Space-time metasurfaces employ a spatial and temporal phase-gradient to impart momentum to input light in space and frequency. In this work, we used a reflective gate-tunable ITO-based metasurface operating at 1530 nm to generate diffraction of frequency-shifted light up to 1 MHz. Our device consists of an array of interdigitated plasmonic nanoantennas with two electrical contacts that are modulated with identical electrical waveforms, time-delayed by half a period, to diffract frequency-shifted light. The simultaneous control of spatial and spectral properties of light enabled by space-time metasurfaces is highly desired in applications such as LiDAR, LiFi, and space communication.

We have demonstrated that a classical metamaterial nanostructure, a two-dimensional array of plasmonic metamolecules supported on flexible nanowires, can be driven to a state possessing all the key features of a continuous time crystal - an eagerly-sought state of matter with properties periodic in time, rather than in space. The phenomenon points to the possibility of a new mechanism for the synchronization of noise-driven, linear oscillators based upon non-reciprocal coupling (non-Hamiltonian optical forces) between metamolecules.

In this talk, I discuss our recent research efforts on determining fundamental and practical limits to various basic properties of optical materials and metamaterials, including (i) upper bounds on the refractive index of transparent optical materials; (ii) upper bounds on nonlinear susceptibilities; (iii) constraints and performance limitations of time-varying metamaterials. Our findings provide insight into the physical bounds of natural and engineered materials and allow identifying application areas for which there are opportunities for large improvements over the state of the art.

We show that optical systems, from imagers to sophisticated metasurfaces, require a minimum thickness. Hence, for example, one thin metasurface layer may not be enough for many optical functions. This thickness arises whenever the input regions for different output pixels overlap. Even before starting design, just from what the optics is to do, we can deduce an overlapping non-locality number C that says how many channels must flow sideways inside the system. The system needs thickness to allow those channels to propagate sideways inside the structure. Several optimized system designs approach this limit but do not surpass it.

Strong instrinsic light-matter coupling was realized within nanostructured bulk flakes of the transition metal dichalcogenide tungsten disulfide. WS2 metasurfaces driven by the concept of symmetry-protected bound states in the continuum (BIC) show sharp resonances, which are tuned over the exciton leading to an, in transmission measurements resolved, anti-crossing pattern with a Rabi splitting of 116 meV. The BIC-inherent control over the systems radiative loss rate leads to a tunable coupling strength, independent of material-intrinsic losses.

For the enhancement of light matter interactions plasmon resonances have been crucial. However, their low quality factors have limited their application in areas requiring high spectral selectivity. A new approach involving the design and 3D laser nanoprinting of plasmonic nanofin metasurfaces has been demonstrated to support bound states in the continuum up to the 4th order. The out-of-plane symmetry of the nanofins can be fine-tuned by altering the triangle angle, enabling control over the ratio of radiative to intrinsic losses. Pixelated molecular sensing in the Mid-IR unveiled the importance of precise coupling tailoring in order to create high performance sensors.

The transportation of in-plane photonic topological states, achieved by mimicking electronic systems, has gained a lot of attention. However, there has not been much exploration of their out-of-plane radiation effects. By treating leakage radiation, the concept of the band topology can be extended to resonant metasurfaces, providing new opportunities to directly measure them and to create new features for free-space applications. In this talk, we examine the basic mechanisms of far-field radiation and resonant excitation in topological junction metasurfaces made of thin film dielectric subwavelength gratings. These structures have distinctive topological characteristics such as complex Dirac masses, Berry phases, and band structures shown through Fano spectral responses. Considering the correlation between the topological quantities and structural parameters, we discuss new optical functionalities discovered in the topological metasurfaces such as Dirac-mass engineered beam shaping.

