-backward scattering which puts a 25% theoretical limit on cross-polarization conversion for ultrathin metasurfaces. We use tunable, multipole-interference-based meta-atoms to address all loss channels simultaneously. The experimentally demonstrated transmission efficiency and extinction ratio of our geometric PM are 42.3% and 7.8dB, respectively. As for dielectric metasurfaces, we demonstrate a new class of metasurfaces where the meta-atoms consist of a simple anti-reflective coating (ARC). ARCs enable the control over the entire 2 pi phase range by varying the dielectric films thicknesses while realizing ~ 99% transmission efficiency even in the visible range. The metasurface consists of patches of ARC meta-atoms with dielectric optical thicknesses much lower than that required in Fresnel optics to control the entire phase range.

Graphene photodetectors’ intrinsically low responsivity (sensitivity) has been a long-standing issue that overshadows graphene’s other excellent optical properties as a photodetection material. The key to improving the graphene photodetector responsivity lies in enhancing the photothermoelectric (PTE) effect, which has already been demonstrated to be the dominant photocarrier generation mechanism. To maximize the PTE current, one would need a strong optically-induced temperature gradient to overlap with a graphene p-n junction spatially. Here, the temperature gradient drives the charge carrier movement, while the graphene p-n junction separates the different charge carrier types (electrons and holes) and makes them drift in opposite directions. In this work, we show that these two conditions can be met simultaneously in a meticulously designed device, combining a gap plasmon structure and a pair of split-gates. The gap plasmon structure absorbs 71% of incident light creating localized heating (thereby large temperature gradient), and the split-gates create a p-n junction at the center of the localized thermal gradient. We fabricated a graphene photodetector with the proposed configuration, and experimentally verified the dominance of PTE effect in photocurrent generation in good agreement with theoretical calculations. More importantly, we obtained a responsivity 70 times higher than the previously reported value from a similar device without plasmon-enhancement. Moreover, originating from the combination of gap plasmon-enhanced optical absorption and optimized p-n junction, our responsivity is 5~7 times higher than reported values for other graphene photodetectors with different types of plasmon-enhancement and no junction control.

We present an original design of a nonlinear metasurface featuring symmetry protected bound states in the continuum (BICs) which enable enhanced photon pair generation via the process of Spontaneous Parametric Down-Conversion (SPDC). We establish both analytical and numerical methods for the optimization of BIC modes which enables the simultaneous enhancement of non-degenerate photon frequencies. We achieve this by inserting oligomer holes into the AlGaAs nanodisks which, along with the symmetry of the lattice, allow us to select from a range of unit cell symmetries. The non-Mie eigenfunctions of the chosen symmetry group will form symmetry protected BICs for zero transverse momentum (Gamma point of the Brillouin zone). Away from the Gamma point, these BICs become high-quality factor Fano resonant modes, which can significantly enhance the photon pair generation. Because the BICs are symmetry protected, we are able to tune the design parameters of the metasurfaces to select pairs of wavelengths for which a non-degenerate SPDC process is enhanced. We further utilize an analytical analysis of the classical-quantum correspondence between sum frequency generation (SFG) and SPDC for metasurfaces and thus predict the SPDC generation of our metasurface via numerical simulations of SFG over the first Brillouin zone. The ultra-thin metasurface thickness removes the conventional restrictions associated with bulk phase-matching. This opens the potential for generation of photons with tailored quantum entanglement at ultra-short time-scales for photons across the visible and infrared spectral regions. Such features of quantum states can underpin advances in nonlinear quantum spectroscopy, low-light sensing, and ghost imaging.

We report on the recent progress in the theoretical understanding of the quantum optical properties of quasi-2D plasmonic nanostructures (metasurfaces and films) of controlled finite thickness [1-4]. While being constant for relatively thick films, the plasma frequency of ultrathin plasmonic films acquires the spatial dispersion typical of 2D materials, gradually shifting to the red with the film thickness reduction [1]. This explains recent experiments done on ultrathin TiN films of controlled variable thickness [5]. The confinement induced plasma frequency spatial dispersion and associated dielectric response nonlocality can result in the new attractive features of the magneto-optical response of the film [3]. The magnetic permeability exhibits a sharp resonance structure shifting to the red as the film aspect ratio increases. When tuned appropriately, the ultrathin films of finite lateral size can be negatively refractive in the IR frequency range. As compared to the thick film, a dipole emitter near the ultrathin film shows a two-order-of-magnitude radiative decay rate enhancement with inelastic electron scattering diminishing the effect. The radiative decay rate can even be enhanced unidirectionally using the ultrathin, periodically anisotropic metal-dielectric plasmonic nanostructures [4]. Last but not least, we discuss the peculiarities such as the overall temperature behavior of the ENZ plasmonic modes and their interaction with light in such systems. We show that the light-matter interaction in close proximity to plasmonic films can be controlled not only by varying the chemical composition and material quality of the film but also by adjusting its thickness and aspect ratio as well as by choosing the deposition substrates and coating layers appropriately. We believe our findings open up entirely new avenues for potential applications of ultrathin plasmonic films in modern optoelectronics. Acknowledgments: NSF-DMR-1830874 (I.V.B.), DOE-DE-SC0007117 (H.M.), ONR-N00014-16-1-3003 (V.M.S.) References: [1] I. V. Bondarev and V. M. Shalaev, Universal features of the optical properties of ultrathin plasmonic films, Optical Mater. Express 7, 3731 (2017). [2] I.V. Bondarev and V.M. Shalaev, Quantum electrodynamics of optical metasurfaces, 2018 International Applied Computational Electromagnetics Society Symposium (ACES), 1-2. [3] I. V. Bondarev, H. Mousavi, and V. M. Shalaev, Optical response of finite-thickness ultrathin plasmonic films, MRS Commun. 8, 1092 (2018). [4] I. V. Bondarev, Finite-thickness effects in plasmonic films with periodic cylindrical anisotropy [Invited], Optical Mater. Express 9, 285 (2019). [5] D. Shah, H. Reddy, N. Kinsey, V.M. Shalaev, and A. Boltasseva, Optical properties of plasmonic ultrathin TiN films, Adv. Optical Mater. 5, 1700065 (2017).

