Efficient light bending with isotropic metamaterial Huygens' surfaces

Matte surfaces with broadband transparency enabled by highly asymmetric diffusion of white light

The long-standing paradox between matte appearance and transparency has deprived traditional matte materials of optical transparency. Here, we present a solution to this centuries-old optical conundrum by harnessing the potential of disordered optical metasurfaces. Through the construction of a random array of meta-atoms tailored in asymmetric backgrounds, we have created transparent matte surfaces that maintain clear transparency regardless of the strength of disordered light scattering or their matte appearances. This remarkable property originates in the achievement of highly asymmetric light diffusion, exhibiting substantial diffusion in reflection and negligible diffusion in transmission across the entire visible spectrum. By fabricating macroscopic samples of such metasurfaces through industrial lithography, we have experimentally demonstrated transparent windows camouflaged as traditional matte materials, as well as transparent displays with high clarity, full color, and one-way visibility. Our work introduces an unprecedented frontier of transparent matte materials in optics, offering unprecedented opportunities and applications.

Revisiting the Design Strategies for Metasurfaces: Fundamental Physics, Optimization, and Beyond

Over the last two decades, the capabilities of metasurfaces in light modulation with subwavelength thickness have been proven, and metasurfaces are expected to miniaturize conventional optical components and add various functionalities. Herein, various metasurface design strategies are reviewed thoroughly. First, the scalar diffraction theory is revisited to provide the basic principle of light propagation. Then, widely used design methods based on the unit-cell approach are discussed. The methods include a set of simplified steps, including the phase-map retrieval and meta-atom unit-cell design. Then, recently emerging metasurfaces that may not be accurately designed using unit-cell approach are introduced. Unconventional metasurfaces are examined where the conventional design methods fail and finally potential design methods for such metasurfaces are discussed.

Controlling the polarization dependence of the complex transmission spectrum using the Brewster effect in metafilms

We propose a method for controlling the polarization dependence of the complex transmission spectrum using the Brewster effect in a two-dimensional array of meta-atoms with finite thickness, which we refer to as a metafilm. We show that the complex transmission spectra of the orthogonal linear polarization components can be controlled independently without reflection by stacking multiple metafilms that exhibit only an electric dipole resonance. As a proof-of-concept numerical demonstration, we design several broadband waveplates with high transmission efficiency based on simple design principles. The proposed method would enable us to easily design anisotropic metamaterials with various complex transmission spectra.

Controlling the directional excitation of surface plasmon polaritons using tunable non-Hermitian metasurfaces

In this work, we propose an efficient approach to controlling the directional excitation of surface plasmon polaritons (SPPs) by dynamically modulating the real-part perturbation in a passive parity-time symmetric metasurface. This non-Hermitian system can experience two exceptional points that can induce two unidirectional excitation states of SPPs along opposite directions. Empowered by its superior modulation depth, the energy ratio and energy intensities of two excited SPP states can be effectively manipulated by this non-Hermitian metasurface. To demonstrate these findings, we design and numerically verify non-Hermitian metasurfaces integrated with an Sb2Se3 phase-change material. Our work provides a promising platform for the controllable engineering of SPP excitations, holding significant potential for the development of new plasmonic devices, including on-chip SPP sources, routers and sorters, and integrated optical circuits.

A Double-Layer Dual-Polarized Huygens Metasurface and Its Meta-Lens Antenna Applications

In this paper, a dual-polarized Huygens unit is proposed, which has a double-layer metallic pattern etched on both sides of one dielectric substrate. Induced magnetism enables the structure to support Huygens' resonance, thus obtaining nearly complete available transmission phase coverage. By optimizing the structural parameters, a better transmission performance can be achieved. When the Huygens metasurface was used for the design of a meta-lens, good radiation performance was exhibited, with a maximum gain of 31.15 dBi at 28 GHz, an aperture efficiency of 42.7% and a 3 dB gain bandwidth of 26.4 GHz to 30 GHz (12.86%). Due to its excellent radiation performance and very simple fabrication, this Huygens meta-lens has important applications in millimeter-wave communication systems.

Multi-Function Reflective Vector Light Fields Generated by All-Dielectric Encoding Metasurface

Traditional optics usually studies the uniform polarization state of light. Compared with uniform vector beams, non-uniform vector beams have more polarization information. Most of the research on generating cylindrical vector beams using metasurfaces focuses on generating transmitted beams using the geometric phase. However, the geometric phase requires the incident light to be circularly polarized, which limits the design freedom. Here, an all-dielectric reflective metasurface is designed to generate different output light according to the different polarization states of the incident light. By combining the two encoding arrangements of the dynamic phase and the geometric phase, the output light is a radial vector beam when the linearly polarized light is incident along the x-direction. Under the incidence of linearly polarized light along the y-direction, the generated output light is an azimuthal vector beam. Under the incidence of left-handed circularly polarized light, the generated output light is a vortex beam with a topological charge of -1. Under the incidence of right-handed circularly polarized light, the generated output light is a vortex beam with a topological charge of +1. The proposed reflective metasurface has potential applications in generating vector beams with high integration.