We propose a metamaterial absorber with a three-dimensional, vertically-oriented metal-insulator-metal (v-MIM) structure for ultra-sensitive infrared spectroscopic gas sensing. The small footprint of the v-MIM structure allows for increased integration density of the MIM structure, resulting in the detection of gas molecules with suppressed background light. We designed and fabricated the v-MIM structure and used it to detect carbon dioxide and butane molecules. The mutual coupling of the vibration of the molecules and the resonant mode of v-MIM creates a Fano resonance, and their distinct peaks are clearly observed in the corresponding transmission dips.

Here, we experimentally demonstrate broadband nonlinear imaging by using Si resonant metasurfaces based on third-order nonlinear processes, including third harmonic generation (THG) and four-wave mixing (FWM). Particularly, the quadratic dependence of the FWM emission on the pump power allows to significantly reduce the dependence on high-power signal input in nonlinear imaging via designing a high-Q resonance at the pump wavelength based on bound states in the continuum (BICs). In the experiment, our metasurfaces support high-performance infrared imaging, including a large acceptance angle, a high frame rate, and a wide range of operating wavelengths (from 900 nm to 2500 nm).

Here, we developed humidity-responsive full color nano-pixels with 700nm resolution using 3D nanoimprinted polyvinyl alcohol (PVA) and disordered silver nanoparticles (NPs). The nano-pixels are designed as Fabry-Pérot structure which is composed of an aluminum mirror substrate, humidity-responsive PVA spacer, and disordered silver nanoparticles as top metal layer. The proposed structure has three main merits as follows. It has 1) high resolution thanks to 3D nanoimprinted PVA technique, and 2) millisecond-response time (441ms), 3) vivid color generation thanks to disordered silver NPs which enhance the penetration speed of water molecules into the PVA layer and have highly absorbing dielectric optical property.

We numerically study several metasurfaces which include ferro-electric material, such as lithium niobate (LN) and explore their ability to control the phase, amplitude and polarisation of light. For each of these parameters, ideally we would like to be able to control them independently, without affecting others. We study metasurfaces containing silicon resonators on LN substrates as well as the patterned LN metasurfaces. We find that the amplitude and polarisation modulation are relatively easy to achieve in a single metasurface, however the best phase modulation we were able to achieve is approximately 70 degrees, in a metasurface which maintains high transmission amplitude at over 90%.

Here, we introduce low-loss hydrogenated amorphous silicon (a-Si:H), whose bandgap is optimized to suppress their extinction coefficient at visible frequencies. The bandgap of a-Si:H has been manipulated by adjusting the deposition parameters of plasma-enhanced chemical vapor deposition. Low-loss a-Si:H have been applied for beam-steering metasurfaces at the entire visible spectrums. Beam steering metasurface has achieved measured efficiencies of 42%, 65%, and 75% at the wavelengths of 450, 532, and 635 nm, respectively.

Here, we developed two types of tunable PVA metaphotonic platforms, which are 1) PVA metasurface that simultaneously displays a far-field holographic image and reflective structural coloration under coherent and incoherent light, respectively, and 2) humidity-responsive structural coloration that enables to generate RGB color in a single pixel. Three types of PVA metasurfaces have been designed: 1) revisable holography/structural coloration with pure PVA structures, 2) relative-humidity sensitive optical decryption with PVA-hydrogenated amorphous silicon structures, and 3) revisable holography/structural coloration with Pt-coated PVA structures. Considering that the aforementioned PVA structures have been fabricated with low-coat single-step nanoimprinting methods, they will be widely applied for various tunable optical components by significantly reducing their production costs.

We develop and experimentally realise a single-layer metasurface that converts unpolarised light into fully polarised light surpassing the efficiency limit of 50% for conventional polarisers. We achieve this by using an inverse-designed metasurface that splits incoming light into multiple outputs with the same polarisation. We predict a greater than 80% conversion efficiency when combining the powers of two or three outputs. We fabricate the freeform silicon metasurface and experimentally measure a combined efficiency of over 60% in converting unpolarised light to polarised light at 1550 nm with an overall extinction ratio 20.