We develop a theory of cooperative emission mediated by cooperative energy transfer (CET) from an ensemble of quantum emitters (QE) to plasmonic antenna at a rate equal to the sum of individual QE-plasmon energy transfer rates. If the antenna radiation efficiency is sufficiently high, the transferred energy is radiated away at approximately the same cooperative rate that scales with the ensemble size. We derive explicit expressions, in terms of local fields, for cooperative Purcell factor and enhancement factor for power spectrum valid for plasmonic structures of any shape with characteristic size smaller than the radiation wavelength. The radiated power spectrum retains the plasmon resonance lineshape with overall amplitude scaling with the ensemble size. If QEs are located in a region with nearly constant plasmon local density of states (LDOS), e.g., inside a plasmonic nanocavity, we demonstrate that the CET rate scales linearly with the number of excited QEs, consistent with the experiment, and can be tuned in a wide range by varying the excitation power. For QEs distributed in an extended region saturating the plasmon mode volume, we show that the cooperative Purcell factor has universal form independent of the system size. The CET mechanism incorporates the plasmon LDOS enhancement as well, giving rise to possibilities of controlling the emission rate beyond field enhancement limits.

Enhancement of local electromagnetic fields is instrumental for engineering of light absorption, emission, scattering, chemical reactions, and other processes. Nanostructured composites with plasmonic inclusions have been shown as promising candidates to concentrate electromagnetic waves in nanometer-sized “hot spots”. Unfortunately, majority of high-performance plasmonic structures are resonance-based, and therefore their performance is relatively narrow-band. Here we present a novel material system that has potential to realize broadband enhancement of local intensity and explain the origin of this behavior. The proposed material platform comprises an array of aligned plasmonic cones arranged in a periodic planar lattice. From the effective medium standpoint, such structure represents a uniaxial material whose effective permittivity varies along the cone. Importantly, there exists a relatively wide range of wavelengths where one component of the effective permittivity tensor crosses zero within the composite. According to previous research, strong enhancement of local field is expected in the vicinity of epsilon-near-zero point in homogeneous materials with spatially varying permittivity, often called transitional metamaterials. We show, however, that due to strong structural nonlocality electromagnetic response of nanocone media does not follow this recipe. In fact, we demonstrate that the incoming radiation is coupled into an additional electromagnetic wave that propagates towards the tip of the cone causing a strong enhancement to the local field. We present a comprehensive description of this phenomenon.

We review the physics of bound states in the continuum and their applications in meta-optics and metasurfaces. First, we discuss strong coupling between modes of a single subwavelength high-index dielectric resonator and analyse the mode transformation and Fano resonances when resonator’s aspect ratio varies. We demonstrate that strong mode coupling results in resonances with high quality factors, which are related to the physics of bound states in the continuum when the radiative losses are almost suppressed due to the Friedrich–Wintgen scenario of destructive interference. Our theoretical findings are confirmed by microwave and optical experiments for the scattering of high-index subwavelength resonators with a tunable aspect ratio. The proposed mechanism of the strong mode coupling in single subwavelength high-index resonators accompanied by resonances with high quality factor helps to extend substantially functionalities of all-dielectric nanophotonics that opens new horizons for active and passive nanoscale metadevices. Next, we discuss how bound states in the continuum can appear in metasurfaces. We reveal that metasurfaces created by seemingly different lattices of (dielectric or metallic) meta-atoms with broken in-plane symmetry can support sharp high-Q resonances that originate from the physics of bound states in the continuum. We demonstrate a direct link between the bound states in the continuum and the Fano resonances, and discuss a general theory of such metasurfaces, suggesting the way for smart engineering of resonances for many applications in nanophotonics and meta-optics.

