Optical Metasurfaces – planarized patterned devices with thickness smaller than or comparable to the operational wavelength – can engineer the light wavefront beyond the limitations of natural materials, and they offer novel opportunities for optical technologies. I will discuss our recent efforts in the design, optimization, fabrication and characterization of dielectric and metallic metasurfaces enabling different nonlinear functionalities, such as nonreciprocal wave propagation, power limiters, efficient SHG generation, and beam steering. I will discuss the main potential and challenges of current approaches and provide an outlook on possible future directions.

Electro-optically tunable active metasurfaces that enable dynamic modulation of reflection amplitude, phase, and polarization using resonantly excited materials and phenomena are powerful design elements for meta-imaging and computation. We describe the role of such active metasurfaces as cascadable elements in lens-less and single-photon imaging systems.

We show how a departure from the old fashioned photonics design approach can lead to optimal designs that are much better than state of the art on many metrics (smaller, more efficient, more robust). This departure is enabled by development of inverse design approach and computer software which designs photonic systems by searching through all possible combinations of realistic parameters and geometries. We show how this inverse design approach can enable new functionalities for photonics, including chip-to-chip on on-chip optical interconnects with error free terabit per second communication rates.

New optical phenomena and light control emerge when discrete nanostructures are arranged into nano-architected materials. We develop a series of deterministic assembly techniques to construct nano-architected materials on demand and explore their emergent properties for wide-ranging applications from optical sensing to cooling.

Among all the new cancer treatment modalities, photodynamic therapy (PDT) is a promising method for its lower systemic toxicity, lower side effects, and improved tumor selectivity. PDT is based on activating light-absorbing molecules, often known as photosensitizers (PS). Here we introduce a new zinc-oxide nanowire (ZnONW) hydrogel and transparent wound healing antibacterial patch to treat superficial ulcerating cancer wounds. Active embedded ZnONWs in hydrogel/patch with the band gap of 3.37 eV would absorb ultraviolet radiations, and after a series of photochemical reactions in the aqueous environment of hydrogel/wound would generate ROS. The produced ROS triggers a series of cellular and molecular processes that have an antibacterial effect as well as an impact on the growth cycle of cancer cells. According to our study, using this hydrogel/patch in conjunction with conventional chemotherapy could speed up the healing of malignant wounds by a factor of two.

Select quantum materials can support polaritons, hybrid light matter waves, with sub-diffraction-limited confinement. In this talk I will overview recent progress on polaritons in hyperbolic materials, which propagate as conical rays throughout the bulk of these crystals. I will discuss polaritons in a class of hyperbolic hetero-bicrystals. Our data reveals negative refraction, spectral gaps and wave localization can occur in these systems.

Arrayed-waveguide-grating (AWG) devices are among the most popular integrated photonic devices for multi-wavelength operations in several applications like optical communications, spectroscopy, and sensing. Here, we introduce a highly miniaturized (overall size < 1 mm2), yet ultra-wideband (wavelength range: 800 nm-1000 nm), silicon nitride AWG design for spectroscopy applications. The fabrication process of the device is CMOS compatible and hence suitable for mass fabrication. The relative uniformity of the response, the small insertion loss of ~1.07 dB, a wavelength resolution of 5 nm, and cross-talk <-25 dB are among the interesting specifications of this design. Combined with a fine narrowband high-resolution stage formed by a microresonator array, this structure can form a wideband high-resolution spectrometer for sensing applications.

In this study we will design photonic structures whose spectral properties are particularly sensitive to the environmental modifications, and we will present measurement system that allows to detect these small variations. For this purpose, we will focus on the phase at the reflection on periodic structures, in particular for the critical coupling points for which the phase variation is abrupt. The measurement method that we develop consists in an original holographic interferometric setup based on wavefront shaping that allow a very stable phase measurement.The fabrication of a temperature sensor based on these two principles has allowed us to experimentally reach phase sensitivities up to 39°/°C.

We demonstrate electrically switchable nanoantennas and metasurfaces from metallic polymers. Such nanoantennas show well-pronounced plasmonic resonances and can be electrically switched fully off and back on by applying CMOS compatible voltages of ±1V. Operation speed is as fast as 30 Hz. Utilizing this concept, we realize, on the one hand, an electrically switchable metallic polymer metasurface for ultra-high-contrast active beam switching. On the other hand, we show an electro-active metaobjective comprising two metalenses-on-demand. By using gel electrolytes, our metadevices can be integrated into state-of-the-art on-chip electro-optic components.

