Metasurfaces enable the shaping of wavefronts in arbitrary ways, by dispersion engineering of the meta-atoms. Here I will review my group research on singularity engineering, including arrays of equally spaced 0D singularities, 2D singular sheets of arbitrary shapes and topoiogically protected singularities in the 4D space encompassing real space and wavelength. In the latter when a perturbation is added to the metasurface the singularity in the focal spot is preserved but shifts in wavelength, signaling topological protection.

I review the recent trends chiral metaphotonics underpinned by the physics of Mie resonances in high-index dielectric nanoparticles. More specifically, I discuss linear and nonlinear circular dichroism of dielectric resonant metasurfaces. We predict and demonstrate experimentally the enhancement of circular dichroism for normal propagation of light due to the excitation of multipolar Mie-type modes in silicon resonators and the formation of a complex vectorial field structure. High-quality Mie resonances satisfying optimal coupling condition can lead to giant circular dichroism in the nonlinear regime observed for the generation of the third-harmonic signal by normally propagating circularly polarized fundamental frequency fields.

Full wavefront control by photonic components requires that the spatial phase modulation on an incoming optical beam ranges from 0 to 2π. All optical components are intrinsically non-Hermitian, often described by reflection and transmission matrices with complex eigenfrequencies. We discovered that crossing the discontinuity branch bridging a Zero and a Pole along the real frequency axis provides a universal 0 to 2π spectral phase variation of an output channel as a function of the real frequency excitation. This basic understanding is applied to engineer electromagnetic fields at metasurfaces. Non-Hermitian topological features associated with exceptional degeneracies or branch cut crossing are shown to play a surprisingly pivotal role in the design of resonant photonic systems.

Metallic and dielectric nanoparticles behave as resonators with resonant frequencies determined by their size, shape, and composition. When these resonators are placed in periodic arrays, they can radiatively couple through in-plane diffraction orders, forming non-local metasurfaces with collective modes called surface lattice resonances (SLRs). SLRs lead to large field enhancements over extended areas, offering an ideal platform for strong-light matter coupling and optoelectronic applications. In this presentation, I will discuss the coupling of SLRs with excitons in organic molecules to form exciton-polaritons (EPs). EPs can condense to the ground state, leading to a coherent emission known as polariton lasing. The threshold for condensation depends on the different mechanisms assisting the relaxation of excitons to EPs, and the optical losses of SLRs, which can be controlled through the choice of materials and the symmetries in the system. Vortex emission is achieved at a very low threshold from symmetry-protected bound states in the continuum.

Metasurfaces—structured planarized optical devices with a thickness thinner than or comparable to the wavelength of light—typically support a “local” response, i.e., they tailor the optical wavefront through the independent response of each meta-unit. In contrast, “nonlocal” metasurfaces are characterized by an optical response dominated by collective modes over many meta-units. In this talk, I will illustrate a rational design paradigm using quasi-bound states in the continuum to realize nonlocal metasurfaces. I will report experimental demonstration of a few device functionalities: (a) devices that produce narrowband spatially tailored wavefronts at multiple selected wavelengths and yet are otherwise transparent, (b) nonlocal metasurfaces based on CMOS-compatible dielectric materials with thermo-optically reconfigurable wavefronts, and (c) nonlinear resonant GaN metasurfaces growth by templated molecular beam epitaxy for efficient sum frequency generation.

Manipulating light on the nanoscale has become a central challenge in metadevices, resonant surfaces, nanoscale optical sensors and many more, and it is largely based on resonant light confinement in dispersive and lossy metals and dielectrics. Here, we uncover a novel paradigm in dielectric nanophotonics: Resonant subwavelength confinement of light in air. Voids created in dielectric host materials support localized resonant modes confined in air and do not suffer from the loss and dispersion of the dielectric host medium. We realize these resonant Mie voids by focused ion beam milling into bulk silicon wafers and experimentally demonstrate resonant light confinement down to the UV spectral range at 265 nm (4.68 eV), highest resolution nanoscale colour printing, as well as nanophotonic refractive index sensing on the single void level with unprecedented small sensing volumes in the range of 100 attoliter and sensitivities on the order of 400 nm per refractive index unit.