We present a new class of thin-film based metamaterials that exhibits Fano resonance with wide range of potential applications. We realize Fano resonance via thin-film interference between a broadband (continuum) and a narrowband (discrete) light absorbers. Fano resonant optical coatings (FROCs) exhibit selective light reflection, similar to distributed bragg reflectors, with narrower bandwidth and overall significantly less thickness for a given wavelength range. Accordingly, FROCs produce vibrant colors superior to structural coloring via selective light absorption that has been demonstrated using metamaterials and thin-film cavities. We control the iridescence of the produced colors and can produce iridescent free, ultra-pure colors spanning the entire visible spectrum. Furthermore, we show that FROCs can be used as narrowband beam splitters, as opposed to being simple color filter similar to metal-dielectric cavities. Finally, we utilize the absorption/reflection properties of FROCs in energy applications and show that by selectively reflecting light within the absorption band of Si photovoltaic cell, while absorbing the rest of the solar spectrum, we obtain higher power from PV cells as opposed to a normal silver mirror, while increasing the FROC temperature significantly. Accordingly, FROCs can play a crucial role in hybrid, solar-PV and solar-thermal power generation which is of major importance in recent years due to limitation on electric energy storage. By coating an Aluminum sheet with FROC, while processing its back side to be superwicking, we demonstrate single-element spectral splitting that generate electricity from a PV cell while using the generated heat for water desalination.

Metamaterial approach is capable of drastically increasing the critical temperature, Tc, of composite metal-dielectric superconductors. Tripling of Tc was observed in bulk Al-Al2O3 core-shell metamaterials. A theoretical model based on the Maxwell-Garnett approximation provides a microscopic explanation of this effect in terms of electron-electron pairing mediated by a hybrid plasmon-phonon excitation. We report the first observation of this excitation in Al-Al2O3 core-shell metamaterials using inelastic neutron scattering. This result provides support for this novel mechanism of superconductivity in metamaterials and explains the 50 year old mystery of enhanced Tc in granular aluminium films.

Photogeneration of significant electrical voltages observed in plasmonic metasurfaces is promising for applications in plasmon-based electronics and plasmonic sensors with compact electrical detection. In order to better understand the role of the surface geometry, we study photoinduced electrical effects in profile-modulated plasmonic surfaces. Photoinduced voltages in strongly modulated plasmonic surfaces demonstrate highly asymmetric angular dependence with polarity switching at the plasmon resonance conditions. The effects are attributed to coupling between localized and propagating plasmons and discussed in the frame of the electromagnetic momentum loss approach.

Plasmonic materials such as conventional metals, semimetals (transition metal nitrides and carbides), highly doped semiconductors, 2D and quasi-2D materials are playing an increasingly important role in emerging technologies for sustainable energy and photocatalysis. For example, enhanced hot electrons generation in transition metal nitride nanoparticles compared to conventional materials such as gold enables more efficient light-to-electricity conversion. Ultra-broadband light absorbers with titanium nitride nanoparticles and quasi-two-dimensional transition metal nitride and carbides (MXenes) that can be obtained through fast, large-scale and environmentally-friendly processes pave the way to sustainable and scalable technologies addressing the global needs for energy and water.

Strategies for ultrafast optical control of magnetism have been a topic of intense research for several decades because of the potential impact in technologies such as magnetic memory spintronics and quantum computation, as well as the opportunities for non-linear optical control and modulation in applications such as optical isolation and non-reciprocity. Here we report the first experimental quantification of optically induced magnetization in plasmonic Au nanoparticles due to the inverse Faraday effect (IFE). The induced magnetic moment in nanoparticles is found to be ~1,000x larger than that observed in bulk Au, and ~20x larger than the magnetic moment from optimized magnetic nanoparticle colloids such as magnetite. Furthermore, the magnetization and demagnetization kinetics are instantaneous within the sub-picosecond time resolution of our study. By controlling the relative polarization difference between the pump and probe beams in an ultrafast time-resolved study, the contribution of the IFE and the optical Kerr effect (OKE) was clearly distinguished. Our experiments measured optical rotation indicative of magnetization that is parallel or anti-parallel with a pump beam depending on the helicity of the excitation. Additionally, we observe optically induced magnetization that is ~1,000 times larger than in bulk Au, and linearly proportional with incident optical power. We anticipate these results may be of great interest in the photonics community for application in ultrafast optical control of magnetic properties, and for all-optical methods of optical isolation that do not require externally applied magnetic fields.

Complex optical and nanophotonic systems require that we think about light and optical devices in new ways. The powerful approach of singular value decomposition lets us describe optical components and communication channels through pairs of input and output functions – communications modes for the best channels, and mode-converter basis sets for optimal device descriptions. Using interferometer meshes, we can also synthesize arbitrary linear optical components this way, including self-configuring, self-correcting, and self-stabilizing systems in a combination of novel architectures and algorithms. The approach also allows new radiation laws and a novel quantization of the electromagnetic field.