A time gating technique based on Four Wave Mixing was developed to measure for the first time the temporal profile of a 1D PhC nanolaser operating in the telecom range at room temperature. Our method enabled a resolution of 1 ps and a detection sensitivity of 200 nW peak power. Our results show a build-up time of the emission of around 30ps and a decay time of 35ps. This shows the potential of these nanolasers for encoding data at a high bit rate (15 GHz).

Photonic crystal (PhC) phosphor is a paradigm-shifting structural platform that the authors’ group has developed. In this study, two major changes are introduced to the existing two-dimensional PhC phosphor: an increase in the refractive index contrast by replacing the PhC backbone material and the planarization of phosphor surface by the squeegee method. Compared with the reference phosphor, the upgraded PhC phosphor exhibits ~59 times enhanced absorption (simulated) and ~7 times enhanced phosphor emission (experimental). Although already impressive, the huge gap between theory and experiment indicates ample room for further improvement through, for example, the refinements in device fabrication.

This conference presentation, originally published on 17 March 2023, was withdrawn on 10 April 2023 per author request.

This conference presentation was prepared for SPIE OPTO, Photonics West, 2023.

We investigate methods for dynamic reshaping of the spectral and directional characteristics of thermal emissivity from warm objects, using micro patterning. In our design work, we propose infrared meta material structures for on/off switching of emissivity peaks. We use index perturbation to break the symmetry of a dark mode formed by coupling between a pair of resonators, allowing the mode to emit. We further introduce coupled cavity structures for which small index perturbations cause a large change in the dominant angle of thermal emission. In our experimental implementations, we demonstrate voltage tuning of spectral peaks in metamaterials based on epitaxial transfer of p-i-n GaAs structures to the intermediate layer of a MIM metamaterial.

We have recently demonstrated that mid-infrared saturable-absorber mirrors and optical power limiters can be constructed using the concept of intersubband polaritonic metasurfaces – devices in which intersubband transitions in a semiconductor heterostructure are strongly coupled with optical modes in nanoresonators. Our original demonstration produced only relatively small (~20%) variation in reflection between low and high intensity illumination. We have now optimized the metasurface design, relying on a GaAsSb-InGaAs heterostructure that provides narrower-linewidth intersubband transitions, increased doping density, and utilized transitions between excited states to significantly improve the experimentally-measured reflectivity contrast, which now spans from 80% to 10% for different illumination intensities.

The quintessential concept of metamaterials is to obtain material properties via structuring, and that of photonic crystals is to master band structures inspired by condensed matter. The combination of the two gives rise to an exciting optical frontier called reciprocal or k-space metaphotonics, with a prominent example of bound states in the continuum (BICs). Here, we will discuss our recent efforts on artificial intelligence to design BICs, utilizing them for optically forbidden excitons, and investigate the topological nature of BICs for nonlinear high-harmonic generations. The high finesse and relative ease of fabrication could render BIC-based k-space metaphotonics promising for applications.

The efficient harvesting of electromagnetic waves by subwavelength nanostructures can result in perfect light absorption in the narrow or broad frequency range. By the use of proper material and design configuration, it is possible to realize these lithography-free light perfect absorbers in every portion of the EM spectrum. This, in turn, opens up the opportunity of the practical application of these perfect absorbers in large scale dimensions. In last couple of years, we adopted these lithography-free techniques in many applications including photoconversion, photodetection, light emission, sensing, filtering and thermal camouflage. This presentation will summarize our recent accomplishments in this field.

If light interacts with media, it will naturally transform into structured light, spatially varying in its properties as amplitude, phase, and/or polarization and carrying information about the interacting medium. Going to the nano-regime, media information is typically hidden in the non-paraxial properties of light, including its 3D polarization states. However, with standard imaging techniques, this information is not directly accessible and stays invisible. We present how customized non-paraxial laser light fields can serve as a sophisticated tool for revealing hidden media information by selective dipole excitation. Additionally, we further advance our system by implementing a dielectric metasurface for information extraction.