Micro-dimensional oscillation-based optimization for a dielectric metalens in the mid-infrared

In the past few decades, there has been significant progress made in metasurfaces and integrated and miniaturized optical devices. As one of the most prominent applications of metasurfaces, the metalens is the subject of significant research. In this paper, for achieving better focusing performance of the initial metalens designed by the Pancharatnam-Berry (PB) phase, a concept of micro-dimensional oscillation is proposed to optimize the geometric parameters of nanopillars. A strategy of grouping iteration is proposed to reduce the loss rate and computational effort in a holistic way. Its essence is to divide an extremely large-scale optimization space into many overlapping groups. Meanwhile, an improved genetic-simulated annealing (IGSA) algorithm is presented for the optimal solution of each group. By introducing the adaptive crossover and mutation probabilities in traditional genetic algorithms, the IGSA algorithm has both strong global searching capability and excellent local searching capability. After optimization, the maximum field intensity of the central hot spot can be increased by about 8% compared to the initial metalens. Moreover, the field intensity of the side lobes around the hot spot is almost constant, and the central hot spot increases, which provides a potential for the realization of high imaging contrast.

Individually tunable array reflector for amplitude and phase modulation

Based on graphene's phase modulation property and vanadium dioxide's amplitude modulation property, we developed an array reflector for terahertz frequencies that is individually adjustable. Starting with a theoretical analysis, we look into the effects of voltage on the Fermi level of graphene and temperature on the conductivity of vanadium dioxide, analyze the beam focusing characteristics, and finally link the controllable quantities with the reflected beam characteristics to independently regulate each cell in the array. The simulation findings demonstrate that the suggested array structure can precisely manage the focus point's position, intensity, and scattering degree and that, with phase compensation, it can control the wide-angle incident light. The array structure offers a novel concept for adjustable devices and focusing lenses, which has excellent potential for study and application.

Broadband metasurface superstrate for polarization-independent wave focusing and gain enhancement at Ka-band

A broadband metasurface flat lens is proposed as a polarization-independent wideband superstrate for wave focusing and gain enhancement at Ka-band. The proposed metasurface structure consists of four metal layers and is designed with diagonally symmetric unit cells to accommodate both the vertical and horizontal polarizations. The focusing ability of the proposed metasurface flat lens is validated via simulation and measurement, where normally incident plane waves are shown to be enhanced by up to 11 dB as a result of wave focusing. Also, the radiation gain enhancement due to the proposed metasurface flat lens is demonstrated via simulation and measurement, where a gain enhancement of up to 10.5 dB is achieved. The results show that the proposed structure maintains the wave focusing and gain enhancement characteristics over a bandwidth of 28–32 GHz. Furthermore, to demonstrate the utility of the proposed metasurface for circular polarization (CP), the gain enhancement of a CP patch antenna as a result of implementing the proposed metasurface as a superstrate is demonstrated via simulation and measurement. It is shown that the proposed metasurface superstrate provides a CP gain enhancement of nearly 10 dB.

Review of Metasurfaces and Metadevices: Advantages of Different Materials and Fabrications

Flat optics, metasurfaces, metalenses, and related materials promise novel on-demand light modulation within ultrathin layers at wavelength scale, enabling a plethora of next-generation optical devices, also known as metadevices. Metadevices designed with different materials have been proposed and demonstrated for different applications, and the mass production of metadevices is necessary for metadevices to enter the consumer electronics market. However, metadevice manufacturing processes are mainly based on electron beam lithography, which exhibits low productivity and high costs for mass production. Therefore, processes compatible with standard complementary metal–oxide–semiconductor manufacturing techniques that feature high productivity, such as i-line stepper and nanoimprint lithography, have received considerable attention. This paper provides a review of current metasurfaces and metadevices with a focus on materials and manufacturing processes. We also provide an analysis of the relationship between the aspect ratio and efficiency of different materials.

Ultrafast metamaterial all-optical switching based on coherent modulation

We report a demonstration of an ultrafast all-optical switching with unique light control effects. The all-optical switching consists of a gold film with asymmetric split rings and a silica substrate. The device effectively controls the transmission and absorption of continuous pulses in the communication band (1200-1800 nm) and short pulses with a pulse duration of 80 fs by using the interaction of two coherent beams on nano-metamaterials with a thickness of only 50 nm. The metamaterial can achieve more than 90 % output control under continuous light irradiation. When the pulse duration is 80 fs, the switching contrast ratio is greater than 3 : 1 and the modulation bandwidth is greater than 12.5 THz. Switching time can be on the order of femtosecond. This paper provides a new structure for ultra-high speed optical data processing components in coherent networks.