Planar optical elements with efficient light control at the nanoscale can be designed based on transdimensional photonic lattices that include 3D-engineered nanoantennas supporting multipole Mie resonances and arranged in the 2D arrays to harness collective effects in the nanostructure. Periodic arrays of nanoparticles have gained special attention because of extraordinary lattice resonances in proximity to the wavelength of diffraction, the so-called wavelength of Rayleigh anomaly. We show the possibility of exciting strong periodic nanoparticle resonances not only in plasmonic arrays but also in the nanoparticle lattices with the high refractive index. We perform the calculation for tungsten disulfides, which belongs to the families of van der Waals materials and transition metal dichalcogenides. Nanoparticles arrangement in a periodic array plays a crucial role resulting in collective array resonances and pronounces features in spectra.

The Unruh effect is the prediction that an accelerating object perceives its surroundings as a bath of thermal radiation even if it accelerates in vacuum. The Unruh effect is believed to be very difficult to observe in the experiment, since an observer accelerating at g=9.8 m/s2 should see vacuum temperature of only 4x10^-20 K. Here we demonstrate that photons in metamaterial waveguides may behave as massive quasi-particles accelerating at up to 10^24 g, which is about twelve orders of magnitude larger than the surface acceleration near a stellar black hole. These record high accelerations, which may be created in terrestrial laboratories, will enable experimental studies of the Unruh effect.

The emergence of metamaterials, a new type of artificial materials exhibiting characteristics that are not available in nature, had a profound impact on the advances in terahertz (THz) science and technology by realizing appropriate electromagnetic responses in the THz frequency. Despite the recent efforts, the still existing needs for the more efficient manipulation of THz waves fuels the development of metamaterial based THz active and passive devices such as filters, polarizers, and modulators. In this work, we present a dual-band THz filter based on self-complementary metasurfaces that are two-dimensional arrays of the unit-cells consisting of an artificial resonator and its complementary counterpart. The unit cell of the self-complementary metamaterial utilized to realize the dual-band filter is based on a combination of a Jerusalem cross and its complementary structure designed to resonate in the THz regime. This structure functions as a selective band-stop filter (BSF) or band-pass filter (BPF) depending on the polarization states of the incident wave. It is also observed that the transmission phases of the two orthogonally polarized waves exhibit 90° phase difference in broad frequency range. It implies that it can be utilized as a quarter wave plate manipulating the polarization of the incident THz waves. The use of self-complementary structures enables the design of a 2-in-1 THz filter device whose function can be chosen between BPF and BSF by changing its orientation. The operating principles and design guidelines of the self-complementary metasurfaces will also be presented by using electromagnetic simulation and equivalent circuit method.

Optical Solar Reflectors (OSRs) form the physical interface between the spacecraft and space and they are essential for the stabilization and uniform distribution of temperature throughout the spacecraft. OSRs need to possess a spectrally selective response of broadband and perfect electromagnetic wave absorption in the thermal-infrared spectral range, while strongly reflecting the solar energy input. In this work, we experimentally show that disordered and densely packed ITO nanorod forests can be used as an excellent top-layer metasurface in a metal-insulator-oxide cavity configuration, and a thermal-emissivity of 0.97 is experimentally realized in the spectral range from 2.5 to 25 μm. The low-loss dielectric response of ITO in the solar spectrum, from 300 nm to 2.5 μm range limited the solar absorptivity to an experimental value of 0.167. These make our proposed design highly promising for its application in space missions due to combining high throughput, robustness, low cost with ultra-high performance.

Using a forced-mode composite right/left-handed resonator, we designed and proved a permittivity sensor at the microwave regime. We use this sensor to measure humidity percent of honey and potatoes. The sensor resonant frequency can be modified without altering its physical length. With this characteristic a bigger frequency bandwidth can be covered. In addition to that, the second and third mode can also be used for measuring increasing even more the sensor frequency bandwidth. It can be clearly appreciated that one mode is magnetic and the other electric. Not only they differ in their amplitude but also in their quality factor. By moving the position that forces the modes, the first mode frequency value will increase while the second one will decrease. This will happen until the modes meet each other and then they will exchange places.

Ballistic metamaterials, metal-dielectric composites with the unit cell size smaller than electron mean free path, represent a new class of composite media with many unique properties, such as hyperbolic response above the plasma frequency.

We proposed and analyzed the electromagnetic propagation response of multilayered metamaterial hypercrystals. The structure is composed by a periodic sequence of dielectric material and a metamaterial based on metal embedded in another dielectric. The final structure can exhibit asymmetrical optical properties and they can be used as mirrors, stop band filters and near unity absorbers for visible and infrared radiation. The propagation properties of the proposed hypercrystal can be tuned by adjusting their optical their geometrical and optical parameters.