We propose a new application of metasurfaces, namely the harnessing of the visual appearance. We demonstrate that disordered metasurfaces composed of high-index resonant metaatoms, once deposited on macroscopic objects, offer visual appearances with novel visual properties.

In our study, we introduce wavelength-multiplexed orbital angular momentum meta-holography, aiming to expand holographic information channels by leveraging light's wavelength and orbital angular momentum (OAM). We selectively employed red, green, and blue as primary colors, and incorporated four unique OAM channels (-2, -1, 1, and 2). Through the utilization of a metasurface comprising three distinct types of wavelength-selective meta-atoms, we successfully encoded the twelve holographic images onto a single metasurface. This advancement holds significant potential for applications in various fields, providing opportunities for advanced holography and augmented reality systems.

We demonstrated a novel depth sensing system that utilizes metasurfaces and photonic crystal surface-emitting lasers (PCSELs), realizing structured light generation and facial recognition in monocular depth sensing. Our single-shot system projects approximately 10,000 infrared spots from a 300^2 μm^2 metasurface area, which is 233 times smaller than the commercial DOE-based dot projector used in the Face ID on iPhone. The system is lens-free and power consumption due to the utilization of PCSEL, reducing 5-10 times of power compared to VCSEL-array based dot projectors.

In this presentation, I will present the findings of a compact depth sensing system for LiDAR that is based on a metasurface. I will explain the underlying design concept of this metasurface, which utilizes structured light to expand the range of view angles for point cloud generation. Two separate cameras, capture the generated point cloud, and depth information is calculated using the stereo-matching method. Additionally, I will propose electrically adjustable metalens that can be easily incorporated into AR/VR systems and electronic systems. These metaphotonic devices offer potential applications in diverse fields such as mobile sensors, biometric security systems, autonomous driving, metaverse, and driver assistance.

Full-color imaging with a large aperture meta-lens remains an important unsolved problem. We employ computationally designed meta-optics to solve this problem and enable ultra-compact cameras. Our solution is to design the meta-optics such that the modulation transfer function (MTF) of all the wavelength across the desired optical bandwidth are the same at the sensor plane. Additionally, the volume under the MTF curve is maximized to ensure enough information is captured enabling computational reconstruction of the image. The same intuition can be employed for different angles to mitigate geometric aberrations as well. In this talk, I will describe our efforts on achieving full-color imaging using a single meta-optic and a computational backend. Starting from traditional extended depth of focus lens, I will describe inverse-designed meta-optics, end-to-end designed meta-optics and hybrid refractive/ meta-optics for visible full-color imaging. I will also talk about how these techniques can be extended for thermal imaging.

In recent years, interest in infrared (IR) imaging has grown, motivated by applications in surveillance, quality control and healthcare. However, conventional IR imaging devices are limited by their low temperature operations and high-noise levels. Nonlinear metasurfaces offer a promising platform for up-conversion IR imaging, potentially allowing multi-color IR imaging in compact devices, at room temperature. Here, we demonstrate up-conversion IR imaging by employing a nonlocal lithium niobate metasurface supporting guided mode resonances. Driven by the resonant enhancement of the incident field, we demonstrate up-conversion of short-wave IR images at 1530 nm to visible images at 550 nm. Our study has important applications in the future development of compact night vision instruments and sensor devices.

We demonstrate meta-optic based accelerators that can off-load computationally expensive operations into high-speed and low-power optics. The key to these architectures are the new freedoms afforded by metasurfaces such as optical edge isolation, polarization discrimination, and the ability to spatially multiplex, and demultiplex, information channels. I will discuss how these freedoms can be utilized for accelerating optical segmentation networks and objection classifiers, both based on incoherent illumination. This approach could enable compact, high-speed, and low-power image and information processing systems for a wide range of applications in machine-vision and artificial intelligence.

In this presentation, I will show our recent progress in development of (i) flat metasurface-based lenses for wide-field-of-view white light imaging for mobile devices and hyperspectral imaging from space, and (ii) tunable metasurface based small-pixel-size spatial light modulators for LiDAR and 3D holographic displays.