Generating ultraviolet perfect vortex beams using a high-efficiency broadband dielectric metasurface

Due to the topological charge-independent doughnut spatial structure as well as the association of orbital angular momentums, perfect vortex beams promise significant advances in fiber communication, optical manipulation and quantum optics. Inspired by the development of planar photonics, several plasmonic and dielectric metasurfaces have been constructed to generate perfect vortex beams, instead of conventional bulky configuration. However, owing to the intrinsic Ohmic losses and interband electron transitions in materials, these metasurface-based vortex beam generators only work at optical frequencies up to the visible range. Herein, using silicon nitride nanopillars as high-efficiency half-wave plates, broadband and high-performance metasurfaces are designed and demonstrated numerically to directly produce perfect vortex beams in the ultraviolet region, by combining the phase profiles of spiral phase plate, axicon and Fourier transformation lens based on geometric phase. The conversion efficiency of the metasurface is up to 86.6% at the design wavelength. Moreover, the influence of several control parameters on perfect vortex beam structures is discussed. We believe that this ultraviolet dielectric generator of perfect vortex beams will find many significant applications, such as high-resolution spectroscopy, optical tweezer and on-chip communication.

Enhanced and unidirectional photonic spin Hall effect in a plasmonic metasurface with S4 symmetry

In this Letter, we report the significant enhancement of the photonic spin Hall effect (SHE) in a plasmonic metasurface with ${\rm S}_4$ symmetry. We find that an enhanced SHE of reflected light can occur in both horizontally and vertically polarized incident beams, and the maximum transverse displacement can approach half of the beam waist. Such a large displacement is caused by the non-resonant and near-zero pseudo-Brewster angles in the plasmonic metasurface. Owing to ${\rm S}_4$ symmetry, a unidirectional SHE is obtained in the metasurface, i.e., large and tiny transverse displacements are realized for a linearly polarized beam incident from the opposite side. This Letter provides a new, to the best of our knowledge, way to achieve an enhanced photonic SHE and offers more opportunities for designing spin-based nanophotonic devices.

High quality factor phase gradient metasurfaces

Dielectric microcavities with quality factors (Q-factors) in the thousands to billions markedly enhance light-matter interactions, with applications spanning high-efficiency on-chip lasing, frequency comb generation and modulation and sensitive molecular detection. However, as the dimensions of dielectric cavities are reduced to subwavelength scales, their resonant modes begin to scatter light into many spatial channels. Such enhanced scattering is a powerful tool for light manipulation, but also leads to high radiative loss rates and commensurately low Q-factors, generally of order ten. Here, we describe and experimentally demonstrate a strategy for the generation of high Q-factor resonances in subwavelength-thick phase gradient metasurfaces. By including subtle structural perturbations in individual metasurface elements, resonances are created that weakly couple free-space light into otherwise bound and spatially localized modes. Our metasurface can achieve Q-factors >2,500 while beam steering light to particular directions. High-Q beam splitters are also demonstrated. With high-Q metasurfaces, the optical transfer function, near-field intensity and resonant line shape can all be rationally designed, providing a foundation for efficient, free-space-reconfigurable and nonlinear nanophotonics.

Compact Double-Layer FR4-Based Focusing Lens Using High-Efficiency Huygens' Metasurface Unit Cells

High transmission efficiency metasurface unit cells have been designed based on surface electric and magnetic impedances derived from Huygens' principle. However, unit cells for low transmission loss (<1 dB) over a wide transmission phase range require at least three metallic layers, which complicates the unit cell design process. In this paper, we introduce high-efficiency Huygens' metasurface unit cell topologies in double-layer FR4 printed circuit board (PCB) by implementing surface electric and magnetic current using the top and bottom metallic patterns and via drills. Eleven unit cells were optimized for wide phase coverage (-150° to 150°) with a low average transmission loss of -0.82 dB at 10 GHz. To demonstrate the high-efficiency of the designed unit cells, we designed and fabricated two focusing lenses with dimensions of near 150 × 150 mm (5λ × 5λ) to focus a spherical beam radiated from short focal distances (f = 100 and 60 mm). The fabricated focusing lens showed 12.87 and 13.58 dB focusing gain for f = 100 and 60 mm at 10 GHz, respectively, with a 1 dB fractional gain bandwidth of near 10%. We expect that the proposed focusing lens based on high-efficiency double-layer metasurface unit cells can help realize compact and high-gain focusing lens-integrated antenna systems.