We study the scattering of plane waves by a cylinder inside a uniaxial hyperbolic medium and show a unique scattering behavior which is quite different from scattering inside an isotropic dielectric medium. The polarization is preserved during the scattering i.e. the scattered waves have the same polarization as the incident wave for all incident angles. The scattering of TM waves is highly anisotropic and can be tuned from the Rayleigh regime, to the resonant regime, to the evanescent regime by simply changing the angle of incidence. In the resonant regime, we also see sharp Fano resonance features even for low index cylinders. Finally, a magnetic cylinder is shown to exhibit a highly angular selective scattering. All these effects can be explained from the unique properties of the uniaxial hyperbolic medium and illustrate the opportunity to tailor the scattering properties by controlling the environment.

In this work, we investigate the guided mode properties of planar anisotropic aluminum-doped zinc oxide waveguides (air/metamaterial/silicon oxide) at the epsilon-near-zero spectral point. Our calculations predict two fundamentally different propagation regimes for the lowest order guided TM and TE modes over a broad spectral range (400-2000 nm). Our study shows that excitation of the TM guided mode is possible for wavelengths higher than epsilon-near-zero spectral point. However the propagation distance for such a TM mode will strongly depend on the spectral position of the mode’s eigenfrequency. E.g., propagation distance is maximized for eigenfrequencies near epsilon-near-zero point and reduced by an order of magnitude as the eigenfrequency is tuned by ~50 nm.

Indium Tin Oxide (ITO) has shown significant potential in becoming a candidate for ε-near-zero (ENZ) metamaterial which can be a host material for EMNZ devices. However, the ENZ ITO material itself has not been thoroughly studied at a device level for several reasons. So far, only relatively thin (hundred nm scale) annealed ITO film has been studied for ENZ purposes. We put an initial effort in characterizing the 2 µm-thick ITO film in respect to its permittivity (ε). The melting point for indium is between 350 C and 400 C, so the annealing temperature falls into this window. A series of 2 µm-thick ITO films were deposited on a 3 µm-thick SiO2 on Si wafer that were annealed at different temperatures and times. These sample were further investigated by a cutting-edge ellipsometry technology. The optical constant depth profile at 1550 nm is measured for various annealing temperature and periods. The results show that both real and imaginary part of permittivity are non-uniform along the growth direction. Under a specific processing window, we are able to achieve a micron-scale of epsilon near-zero ITO film. We also conducted a TEM study to investigate the physical structure of the material. We find the evidence of different partial crystallization across the entire ITO film. The cross-section TEM with low magnification to show entire depth profile of the ITO from the SiO2 interface to the top surface. TEM images show evidence for the different crystal morphology across the ITO film, as the crystal grains varies for different regions of ITO.

Metal-insulator-metal metasurfaces (MIM-M) structures can achieve near-unity absorptance at sub-wavelength volumes. A conventional approach to tune the absorption wavelength and the bandwidth of MIM-Ms is to change the geometrical parameters of the structure. In this research, we investigate numerically a new approach to achieve enhanced absorption in MIM-Ms in the mid-infrared spectral region by using both lossy metals and a lossy dielectric. The absorption in the dielectric spacer layer is attributed to the Berreman and epsilon near-zero modes in combination with the dielectric intrinsic loss. The simulations have revealed the presence of additional absorption peaks compared with the case of lossless dielectrics. In particular, the effects of the dielectric thickness and the radiation incident angle on the absorption spectra of the proposed MIM-M structures have been investigated.

Metasurfaces enable arbitrary control of the wavefront of light by locally manipulating polarization in addition to amplitude and phase. As a result, multiple optical functions can be encoded with greatly reduced complexity that be accessed by changing the input polarization, wavelength and k-vector. Unique ways to generate structured light, a new polarization optics that greatly surpasses the capabilities of the standard and a new class of lenses that correct aberrations without requiring multiple stacked lenses have emerged from this approach .1-5 I will present spin-to-total angular momentum converters (J-plates) that create complex entangled states with applications in quantum optics and other fields6 , new polarimeters and polarization state generators7 and broadband achromatic lenses8. 1. N. Yu and F. Capasso Nature Materials 13, 139 (2014) 2. N. Yu et al. Science 334, 333 (2011) 3. M. Khorasaninejad et al. Science 352, 1190 (2016) 4. M. Khorasaninejad and F. Capasso Science 358,1146 (2017) 5. J. P. Balthasar Mueller et al. Physical Review Letters 118, 113901 (2017) 6. R. C. Devlin et al. Science 358, 896 (2017) 7. Noah A. Rubin, et al. Optics Express 26, 21455 (2018) 8. Wei-Ting Chen et al. Nature Nanotechnology (2018) doi:10.1038/s41565-017-0034-6

The paper demonstrates spatial filtering in reflection based on meta-mirrors, composed of periodic subwavelength gratings. The periodic modulation of the refraction index on the sub-micron scale exhibits optical beam shaping with transverse invariance. The paper starts with a theoretical model for the proposed metamirrors based on multiple scattering theory. The results from the proposed analytical model coincide with the that from FDTD simulations. The analytical studies in the paper show that the filtering performance is enhanced by structured cavities where Mie resonances occur. Observations of different Mie resonances for varying units of the meta-mirrors are also presented in this paper. The metamirror may serve as a versatile tool for narrowing beam with high efficiency and transverse invariance.