Augmented reality glasses have been considered a promising candidate for next-generation mobile hardware platforms. However, the bulky form factor and high-power consumption of their display systems using conventional optical components hinder the commercialization of augmented reality glasses. In this work, we demonstrate metasurface-based waveguide as an image combiner having high efficiency and large field of view. To design the metasurface-based waveguide consists of metasurface grating we propose an inverse design method based on gradient-descent optimization. In addition, our design method exploiting high degrees of freedom in meta-atom design can precisely control wave propagation through waveguides, enabling two-dimensional pupil expansion. As proof of concept, we fabricated metasurface-based waveguide providing a high efficiency of 500nit/lm and a large field of view of 50 degrees. We expect that the proposed metasurface-based waveguide opens up a new route for the development of augmented reality display systems with glass-like form factors suitable for daily wear.

We introduce high-resolution speckle-free holograms, created using polarization manipulating metasurfaces. The holograms enable clear projection of 2D and 3D images, in contrast to conventional holograms with significant speckle noise. The approach uses a simple non-iterative algorithm at a low computational cost. As a proof of concept, we demonstrate a reflective hologram composed of silicon nitride nanoposts on an aluminum layer that projects a high-resolution grayscale image. By implementing three holograms for primary colors, full-color far-field and 3D holograms become feasible, with potential applications in anticounterfeiting tags.

Recent advancements in the integration of flat metaoptic components with light sources and detectors have created exciting possibilities for developing compact optical measurement devices. We have demonstrated monolithic integration of curved GaAs metagratings on vertical-cavity surface-emitting lasers (VCSELs), creating an ultra-compact illumination module for both total internal reflection and dark field microscopy techniques. Based on an unconventional design that circumvents the aspect ratio-dependent etching problems associated with monolithic integration, our integrated metagratings VCSELs generate a quasi-collimated off-axis beam centered at 60° in air and 63° in glass and achieves relative deflection efficiencies of 90% and 70%, respectively.

Electro-optically tunable active metasurfaces that enable dynamic modulation of reflection and transmission amplitude, phase, and polarization using resonantly excited materials and phenomena are powerful design elements for meta-imaging and computation. As flat, low-profile optical elements, active metasurfaces have potential serve as cascadable, programmable components in optical meta-imaging systems, such as lens-less cameras and single-photon imaging systems. In this talk, I will discuss metasurfaces with high quality factor, local, resonant elements capable of two-dimensional phase gradient generation, in both passive and active metasurface designs. I will also describe active metasurfaces with both spatial and temporal phase gradients, and an active metasurface as a lens-less imaging system, and compare the characteristics to conventional lens-coupled image sensors.

Here, we present an electrically tunable metasurface based on indium tin oxide (ITO) that enables dynamic manipulation of light wavefronts at a theoretical frequency range of 15 to 400 GHz. This all-solid-state spatial light modulator (SLM) modulates both the phase and amplitude of incoming light around the 1550 nm wavelength by utilizing an ITO layer and a periodic metallic grating structure separated by a thin AL₂O₃ capacitive layer. When a voltage is applied, a charge accumulation layer forms in the ITO and the ITO-oxide interface, effectively modulating the complex refractive index of the ITO. The optical transparency of the ITO layer at 1550 nm surpasses conventional reflected SLM designs, making it suitable for seamless integration with Vertical-Cavity Surface-Emitting Lasers (VCSELs), potentially reducing power consumption, heat generation, and increasing modulation bandwidth. Furthermore, the compact design and simple fabrication process allow easy integration into Photonic Integrated Circuits (PICs) as an on-chip laser source with dynamic beam steering capabilities. This unique set of properties opens up possibilities for advanced applications in on-chip computing, optical communication, environmental sensing, health diagnostics, and the development of next-generation optical computing architectures and neuromorphic computing systems.

Large-area metasurfaces offer dramatically new functionality, but that promise raises questions: what is possible? What are the extreme limits? We describe theoretical techniques for tackling these questions. First we consider the special case of large-area plasmonic field enhancement (for applications from imaging to ARPES studies), where we offer analytical bounds and meta-grating designs approaching them. Second we consider the general case of arbitrary functionality, where we describe a systematic conservation-law approach to bounds and design.