In this presentation, I will introduce the result of a metasurface-based compact depth sensing system that can be applied to LiDAR. I will explain the design principle of this structured light metasurface which can spread the point cloud with a large field of view angles. This spread point cloud is captured by two distinct cameras, and depth information is computed by the stereo-matching method. And I will suggest electrically tunable metalens which can be easily integrated with not only AR/VR systems but also electronic systems. These metaphotonic devices have the potential to be applied to various fields such as mobile sensors, biometric security systems, autonomous driving, metaverse, and driver assistance.

Strong light-matter coupling enabled polariton states were extensively exploited for enhanced optical nonlinearities, development of low threshold lasers and quantum simulators. Here, we will report our recent work on demonstration of novel topological polaritonic phases by leveraging the strong coupling between the photonic topological boundary states and two different material degrees of freedom in 2D Van der Waals materials; first transverse optical phonons in a hexagonal boron nitride (hBN) thin film and second excitons in 2D WSe2 monolayer. Our results will demonstrate emergence of topological boundary states of phonon-polariton and exciton-polariton character, and their resilient unidirectional propagation around sharp corner with avoided backscattering.

We present our recent results related to active metasurfaces. Several mechanisms for implementing active devices will be discussed. We will describe the demonstration of tunability in dielectric and metallic metasurfaces for diverse applications using tunability mechanism such as MEMS technology integrated with metasurfaces and the electro optic effect in lithium niobate integrated with metasurfaces, as well as the tunability of a metasurface by controlling an external medium. We also discuss the role of nanoscale structures in enhancing functionalities such as light emission and light detection.

Single-photon detectors have been widely studied for decades because of their unique capability to resolve photon numbers, enabling many applications in quantum information technologies. Compared to other types of single-photon detectors, niobium nitride (NbN) superconducting nanowire single-photon detectors are promising candidates and are commercially available today. However, the absorption coefficient of superconducting NbN in the visible range is typically low, resulting in low quantum efficiency and low signal-to-noise ratios. In the present work, we use a novel approach to enhance the visible-light photoresponse in NbN superconducting microwire photon detectors (SMPDs) by integrating them with gap plasmon resonators (GPRs). This talk describes how we observe the plasmonic NbN SMPDs can achieve a 233-fold enhancement in the phonon-electron interaction factor (γ) compared to pristine NbN SMPDs under resonant conditions with illumination at 532 nm. The nonlinear photoresponse in the visible region is attributed to the gap-plasmon-induced heating that breaks the superconducting state to normal. Our results open new opportunities for ultrasensitive single-photon detection for quantum information processing, quantum optics, imaging, and sensing at visible wavelengths. The detailed mechanisms and possible applications will be discussed. Finally, I will discuss emerging plasmonic platforms based on transition metal nitrides and their potential applications.

The development of reconfigurable metalenses is crucial for various applications including scanning microscopy and 3D imaging. While the focal length of most metalenses is fixed once they are fabricated, realizing dynamically-tunable metalenses can save costs and provide greater flexibility. In this study, an interleaved metasurface was designed to achieve continuous focal length tunability in the visible window. The results showed that the proposed metalens can effectively alter the focal length, offering a promising avenue for developing more versatile and adaptable optical devices.

We present a new approach to address the phase error induced by the near-field coupling effect among meta-atoms in metasurfaces. Our method estimates the actual phase shifts of the meta-atoms by utilizing a periodic model that considers the meta-atom of interest and its closest neighbors. The estimated phase is then used to replace the meta-atoms via an iterative process. To demonstrate the effectiveness of our approach, we designed an achromatic metalens as a numerical example. We anticipate that our work will offer a valuable way to accurate metasurface design, leading to enhanced performance of meta-optical devices.

Developing approaches to power efficient and high-throughput computing is critical for meeting ever-growing computational requirements. There is significant interest in using optics to perform computational tasks such as performing image processing tasks in analog, eliminating the need for computational resources. In this work, we demonstrate inverse-designed meta-optics capable of isotropic 2D edge-detection with large numerical apertures. To highlight the versatility afforded by the optimization-based approach, we design a multifunctional device that performs a variety of processing tasks based on polarization, wavelength, and diffraction order. These devices can be used in novel computer vision and computational imaging applications.