This talk is dedicated to the use of Fourier techniques for the purpose of designing dielectric metalenses with reduced sidelobes. Metalenses act as submicron scale spatially varying phase plates that apply a quadratic phase shift to a signal, thereby focusing the beam. Abnormalities arise due to the incredibly small scale of the lens; however, spatial filtering techniques, similar to techniques used in optical signal processing, can be applied to counteract these abnormalities. These techniques lead to new strategies in metalens design to remove unwanted effects, such as sidelobes, and a possibility of new functionality for metasurfaces in general.

The ability to actively precisely control the absorption, emission, and flow of light is one of the main features of nanophotonics research. Phase change materials (PCMs) hold promise for achieving energy efficient, set-and-forget reconfigurability over long times which can be used for a range of applications in optical memory, neural networks, trimming, programmable and reconfigurable photonics. To achieve non-volatile control over optical phase without inducing large losses, a next generation of low-loss optical PCMs is needed. Recently, antimony-based materials Sb2S3 and Sb2Se3 have been introduced as a family of PCMs of particular interest for applications in telecoms, near-infrared and visible range. I will present an overview of our work on exploring the properties of Sb2Se3 including time resolved switching, endurance, and applications in silicon photonics and free space optical devices.

Phase-change materials (PCMs) offer a compelling platform for active metaoptics, owing to the large index contrast and fast yet stable phase transition attributes. In this talk, I will demonstrate our recent progress on in situ electrically driven tunable metasurfaces by harnessing the full potential of both archetypal PCM alloys (e.g., Ge2Sb2Te5) and less explored classes of PCM compounds (e.g., Sb2S3 and Sb2Se3), in order to realize non-volatile, reversible, multilevel, fast, and remarkable optical modulation in the near-infrared and visible spectral range. Dynamically reprogrammable flat optical devices including hybrid plasmonic-PCM metasurface and dual-view non-volatile micro-displays with record characteristics are presented.

We design a non-reciprocal infrared thermal emitter that exhibits unequal absorptivity and emissivity. A graphene grating over a slab is used to create high Q-guided resonances. Non-reciprocity is induced by dynamic modulation of the Fermi energy of graphene to drive interband photonic transitions. We show that strong contrast between absorptivity and emissivity can be obtained with realistic modulation frequencies of 10’s of GHz. This study presents a graphene-based platform for non-reciprocal thermal emitters. These results open up exciting new possibilities for electrical control over absorptive and emissive characteristics in the infrared.

Controlling a state of material between its crystalline and glassy phase has fostered many real-world applications. Switching between these states is particularly interesting if accompanied by a significant change of optical properties. Phase change materials provide the combination of fast switching and a pronounced property change. For nanophotonic applications it is crucial to tailor the crystallization kinetics, the length of the switching processes and as the optical contrast. We devise design rules for crystallization and vitrification kinetics and the contrast of optical properties. We discover a clear stoichiometry dependence along a line connecting regions characterized by two fundamental bonding types, metallic and covalent bonding. Increasing covalency slows down crystallization siginficantly and promotes vitrification. A quantum-chemical map explains these trends and provides a blueprint to design crystallization kinetics, nanostructure control and property contrast.

Metamaterials enable tailoring of light–matter interactions, driving discoveries which fuel novel applications. Deep neural networks (DNNs) have shown marked achievements in metamaterials research, however they are black boxes, and it is unknown how they work. We present a causal DNN where the learned physics is available to the user. Here, the condition of causality is enforced through a deep Lorentz layer which takes in the geometry of an all-dielectric metamaterial, and outputs the causal frequency-dependent permittivity and permeability. The ability of the LNN to learn metamaterial physics is verified with examples, and results are compared to theory and simulations.

In this work, we present a new approach based on metric learning for defining new similarity measures that are well-matched for design tasks in nanophotonics. Majority of the existing approaches use mean squared error (MSE) or mean absolute error (MAE) as the similarity measure to compare the desired and optimal spectra while it is clear that point-wise distance cannot capture the important features of the responses. Here, our goal is to use deep metric learning to provide a systematic approach for defining new metrics in nanophotonics.

This conference presentation was prepared for SPIE OPTO, Photonics West, 2023.

In this talk, I will discuss new opportunities in the design and implementation of freeform metasurfaces. In the first part, I will discuss progress we have been making in developing all neural-based simulators and optimizers, which can be used to perform large area freeform metasurface design orders-of-magnitude faster than conventional methods. In the second part, I will discuss applications of freeform metasurfaces in the domains of beam steering, large numerical aperture optics, and polarization control.