Polarization components play a vital role in a variety of consumer products, including displays, AR/VR devices, sensors including LIDAR, as well as in modern photonics systems. Traditional polarizing beam splitters based on birefringent crystals are too bulky and expensive for mass production. Multi-layer polarizing beam-splitting structures are limited to the transparency window of the substrate and the layer stack and often have low radiation resistance. Plasmonic structures, such as wire grid polarizers, are often the only viable choice in the infrared (IR). Multi-layer polarizing beam splitters and wire grid polarizers require folded configurations for their integration into photonics systems. This work provides design and performance optimization details of meta-optics all-dielectric polarization beam splitters for the IR spectral region operating in transmission for both polarization states. Their performance is compared to that of wire grid polarizers designed for the same spectral range, and practical considerations for their manufacturability and system integration are also presented.

The design of metasurfaces allows to tailor their optical response with specific spectral and polarization features. We will see how the spectral response of metasurfaces can be fully understood and predicted thanks to the singularities in the complex frequency plane of the scattering matrix elements. This approach turns out to be especially relevant for deriving the scattered field in the time domain providing fruitful insights in the time dynamics of the metasurface. The singularity expansion method offers a rigorous approach for deriving analytic expressions of dispersive dielectric permittivities that share the properties of linear transfer functions. This approach can be used to derive an improved expression of the Debye Drude Lorentz model that complies with the mathematical properties of complex analysis and exhibiting excellent fits with experimental data.

Nonlocal metasurfaces—metasurfaces with an engineered momentum-dependent (spatially-dispersive) response—represent an emerging direction in the field of flat optics, with applications ranging from optical computing to ultra-compact imaging systems. In this talk, we present our recent efforts on this exciting topic at the frontier of the field of metasurfaces. We broadly discuss how nonlocal designs afford new degrees of freedom that enable functionalities and performance metrics unattainable using local metasurfaces and conventional optical systems. We then present some recent advances on nonlocal metasurfaces for space compression ("spaceplates"), which represent an important step in the quest toward the ultimate thickness limits of optics.

Physics-augmented training schemes have enabled the use of ultra-fast deep learning surrogate solvers in adjoint optimization algorithms, accelerating design by many orders of magnitude. However, the utility of these solvers for device design is severely limited by their inability to function outside of fixed simulation parameters. We present a foundational method for conditioning deep learning surrogate solvers on arbitrary parameters, such as the source incidence angle. We then demonstrate the capability of a conditional deep learning model to optimize high-efficiency aperiodic metapixel deflectors that are fashioned to create a large-area metalens.

Moxtek has attained the required feature fidelity for visible wavelength metalenses through e-beam lithography based mastering for a Nanoimprint Lithography (NIL) and Nb2O5 etching based manufacturing process. An overcoat is also added to the metalenses, which boosts performance and protects against handling damage. Metalens and associated test structure metrology results including MTF, veiling glare, and efficiency will be presented for various designs spanning the visible wavelength range with NA’s varying from 0.02 to 0.71. Collectively, Moxtek has demonstrated volume manufacturing of metalenses for the visible regime, which was made possible by high precision NIL and Etch processes.

Self-assembly provides a powerful approach for fabricating metasurfaces using bespoke colloidal nanocrystals (NCs) whose sizes, shapes, and surface chemistry can be rationally designed to achieve complex, heterogeneous, and hierarchical photonic architectures. I will present our group’s work in the area of NC assembly for plasmonic metasurfaces, where shaped metal NCs serve as the building blocks for resonant optical nanojunctions. We have demonstrated that metal NCs can be assembled into large-scale periodic arrays using interfacial assembly, or into more exquisite architectures (e.g. chains, lattices) using entropy-driven steric forces. Uniquely, these metal NCs possess single-crystalline surfaces with low roughness that enable the generation of low-loss structures that support phenomena such as intense electromagnetic field localization, non-linear optical response, and inelastic electron tunneling.

In this talk, we present our recent findings on compound semiconductor-based nonlinear metasurfaces for all-optical signal processing. Nonlinear metasurfaces have revolutionized the field of nonlinear optics by enabling a radically different way to control light-matter interactions at the subwavelength scale. In this approach, nonlinear optical processes can be maximized by carefully choosing the shape, orientation, and arrangement of subwavelength-scale artificial atoms, called meta-atoms. By introducing Kerr nonlinearity from compound semiconductor materials, such as AlGaAs/GaAs, into a high-quality resonant metasurface, power requirement to achieve optical bistability can be greatly reduced. Optical bistability can has been actively studied due to its potential applications for all-optical switching and optical logic gates. In our research, we will utilize intensity-dependent refractive index in a semiconductor metasurface to realize refractive bistability for all-optical signal processing. Different design strategies will be discussed to excite quasi-bound waves with a high-quality factor and a small mode volume.