We show that thermal radiation from a topological insulator carries a nonzero average spin angular momentum.

The growing demand for high-capacity optical-transmission technologies sparked the growth of integrated and silicon photonics. Efficient on-chip manipulation of optical signals requires development of high-fidelity Y-junctions, photonic lanterns, mode filters and multiplexers, and interferometers. The concept of supersymmetry (SUSY) originated in the fields of particle physics and enabled treatment for bosons and fermions on equal footing. Supersymmetry has expanded to quantum mechanics, and optics where it can be used, for instance, to design (de)multiplexing arrays of waveguides. To date, the majority of optical applications employed the unbroken SUSY that relates partners supporting the same set of eigenstates with the exception of the fundamental state. We propose a design of a mode sorter made of fully iso-spectral permittivity profiles related by a continuous SUSY transformation in the broken regime. This ensures that the propagation constants of the all the modes to be sorted are preserved along the length of the device. As a result of this global matching of the propagation constants, the SUSY design allows for reduction of the modal cross-talk by two orders of magnitude compared with a standard asymmetric Y-splitter. Moreover, the SUSY mode sorter operates for both transverse-electric and transverse-magnetic light polarization, and it shows low losses and modal cross-talk over a broad wavelength range (1300-1700 nm). Compared with the previous SUSY based modes sorters, our design offers similar performance with an order of magnitude smaller sorter length and can separate modes without losing energy via radiative modes.

We report on a discovery that homogeneous metallic non-diffracting metasurfaces of a certain type allow robust speckle-free discrimination between different degrees of the spatial coherence of light. The effect has no direct analogue in natural materials and has been previously unseen in metamaterials (and metasurfaces in particular). It results in a qualitative change of the optical response of metasurfaces, whereby their transmission (and reflection) spectra acquires different spectral components, depending on whether the nano-structures are illuminated with spatially coherent or incoherent light. This effect is robust and exceptionally strong (e.g., the resulting absolute change in transmission exceeds 50%), which makes it immediately suitable for practical applications, such as optical metrology, imaging and communications. Among the metasurfaces that have been found to exhibit the new effect are planar metamaterials featuring a continuous periodic zigzag pattern. The reported samples were designed to operate in the near-IR part of the spectrum and composed of arrays of continuous zigzag nano-wires, as well as their inversion, i.e., continuous zigzag nano-slits, covering the area of ~20x20mkm2. The measured data suggest that these apparently trivial metasurfaces, while non-diffracting, can indeed behave differently under spatially incoherent and coherent illumination. The systematic experimental investigation and rigorous theoretical analysis of this phenomenon (the results of which will be presented at the conference) reveal that the effect is underpinned by strongly non-local response of the metasurfaces. Its mechanism involves interference of light scattered via non-dispersive delocalised plasmon modes uniquely supported by the fabric of the metasurfaces.

Given some amount of material, what optical functionality is possible? I will present two approaches to this question: (1) bottom-up large-scale computational “inverse” design, achieving e.g. broadband, high-NA metalense, and (2) top-down theoretical bounds, whereby it is possible to discover fundamental limits for spontaneous emission, radiative heat transfer, and more.

We show that optical waves passing through a nanophotonic medium can perform artificial neural computing. Complex information, such as an image, is encoded in the wave front of input light. The medium continuously transforms the wave front to realize highly sophisticated computing tasks such as image recognition. At the output, optical energy is concentrated to well defined locations, which for example can be interpreted as the identity of the object in the image. These computing media can be as small as tens of wavelengths in size and thus offer extremely high computing density. They exploit sub-wavelength linear and nonlinear scatterers to realize sophisticated input-output mapping far beyond traditional nanophotonic devices. To enable these complex neural computing, we draw inspiration from artificial neural network and use stochastic gradient decent to optimize nonlinear nanophotonic structures with structural gradient computed from adjoint state method.

Advances in design and nanofabrication are enabling the ability to realize nanophotonic structures that achieve optical functionalities not possible with conventional devices. Some of the most interesting structures being developed are based on metasurfaces comprised of sub-wavelength unit cells intelligently patterned to locally manipulate an electromagnetic wavefront in a desired way. However, designing a metasurface to achieve a specific set of desired functionalities is extremely challenging especially in the presence of coupling between unit cells, which can affect system performance and lead to unexpected optical aberrations. To overcome this challenge, designers typically employ only canonical structures (e.g., loaded-dipoles, v-antennas, split-ring resonators) to synthesize a metasurface to achieve their desired functionality. However, metadevices based on these canonical structures do not always achieve optimal performance especially when broadband and/or wide-field-of-view functionality is desired. Additionally, different material combinations as well as fabrication effects and tolerances can make the optimal unit cell topology selection difficult. Therefore, the ability to generate a diverse set of unintuitive metasurface unit cells and evaluate their electromagnetic behaviors for achieving a specific functionality is highly desirable. Moreover, doing so in an efficient and intelligent manner is extremely important in order to maintain a tractable inverse design process. To this end, we employ a custom nanoantenna unit cell topology generation routine in conjunction with robust state-of-the-art multi-objective and surrogate-assisted optimization algorithms to find optimal designs that achieve an arbitrary number of user-specified performance criteria. Finally, when paired with advanced full-wave forward solvers these design and optimization algorithms enable an efficient inverse design toolkit for high performance metasurface synthesis.