The exceptional resonance was excited by symmetry-protected quasi-bound states in continuum (Q-BICs). Q-BIC absorption ~ 90% at the communication wavelength was demonstrated by a germanium nanohole metasurface without a back reflector involved.

We propose a dual-beam pumping scheme whereby a pulse of telecom frequency ω (1550 nm wavelength) is mixed with its frequency-doubled replica at 2ω. This brings about sum-frequency generation at ω+2ω = 3ω on top of third harmonic at ω+ω+ω = 3ω. We exploited the diffraction by an AlGaAs metasurface for realizing the symmetry breaking which is key to avoid averaging out the interference between the two coherent frequency-tripling pathways. The upconverted light can thereby be routed into different diffraction orders with high selectivity, based on the pumps’ relative phase, providing a platform for GHz-rate amplitude modulation and information encoding.

Photo-induced thermo-optical effects in dielectric nanoresonators enable contactless modulation of their optical response. We report results on the modelling and design of thermally reconfigurable metalenses as well as insight into the interplay of self- and collective heating effects in these thermonanophotonic devices.

Fabrication of structures with high aspect ratios (HAR) enables creation of metasurface optics with ever-increasing functionality. Dielectric meta-atoms consist of transverse (x,y) shapes extruded in the z-dimension. The phase discontinuity imparted by the meta-atom is a function of the dielectric constant of the material and the z-height of the meta-atom. The performance of the meta-optic improves with larger phase swings. Here we demonstrate HAR etched structures in silicon and gallium arsenide. Initial etch development employs interferometric lithography while final patterns are realized using e-beam patterning. The paper will discuss this etch development process for both material sets.

We report novel phonon-polariton states induced by spatial defects in topological metasurface integrated with hexagonal boron nitride (hBN). The introduction of topological defects, created by stitching domains with different choices of unit cells leads to the emergence of spatially localized modes, while the coupling of these trapped modes with phonons in hBN gives rise to the formation of polaritonic states. We designed and fabricated a mid-IR-operating hybrid system that consists of a photonic metasurface with a thin layer of hBN on top of it. Topological defect modes of the fabricated structure were probed using direct imaging in both real- and Fourier-space.

We introduce the concept of adiabatic topological photonic structures, which allows us to overcome many of the limitations of topological photonic metasurfaces. We demonstrate that topological metasurfaces with slowly varying gauge fields significantly improve trapping of optical modes, and also offer excellent guiding features in both spin-Hall and valley-Hall topological photonic structures. Adiabatic variation of the mass terms at the domain walls makes topological boundary modes less sensitive to details of the lattice, perceiving the structure as an effectively homogeneous Dirac metasurface. As the result, the modes exhibit longer radiative lifetime and propagation distance, while retaining their topological resilience. At the same time, localized modes trapped due to the 2D variation of the mass term exhibit high quality factors and controllable radiative properties, which, along with non-zero angular momentum of their far field, makes them of great interest for applications.

Structured light with an asymmetric phase distribution emerges as an enabling tool to control the light-matter interactions in optical nanostructures. In this work, we first investigate structured light interactions with the all-dielectric meta-atoms of different geometries and aspect ratios and demonstrate that the phase asymmetry of Laguerre-Gaussian (LG) beams of various orders facilitates the excitation of higher-order radiative modes that are not accessible via conventional Gaussian beam or plane wave. In particular, we show that using an LG beam can excite the quadrupole moments within the designed nano-resonators and can alter the induced moments' strength. It is also demonstrated that the geometry and orientation of the meta particles, as well as the illumination angle of the LG beam, can strongly affect the magnitude and spectral location of the induced radiative modes within the subwavelength meta-atom. Next, we demonstrate the design of a meta-atom that supports new non-radiative states, called anapoles, formed due to the destructive interference between waves generated by certain multipoles. While most existing studies focused on the electric anapoles, here we design and demonstrate an all-dielectric cuboid meta-atom that supports magnetic and hybrid anapoles formed due to the overlap of electric and magnetic multipoles with their toroidal counterparts. In addition, we demonstrate that with a careful design, such meta-atom can support non-radiative states up to quadrupole moments. We also show that changing the illumination angle can excite and manipulate various anapole orders. Furthermore, by changing the topology of the cuboid meta-atom to a pyramid particle, we demonstrate the satisfaction of different Kerker conditions, such as unidirectional scattering and the establishment of a superscattering regime.