We report a holographic alignment system for the precise alignment of 3D semiconductor devices and compound flat optics. The system uses cascaded metasurfaces to project two holographic patterns, and by interfering the patterns in the far field, small misalignments can be measured without the need for a high-resolution microscope. Operating at 850nm, the technique achieves lateral and axial accuracies of 1 nm and 1 µm, respectively, surpassing the lateral diffraction-limit accuracies of microscopic imaging methods by two orders of magnitude. The technique has potential applications in high-precision alignment detection and registration of multilayer patterns and separate samples and wafers.

The ability to achieve spatiotemporal control of incoherent (thermal and quantum) light emission has been a critical challenge in the field of optics with far ranging applications from remote sensing, holographic displays and quantum information processing. Here we present our results on two light emitting metasurfaces systems: 1) Ultrafast dynamic steering of spontaneous light emission from high-density InAs quantum dots (QDs) embedded inside GaAs resonators under structured optical pumping and 2) Enhancement of single-photon emission from single GaAs local-droplet epitaxial QDs from within AlGaAs Huygens’ metasurfaces.

We demonstrate arbitrary spatiotemporal synthesis of ultrafast optical transients by leveraging the multifunctional control of light at the nanoscale offered by metasurfaces, enabling ready-synthesis of complex space-time wave packets over an ultrawide bandwidth.

While vertical external-cavity surface-emitting lasers (VECSELs) provide several superior laser properties, the heat generated by the pump laser within the active region proves to be a limiting factor for achieving higher output powers. We present our recent progress in the development of an AlGaInP-VECSEL based on a grating waveguide structure. The heat spreader is placed below the active region membrane while a grating structure is etched into the top layer of the active region. This improves the heat removal from the membrane, while the guided-mode resonances should provide good coupling of the pump and laser field as well as a high reflectivity. The VECSEL cavity is completed with an external mirror, serving also as an output coupler.

In this talk, I will share our recent progress in two major research directions. One is the role of the imaginary permittivity in various metaphotonic structures and the other is on the twist degree of freedom in photonics and how it introduces emerging photonic behaviors in properly designed photonic structures.

Calcite is a birefringent material with optical anisotropy that becomes extreme in the infrared, allowing for the excitation of highly-directional, sub-diffractional hyperbolic modes. In this talk, I will discuss our recent work that focuses on understanding the optical behavior of hyperbolic modes supported within asymmetric nanostructures formed in calcite crystals with in-plane anisotropy, including our recent findings that demonstrate how the resonant frequency and directional power flow can be tuned by simply rotating gratings with respect to the crystal axes of calcite – without changing the shape of the gratings.

In nanophotonic, small mode volumes, narrow resonance linewidths and field enhancements, fundamentally scales with refractive index values and are key for many implementations involving light-matter interactions. Topological insulators (TI) are a class of insulating materials that host topologically protected surface states, some of which exhibit very high permittivity values. In this talk, I will present my group’s latest results on chalcogenide metaphotonics. I start by discussing Chalcogenide Bi2Te3 and Bi2Se3 TIs nanostructures. Using polarized far-field and near field Nanospectroscopy we reveal that Bi2Se3 nanobeams exhibit mid-infrared resonant modes with 2π phase shifts across the resonance. We further demonstrate that Bi2Te3 metasurfaces exhibit deep-subwavelength resonant modes utilizing their record high index value peaking at n~11. Finally we discuss how the anomalous thermo-optic effect in lead chalcogenide can be harnessed for implementing temperature invariant metasurfaces

Adjoint optimization is used to construct libraries of varying numbers of arbitrarily shaped elements. These libraries are then used to create and compare four metalens designs. Each of the 0.5 mm diameter metalens are designed to have a NA of 0.5 and is optimized for wavelengths from 4 to 5 µm. Based on libraries consisting of 4, 8, 16, and 32 elements, the different silicon MWIR metalenses show little to no degradation in performance with increasing library size. Across the different library sizes, the transmission increases by 2%, the focal effieciency increases by 1% and the FWHM decreases by less than a percent.