Metasurfaces have broad utility in spectroscopy, integrated optics, optical filtering, and holography. In this talk, I will discuss the development and utility of numerical tools for high performance design. For the first part, I will discuss the application of topology optimization to periodic and aperiodic metasurfaces. These methods are based on mathematical gradient descent, and they enable the discovery of new, freeform designs consisting of nonintuitive nanoscale patterns. For certain classes of devices, these metasurfaces have performance metrics (i.e., efficiencies and capabilities) that far exceed the performance of conventional metasurfaces based on phased arrays. Upon reverse-engineering these devices using coupled Bloch mode analysis, we find that the origins of high efficiency wavefront engineering is due to complex intramode and intermode coupling between the optical modes of the system. For the second part, I will discuss the potential of machine learning to learn features in high performance devices and its use in automated device design. Machine learning is applicable to metasurface design and nanophotonics more broadly because there are clear relationships between geometric structure and optical response. I will show that these non-linear correlations between geometric structure and optical response can be learned in a neural network, and that a properly trained network can produce devices beyond the parameter space of the training data set. This reported research sets the foundation for the future of metasurface and nanophotonic design: one where computers and algorithms identify new design regimes of light-matter interaction unattainable by designs based on human intuition.

Publisher’s Note: This conference presentation, originally published on 9 September 2019, was withdrawn on 29 October 2019 per author request.

We introduce a physical mechanism to perform machine learning by demonstrating a Diffractive Deep Neural Network architecture that can all-optically implement various functions following the deep learning-based design of passive layers that work collectively. We created 3D-printed diffractive networks that implement classification of images of handwritten digits and fashion products as well as the function of an imaging lens at terahertz spectrum. This passive diffractive network can perform, at the speed of light, various complex functions that computer-based neural networks can implement, and will find applications in all-optical image analysis, feature detection and object classification, also enabling new camera designs and optical components that perform unique tasks using diffractive networks.

Discovering novel, unconventional optical designs in combination with advanced machine-learning assisted data analysis techniques can uniquely enable new phenomena and breakthrough advances in many areas including on-chip circuitry, imaging, sensing, energy, and quantum information technology. Topology optimization, which has previously revolutionized aerospace and mechanical engineering by providing non-intuitive solutions to highly constrained material distribution problems, has recently emerged as a powerful architect for advanced photonic design. Compared to other inverse-design approaches that require extreme computation power to undertake a comprehensive search within a large parameter space, topology optimization can expand the design space while improving the computational efficiency. This talk will highlight our most recent findings on 1) merging topology optimization with artificial-intelligence-assisted algorithms and 2) integrating machine-learning based analysis with photonic design and quantum optical measurements. Particularly, we will discuss our studies on implementing deep-learning assisted topology optimization for advanced metasurface design development, focusing on highly efficient thermal emitter/absorber development for thermal-photovoltaics applications. We will summarize our research on merging topology optimization technique with quantum device design for achieving ultrafast single-photon source that offers efficient on-chip integration. Finally, we will also describe our recent works on implementing a novel convolutional neural network-based technique for real-time material defect metrology at the quantum level that outperforms all existing approaches in terms of speed and fidelity. This new method rapidly extracts the values of the single-photon autocorrelation function at zero delays from sparse data and ensures one order speed up on solving “bad”/”good” emitter classification problem in comparison with conventional techniques.

Efficient theoretical modeling of metasurface is highly desired for designing metasurfaces. However, most of current modeling of metasurfaces relies on full-wave numerical simulation methods that solve the Maxwell’s equations. As a metasurface typically consists of many meta-units, solving Maxwell’s equations is computationally expensive and thus inefficient for designing metasurface. Here, we develop a general theoretical framework for modeling metasurface based on the coupled mode theory (CMT), which fully describes the interaction between the meta-units and light by a simple set of coupled-mode equations. Consequently, the CMT formulism is far less computationally demanding than the Maxwell’s equations. We show that our CMT approach allows us to quickly and efficiently optimize the design of a beam-steering metagrating. The optimal design obtained from our CMT model is further validated by numerical simulation. The proposed CMT model provides an efficient tool to model and design optical devices based on multiple optical resonators.