When Nobel laureate Herbert Kroemer coined the phrase “the interface is the device”, he referred to the devices based on thin semiconductor films. In this talk, I will describe how interfaces between several topological crystals can be used for developing a wide range of devices: from cavities with “sticky” mirrors that can localize light, to interferometric devices that can be used as compact reflectionless beam-splitters. Examples of interface-based devices operating across the electromagnetic spectrum will be presented, and future direction will be discussed.

We present the solution to the long-standing problem of the analytical calculation of the effective dielectric permittivity for an arbitrary composite material.

Chiral nanophotonic structures can significantly enhance the chiral optical responses. However, achieving extreme chirality remains challenging. Here, chiral quasi-bound states in the continuum are realized in the visible range by controlling the etching depths in the substrate and inducing out-of-plane symmetry breaking. An extremely high level of chiral emission from a perovskite metasurface is experimentally achieved in the normal direction. Chiral emission is maximally enhanced for one helicity via critical coupling while strongly suppressed for the other helicity. Approaching the ultimate limit of chiral interactions may lead to far-reaching consequences in a variety of important applications.

Multimodal perception is crucial for developing perceptual intelligence. Conventional multimodal sensing devices have limitations in device volume, the field of view, resolution, and imaging speed. Meta-devices, a new class of optical components, offer significant potential to revolutionize perception by manipulating light through nano-antennas. Our research demonstrates that meta-lenses can be used to integrate multiple perception modalities into a single compact device, with applications in robotics, autonomous vehicles, and mixed reality. Integrating meta-lens into perception systems could transform the field by providing low-cost, compact solutions for capturing information about the world in multiple ways.

Metamaterials provide unique opportunities for manipulation of dispersion of light waves and, therefore, polarisation and phase, as well as amplitude of transmitted and reflected waves. Here we report on using linear and nonlinear properties of nanorod and nanotube based metamaterials for shaping ultrashort optical pulses. Intensity limiters, temporal pulse shape control, as well as polarisation switching will be presented. The role on nonlocal effects in pulse propagation in metamaterials will be discussed. Using nanotube based metamaterials allows to introduce additional degree of freedom for passive and active tunability of the optical response.

We show that metallic wires in CMOS chips can provide dual functionalities as electronic interconnects and as plasmonic/metamaterial devices. We demonstrate plasmonic resonances in a chip fabricated in a bulk Si CMOS foundry (TSMC, 65 nm node). Through minimal post processing, we integrate the designed nanophotonic CMOS chip with liquid crystals and demonstrate a high-speed liquid crystal-based electro-optic modulator.

Placing a GaAs quantum dot (QD) within a Mie metasurface permits fine tailoring of the dot emission properties. We aligned the electric and magnetic dipole modes with the QD's emission wavelength to enhance the emission in comparison to an unstructured layer. This enhancement is analyzed using numerical simulations of modes supported by the metasurface and the effects QD position and polarization. We will discuss the comparison between the numerical results and experimental photoluminescence measurements.

Experimental examination of light-matter interactions at the nanoscale is challenging since the corresponding electromagnetic near-field is often confined within volumes below the resolution of conventional optical microscopy. Here, we demonstrate that photoemission electron microscopy (PEEM) can image near-field optical fields in nanophotonic structures. We present concurrent spectroscopy and imaging of the near-field distribution of resonance modes supported by broken-symmetry silicon metasurfaces. Additionally, we can examine collective modes and deduce that coupling between eight resonators establishes the collective excitations. Altogether, the high-spatial resolution of this hyperspectral imaging approach is valuable for the metrology of collective, non-local optical resonances in nanophotonic systems.