The ability to design multi-resonant thermal emitters is essential to the advancement of a wide variety of applications, including thermal management and sensing. These fields would greatly benefit from the development of efficient tools for predicting the spectral response of coupled, multi-resonator systems. In this work, we propose a semi-analytical prediction tool based on coupled-mode theory. We demonstrate the accuracy of our method by predicting and optimizing spectral response of a coupled, multi-resonator system based on hBN ribbons. Our approach greatly reduces the computational overhead associated with spectral design tasks in multi-resonator systems in addition to providing valuable physical insights.

This conference presentation was prepared for SPIE OPTO, Photonics West, 2023.

Conventional design of engineered nanostructures for controlling and manipulating the propagation of electromagnetic waves has been established in the literature over the past several years. Challenges in design, optimization, fabrication, and characterization of such structures have motivated extensive research activities in the field of engineered nanostructure materials and devices. In the present work we utilize an evolutionary optimization algorithm to design a metasurface. The metasurface considered exhibits achromatic performance over the visible range of electromagnetic spectrum.

Light field, or plenoptic, imaging involves gathering the intensity and incoming angle of light at each pixel, which can enable image refocusing, changing the point of view, or reconstructing the depth map. Current light field imaging techniques, such as micro-lens arrays or stacked transmission gratings lead to bulky and sophisticated systems with limited angular sensing range and resolution. Here we introduce a fast metasurface-based angle sensitive pixel design procedure using the discrete dipole approximation. Compared with traditional FDTD-based design procedures, we can find optimal designs in approximately an order of magnitude less time.

I will discuss progress on the development of ultra-low loss thin film lithium niobate (TFLN) photonic platform and its application in classical and quantum information processing. Examples of devices to be presented include low power consumption and high bandwidth electro-optic (EO) modulators and EO frequency comb sources, as well as their integration with high power lasers, of interest for applications in optical communications and microwave photonics. TFLN is also promising for spectral and temporal control of non-classical light. In particular, I will discuss the development of quantum transducers and photonics needed for realization of multiplexed quantum repeaters.

Here we describe advances in laser frequency microcombs in dispersion-engineered microresonators. We examine the phase/frequency-noise and jitter characteristics of microcomb states. When referenced to high-purity sources, the microcomb preserves tooth-to-tooth relative frequency stabilization to a 50 mHz and 2.7×10−16 uncertainty. We also examine the real-time transition dynamics and stability of these dispersion-engineered microcombs. We further describe advances in coherent THz-mmWave radiation generation via frequency microcombs, with 2.8-octaves (330 GHz to 2.3 THz) tunability. With photomixers from Jarrahi group at UCLA, coherent sub-100-Hz linewidth radiation is demonstrated with 10-15 residual frequency instability, supporting applications in high-frequency metrology, sensing, imaging and communications.

The lack of knowledge of the precise geometry of hollow whispering gallery mode (WGM) resonators prevents us from having a theoretical model to analyze the WGMs accurately. Very recently, we applied X-ray imaging combined with focused ion beam milling to obtain detailed information on the geometry of a WGM microbubble resonator and developed a theoretical model for the optical modes that exactly matches the measured geometric parameters. The developed theory can be used for any hollow resonators. It is based on perturbation theory used for SNAP bottle resonators and waveguide theory used for rolled-up bottle resonators.

We demonstrate 2D diamond photonic crystal cavities operating at telecommunication wavelengths in a single crystal diamond membrane. Fully suspended 2D PhC cavities with a theoretical high Q factor of ~ 8×106 and relatively small V of ~2 (λ/n)3 are fabricated in a thin diamond membrane, which is supported by a polycrystalline diamond frame. We observe cavity resonances in the telecommunication O- and S-bands (1360-1470nm), measuring Q factors up to ~1800. Our results pave the way for developing diamond photonic devices at telecommunication wavelengths.