The interaction of light with nanostructures having variation in the refractive index on the order wavelength or subwavelength generates so many rich physical concepts that cannot be easily observed in the conventional medium. As inverse design methods provide effective optimization of the refractive index distribution that is not possible by conventional methods based on the intuition of researcher, they have been recently used in the design of nanophotonic devices. In this study, 2D integrated photonic devices which split optical power equally and exhibit the negative refraction and photonic nanojet were designed through the objective-first inverse design algorithm. Firstly, the optical power splitters (1×N) separate the optical power of the TE or TM fundamental mode at 1.55 μm wavelength up to the four output waveguides. The output powers are approximately equal (± 3%) and their modes are the same input signal modes. Secondly, the negative refractive index medium is designed in the wavelength range of 1.5-1.6 μm, and incident angle of 45°- 60°. Finally, a beam with full width at half maximum (FWHM)<λ/3 and depth of field>2λ is performed in the scope of the photonic nanojet. Also, the designed structures are discretized by acceptable performance losses considering the production conditions. As a result, 4-channel optical power splitter, negative refractive index medium, and photonic nanojet are revealed using the objective-first algorithm for the first time to the best of our knowledge.

In this work, a novel method to obtain all-dielectric toroidal response metasurfaces in the W-band and THz range is demonstrated. Two designs are proposed, a symmetric and asymmetric disk metasurface. The first design is intended to corroborate the theoretical analysis, demonstrating the excitation of a strong toroidal mode resonance at 93.2 GHz. Then, the second design is used to demonstrate that symmetry-breaking variations in the disk dimensions, could lead to birefringent metasurfaces, affecting the polarization of the impinging light. Two structures are designed, a polarization beam splitter and a polarization converter. Such devices are difficult to obtain at the target frequency range with low absorption, so they could be of particular interest for the next generation of 5G communications and THz devices.

Metasurfaces allow unprecedented control of light through engineering the amplitude, phase and polarization across arrays of meta-atom resonators. Adding dynamic tunability to metasurface components would boost their potential and unlock a vast array of new application possibilities such as dynamic beam steering, LIDAR, tunable metalenses and reconfigurable meta-holograms, to name a few. We present here high-index reconfigurable metaatoms, resonators and metasurfaces that can dynamically and continuously tune their frequency, amplitude and phase, across the near to mid-infrared spectral ranges. We highlight the importance of narrow linewidth resonances along with peak performance of tunable mechanisms for efficient and practical reconfigurable devices.

Perfect absorption is desired in many photonic devices, in particular in optoelectronic switches, where the ability to change electrical conductivity with photoexcitation enables fast detectors and modulators. A metallic layer is typically introduced underneath the absorbing layer to realize perfect absorption, however this approach is often impractical for photoconductive devices. Here, we demonstrate perfect absorption using an all-dielectric metasurface consisting of a network of electrically connected nanoscale GaAs resonators. We develop a metasurface structure supporting two critically-coupled and degenerate magnetic dipole modes, with their effective magnetic dipole vectors in and out of the metasurface plane. Since the latter mode is symmetry-protected for incident waves at normal incidence, we break the resonator symmetry to enable excitation of the two modes simultaneously. We provide a physical model for the metasurface design and support it with detailed numerical simulations and experimental verification. We show that this metasurface can be switched between conductive and resistive states with extremely high contrast using an unprecedentedly low level of optical excitation. Funding statement: 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-NA-0003525.

In this paper, an all-dielectric metasurface for surface enhanced Raman spectroscopy (SERS) is presented. The proposed design constitutes of an array of silicon (Si) dimers on top of thin film of silicon nitride (SiN), deposited on a glass substrate. The coupling mechanism between the dimers is based on two orthogonal guided waves in the SiN film. These guided modes lead to a strong separation between excitation and emitted Raman signal in the waveguide. The new design is compared to a previously published dielectric design with a gold backing mirror. The comparison takes into account manufacturability, field enhancement and thermal aspects. This shows that the all-dielectric design has about 7 times less power dissipation but the enhancement factor is about 20 times smaller.

In this work a novel design of an all-dielectric metamaterial perfect reflector has been proposed which exhibits 100% reflectance at a particular frequency and high reflectance in a broad range of wavelengths. This design is purely dielectric, hence, is free from resistive losses which are intrinsic to metallic thin film type reflectors. The design is compatible to on-chip fabrication techniques and can be realised by using well known methods such as optical lithography or electron beam lithography. The proposed structure can augment the performance of low loss cavity resonators, frequency selective filters, sensing, directive emission of radiation, etc.

Measuring light’s information of polarization and phase in real time is very important in optics. Since metasurfaces enable the wavefront manipulation, which can replace some conventional optical components and make the system extremely compact. Here, we apply the concept of metasurface to system level, creating a generalized Hartmann-Shack array based on 3×2 sub-arrays of silicon-based metalenses for optical multi-parameters detection, which not only measures phase and phase-gradient profiles of optical beams but also measures spatial polarization profiles at the same time. The silicon-based metalenses, with a numerical aperture of 0.32 and a mean measured focusing efficiency in transmission mode of 28% at a wavelength of 1550 nm. Furthermore, we demonstrate detections of a radially polarized beam, an azimuthally polarized beam and a vortex beam.