Photonic funnels, conical waveguiding structures with hyperbolic metamaterial cores, have been proposed – and recently demonstrated at mid-infrared frequencies – as optical links between macro- and nano-scales. All recent realizations of the funnels utilize highly conductive claddings to prevent the leakage of light out of the core. Here we demonstrate that clad-less funnels can significantly outperform their PEC-clad counterparts due to excitation of novel surface modes. We also analyze funnel-light interaction in the time domain and demonstrate temporal separation between diffraction-limited and nano-confined signals. Perspectives of temporal shaping of the nano-confined radiation are also discussed.

We investigate the laser damage resistance and optoelectronic properties of thin metallic and semiconducting films used in high power beam steering metasurfaces. We find that gold exhibits the best laser damage resistance, and large absorption losses arising from high carrier concentrations and low mobilities in alternative plasmonic thin films lead to exacerbated degradation and poorer laser damage performance. We will also present results of laser-induced dewetting on thin metallic films deposited on substrates with different optical environments (such as plasmonic films and cavities). We observe that the morphology of dewetting is sensitive to the nonlocal optical environment. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. LLNL-ABS-845919. DE-NA0004007.

We discuss new approaches to the synthesis, design, and exploitation of two-dimensional materials for nanophotonics, including plasmonics in ultrathin crystalline metals, a disruptive class of quantum-phase materials, recent advances in the solution to the problem of coupling between free-space light and ultra-confined optical excitations, and the application of these developments to the study of ultrafast nonlinear phenomena and quantum optics at the atomic scale.

We present ultrafast vectorial electric field measurements of plasmonic skyrmions, merons, quasicrystalline structures, and skyrmion bags. 10 nm spatial resolution, sub-fs temporal resolution, and vectorial properties of the surface plasmon field on single crystalline, atomically flat gold surfaces are presented. Our techniques combine ultrafast vector PEEM and interferometric s-SNOM. Our tailored model systems reveal a rich set of topological phenomena, reaching even into 4-dimensional topology.

Tailored beam topology can be used to tune spatial and spectral components in structured light. The flat optical devices and experimental setups will be described that allow to establish spatial and temporal correlations to produce structured light beams for advanced photonics applications.

Computational techniques such as photonics inverse design, along with new nanofabrication approaches and photonic materials, play a crucial role in addressing challenges of building scalable quantum and nonlinear photonics. We illustrate this with several recent examples, including on chip nonlinear optical isolators, miniaturized Ti:sapphire lasers, chip-to-chip and on-chip optical interconnects with error free communication rates exceeding terabit per second, and scalable quantum systems based on color centers in wide band gap semiconductors such as diamond and silicon carbide.

Toroidal moment, induced by currents flowing on the surface of a torus, plays an important role in the fundamental physics of light-matter interaction. However, natural toroidal response was usually overlooked due to weak interaction with other electromagnetic resonances. Recently, toroidal moments can be effectively excited in artificial metamaterial by utilizing complex nanostructures or special light sources. Here, we have proposed periodic amorphous silicon cylinders embedded in spin-on-glass layer that can successfully generate transverse toroidal dipole (TD) in optical regime under normal incident illumination. Both experimental and simulation results indicate that such TD mode sustains a large structural tolerance and can be spectrally tuned by stretching the cylindrical axis perpendicular to the light polarization. In addition, the excited TD mode also displays ultrahigh refractive index sensitivity. This approach provides a simple and straightforward way to implement TD metamaterials and serves a powerful platform for high-sensitivity biosensors and nonlinear optics.