Evident from more than 50 years of table-top nonlinear optics, utilizing strong quadratic nonlinearities in integrated photonics can significantly expand the potentials of photonics for applications ranging from sensing to computing, especially in the ultra-short-pulse regime. In the past few years, nanophotonic lithium niobate (LN) has emerged as one of the most promising integrated photonic platforms with strong quadratic nonlinearity. In this talk we present some of our recent experimental results on realization and utilizing of dispersion-engineered and quasi-phase-matched devices in nanophotonic LN for intense optical parametric amplification, ultrafast ultra-low-energy all-optical switching, and few-cycle vacuum squeezing. We show a path for realization of large-scale ultrafast nanophotonic circuits in the classical and quantum regimes and discuss how networks of such resonators can lead to topological and non-Hermitian dynamics, and all-optical quantum information processors.

Bound states in the continuum (BIC) to achieve highly efficient frequency conversion using high quality-factor (high-Q) metasurfaces have been demonstrated using symmetry-broken structures with high robustness; however, the breaking-symmetry tactics are typically limited to one of the dimensions of the structures, which restricts the nonlinearity with BIC. In this work, we present a new metasurface structure in the form of an array of unit cells composed of two identical nano-bars with two mirror-symmetric corners cut into each nano-bar to break this limit. By using the high refractive index and large third-order nonlinearity of amorphous silicon (a-Si), we demonstrate ultra-high theoretical Qs up to ~ 2×10^5. Owing to the large nearfield enhancement in the meta-atoms, we observe optical Kerr effect in efficient third harmonic generation from the a-Si BIC metasurfaces via different levels of pump power, which paves the way for variational quasi-BIC for switchable nonlinear generation.

Localized surface plasmon resonance (LSPR) of noble metal nanoparticles is an optical phenomenon to enhance the electro-magnetic field near a particle and has the potential to enhance the performance of solid-state triplet-triplet annihilation photon upconversion (TTA-UC). In order to fully utilize an enhanced electro-magnetic field generated by LSPR, the spatial arrangement of dye molecules near noble metal nanoparticle plays an important role. In this study, donor and acceptor molecules are sequentially introduced on the plasmonic nanoparticles and investigated the correlation between the spatial arrangement of dye molecules and the UC enhancement behavior in solid-state TTA-UC.

Metal nanoparticles separated by nanogap spacings generate a significantly enhanced electromagnetic field caused by the light-induced localized surface plasmon resonance. This enhanced electromagnetic field, so-called a hot spot, amplifies the optical absorption and emission of dye molecules in nanogaps. In this study, we fabricated large-area plasmonic Au nanogap arrays with highly integrated hotspots by utilizing the spontaneous nanophase separation of block copolymers. These arrays showed the amplification of upconversion emission based on solid-state triplet-triplet annihilation (TTA-UC).

Metal nanoparticles can strongly enhance such nonlinear optical processes as harmonic generation and multiphoton photoluminescence due to their high electronic polarizabilities, intense optical resonances and high surface-to-volume ratio. Heavily doped semiconductor nanoparticles, such as metal chalcogenides, can also exhibit plasmonic resonances in addition to their excitonic response. Here we describe frequency upconversion in bilayer nanoparticle thin-film structures comprising copper sulfide and gold nanoparticles separated by insulating ligands or thin films when excited by near-infrared femtosecond laser pulses. The surprisingly large third-harmonic signal is due to coherent excitation of the LSPR in both semiconducting and metallic nanoparticles – a mechanism validated by a dipole-dipole model calculation

Thin-film Black Phosphorus (BP) has shown considerable promise as a material for mid-wave infrared photodetection. BP exhibits attractive materials properties that include a high photoresponse in the mid-wave infrared and an electrically tunable bandgap. However, bandgap tunability requires the material to be kept sufficiently thin, which limits the thickness, and therefore absorption, of a BP active layer in a photodetector. We have designed and characterized metamaterial gratings that increase the absorption in BP. We show that the metamaterial grating, when integrated into a photodetector, increases the photodetection capabilities of thin-film BP.

There is a growing interest in the use of chalcogenide phase-change materials (PCMs) for reconfigurable metasurfaces to realize next-generation compact adaptive optical systems. However, the application of the classic PCM composition such as Ge2Sb2Te5 for near infrared metasurfaces has been limited due to its high absorption in the crystalline state. Here, by using an ultralow-loss and high-index phase-change material Sb2Se3, we show reconfigurable metasurfaces can manipulate light efficiently in near infrared region with comparable efficiencies in both the amorphous and crystalline states of the material.