Geometrical phases such as Pancharatnam and Berry phase, have been playing important role in classical wave and quantum physics. In topological photonics, the geometrical phases can be controlled with artificial gauge fields by designing lattice geometry of photonic crystals. Here, we theoretically and experimentally demonstrate that geometric phases can give rise to a new class of resonant states in topological ring resonators. Our simulations and analytical model reveal a hierarchy of the resonant modes and transformation of polarization states. Also, we provide experimental observations of the resonant states by infrared spectroscopy.

Transverse spin angular momentum (t-SAM) is a spin with a vector perpendicular to the light propagation direction, naturally appearing in tightly confined electromagnetic waves. For the first time, we successfully generated t-SAM in a spatial point of maximum intensity of focused light using an asymmetric metasurface. The designed metasurface provides complete phase and polarization control and allows for the manipulation of both electric and magnetic transverse spin distribution. The results were obtained through both theoretical and experimental methods, demonstrating the metasurface's ability to generate a strong transverse spin in a focal point ensuring non-zero net spin in the focal plane. This discovery has potential applications in optomechanics, quantum optics, nanometrology, and directional scattering.

Space-time modulation has attracted significant attention in recent years, both from the fundamental science point of view as well as due to potential applications (e.g., optical isolators). In this talk I will point to similarities of modulating in space and time as well to the differences, most important from the power consumption point of view. Also, the relation between time reflection and time crystals and optical parametric processes will be considered.

The emergence of van der Waals (vdW) magnets with strongly coupled electronic and spin degrees of freedom offers an intriguing platform to study light-matter interaction in low-dimensional magnets. Specifically, magnetic materials that host excitons in the visible frequencies with strong correlation to underlying magnetic order are very attractive in this context. I will present our recent work on realizing exciton-polaritons (strongly coupled light-matter states) in such magnetic systems and the prospects of controlling the magneto-optical response with light [1, 2]. I will conclude with a discussion on realizing active nanophotonic devices using vdW magnets. [1] “Observation of spin-correlated exciton-polaritons in a van der Waals magnet, F. Dirnberger, et al. Nature Nanotechnology 17, 1060 (2022) [2] “Cavity-controlled magneto-optical properties of a strongly coupled van der Waals magnet,” F. Dirnberger et al. arXiv:2301.07593 (2023)

In the past decade, photonics and optoelectronics have significantly progressed in developing new nanofabrication techniques for optical metamaterials. However, the optimal design of such artificial nanostructures remains complex and resource-intensive, often relying on intuition-based models. In this context, machine learning-assisted optimization techniques emerged as a promising approach to achieving high-performance and practical solutions. We discuss diverse inverse design approaches that use machine learning algorithms to optimize the design of nanophotonic metadevices, including the high-efficiency coupling of single-photon sources with photonic waveguides, variable-index multilayer films, active nanophotonic devices and systems, other photonic metastructures with optimized complex topologies.

This talk will discuss two methods to improve all-dielectric metasurfaces. The first method is a structural design paradigm based on lattices with six-fold rotation symmetry to create symmetry-guaranteed pairs of bound states in the continuum (BICs) for controlling mode splitting and quality-factors. The second method is centered around a search heuristic for identifying super-Mossian materials with refractive indices that surpass the common form of the Moss rule. This discussion will present results from an experimental realization of a super-Mossian metasurface using iron pyrite with a refractive index greater than 4.37 and a bandgap of 1.03 eV. *Work supported by the US Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering (grant BES 20-017574). The work was performed, in part, at the Center for Integrated Nanotechnologies, an Office of Science User Facility operated for the US Department of Energy, 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 US Department of Energy's National Nuclear Security Administration under contract DE-NA0003525. This paper describes objective technical results and analysis. Any subjective views or opinions that might be expressed in the paper do not necessarily represent the views of the US Department of Energy or the United States government.

Nonlinear optical effects in epsilon-near-zero materials have become an area of significant interest to the community within the last 10 years. From demonstrations of refractive index tuning, enhanced harmonic generation, and time varying interactions, a wide range of effects and nonlinear properties have been shown to be influenced by the epsilon-near-zero condition. In this talk, we will a higher level look at refractive index tuning in epsilon-near-zero materials (predominately free-carrier-based materials), highlighting why the epsilon-near-zero conditions provides increased index tuning as well as what features are intrinsic to the epsilon-near-zero condition and what features are dictated by external effects. Breaking down the interaction is intended to highlight what properties are difficult to control and what properties can be readily tuned to maximize effects. This is intended to highlight trade-offs in performance and implementation and facilitate discussion related to selecting the best materials, excitation conditions, and designs as epsilon-near-zero materials attempt to transition from laboratory curiosities to potential applications.

Epsilon-near-zero (ENZ) materials such as indium tin oxide (ITO), have recently emerged as a new platform to enhance optical nonlinearities. In this talk I will report on our recent theoretical and experimental results on the origin of ultrafast nonlinearities in ITO films that are dominated by intraband and interband transitions. We show that there are two competing factors that jointly contribute to a spectrally-invertible nonlinearity of ITO near its ENZ region i.e. the nonparabolicity of the bands that results in a larger effective mass and the Fermi energy shift, which determines the free carrier density. Our work reveals the relationship between the large nonlinearity and the intrinsic material properties of the ITO films which will enable design and development of photonic materials and nonlinear devices made of transparent conductive oxides.

Here, we propose liquid-crystal (LC) integrated metasurfaces to create intensity-tunable metaholography in the visible spectrum. The metasurface is composed of gap-shifted split ring resonators, which exhibit different optical responses depending on the input polarization states of light. The gap-shifted split ring resonators enable the perfect absorption of right-circularly polarized light (RCP), while they reflect left-circularly polarized with varied retardation phases. the integration of LC enables electrically tunable metaholograms, where the intensity of the holographic images can be adjusted by controlling the applied voltages on LC. The circular dichroism in reflection reaches (experiments) 0.91 and (simulations) 0.99.

Here, we demonstrated vivid structural coloration and polarization-sensitive color metasurfaces using bandgap-engineered a-Si:H, whose extinction coefficient is near zero at the entire visible spectrum. The scattering response of high-index nanostructures is numerically analyzed with multipole expansion, and we vitrify that the low-optical losses of bandgap-engineered a-Si:H significantly improve color coverage of metasurface, achieving comparable coverage with the Adobe RGB gamut. Also, we demonstrated the application of optical encryption with polarization-sensitive structural coloration, achieving near-zero reflection when optical information is encrypted. We believe that structural coloration with low-loss a-Si:H will be widely used with its advantageous benefits compared to chemical pigments.

Plasmonic topological insulators using dual layer hexagonal complementary structure provide TE/TM pseudospin edge states. We use finite-difference time-domain method to analyze its nontrivial band structure and the transmission efficiency of topological protected edges states. Chern numbers are calculated by both Berry curvature and Willson loop method. The effect of defects, sharp bendings and scattering loss in one-way transmission are also discussed.

We present coherent perfect absorbers based on topological guided-mode resonances at a topological junction of two thin-film waveguide lattices. The proposed structure provides a fully confined resonance state in both vertical and lateral dimensions, creating a compact coherent absorber possibly operating at the critical coupling condition for the complete annihilation of certain optimized coherent incoming waves. We provide a promising Si-photonic design, incoming wave configuration, and theoretical performance for its operation as a compact coherent absorber in the integrated optics platform.

The ability of a naturally hyperbolic van der Waals material α-MoO₃ to sustain hyperbolic phonon-polaritons allows extreme localization of the mid-infrared electromagnetic field on a subwavelength scale. We propose periodic nanohole arrays in exfoliated α-MoO₃ nanoflakes to tune the wavelength of phonon-polariton resonances. We report on the optical properties of α-MoO₃ nanohole arrays and their dependence on their geometry. Results have been confirmed using the finite-difference time-domain simulations and near-field mapping of phonon-polaritonic modes. We observe an unusual behavior of the transmission spectra in the fundamental absorption band, where the phononic absorption can be almost entirely alleviated.