Single-Element Diffraction-Limited Fisheye Metalens

Wide field-of-view (FOV) optical functionality is crucial for implementation of advanced imaging and image projection devices. Conventionally, wide FOV operation is attained with complicated assembly of multiple optical elements known as "fisheye lenses". Here we present a novel metalens design capable of performing diffraction-limited focusing and imaging over an unprecedented near 180° angular FOV. The lens is monolithically integrated on a one-piece flat substrate and involves only a single layer of metasurface that corrects third-order Seidel aberrations including coma, astigmatism, and field curvature. The metalens further features a planar focal surface, which enables considerably simplified system architectures for applications in imaging and projection. We fabricated the metalens using Huygens meta-atoms operating at 5.2 μm wavelength and experimentally demonstrated aberration-free focusing and imaging over the entire FOV. The design concept is generic and can be readily adapted to different meta-atom geometries and wavelength ranges to meet diverse application demands.

Continuously tunable metasurfaces controlled by single electrode uniform bias-voltage based on nonuniform periodic rectangular graphene arrays

Metasurfaces, the two-dimensional artificial metamaterials, have attracted intensive attention due to their abnormal ability to manipulate the electromagnetic wave. Although there have been considerable efforts to design and fabricate beam steering devices, continuously tunable devices with a uniform bias-voltage have not been achieved. Finding new ways to realize more convenient and simpler wavefront modulation of light still requires research efforts. In this article, a series of novel reflective metasurfaces are proposed to continuously modulate the wavefront of terahertz light by uniformly adjusting the bias-voltage. By introducing the innovation of nonuniform periodic structures, we realize the gradient distribution of the reflected light phase-changing-rate which is the velocity of phase changing with Fermi energy. Based on strict phase distribution design scheme, a beam scanner and a variable-focus reflective metalens are both demonstrated successfully. Furthermore, dynamic and continuous control of either the beam azimuth of beam scanner or the focal length of metalens can be achieved by uniformly tuning the Fermi energy of graphene. Our work provides a potentially efficient method for the development and simplification of the adjustable wavefront controlling devices.

Optical Limiting Based on Huygens' Metasurfaces

Optical limiting is desirable or necessary in a variety of applications that employ high-power light sources or sensitive photodetectors. However, the most prevalent methods compromise between on-state transmission and turndown ratio or rely on narrow transmission windows. We demonstrate that a metasurface-based architecture incorporating phase-change materials enables both high and broadband on-state transmission (-4.8 dB) while also providing a large turndown ratio (25.2 dB). Additionally, this design can be extended for broadband multiwavelength limiting due to the high off-resonance transmittance and readily scalable resonant wavelength. Furthermore, our choice of active material allows for protection in ultrafast laser environments due to the speed of the phase transition. These benefits offer a strong alternative to state-of-the-art optical limiters in technologies ranging from sensor protection to protective eyewear.

Polarization-division and spatial-division shared-aperture nanopatch antenna arrays for wide-angle optical beam scanning

Chip-based optical beam scanners hold promise for future compact high-speed light detection and ranging (LIDAR) systems. Many of the demonstrated chip-based optical beam scanners are designed based on diffraction-based waveguide gratings as on-chip antennas. The waveguide grating antenna, however, only provides a typical field-of-view (FOV) of roughly 10° by tuning the input light wavelength. In this paper, polarization-division and spatial-division multiplexed nanoantenna arrays are proposed to expand the FOV of on-chip antennas. The proposed device, based on silicon-on-insulator (SOI) platform, consists of three nanoantenna groups which are densely packed and fed by a common silicon nanostrip. It is demonstrated that the combination of the optical mode-multiplexing technique and the antenna engineering allows independent controls over the interactions between multiple nanoantenna groups and the waveguide. By proper engineering of the antenna dimensions, the proposed device achieves a FOV of over 40° within a 100 nm wavelength tuning range, almost tripling that of the conventional waveguide grating antenna.

High-Efficiency, Broadband, Near Diffraction-Limited, Dielectric Metalens in Ultraviolet Spectrum

Ultraviolet (UV) optical devices have plenteous applications in the fields of nanofabrication, military, medical, sterilization, and others. Traditional optical components utilize gradual phase accumulation phenomena to alter the wave-front of the light, making them bulky, expensive, and inefficient. A dielectric metasurface could provide an auspicious approach to precisely control the amplitude, phase, and polarization of the incident light by abrupt, discrete phase changing with high efficiency due to low absorption losses. Metalenses, being one of the most attainable applications of metasurfaces, can extremely reduce the size and complexity of the optical systems. We present the design of a high-efficiency transmissive UV metalens operating in a broadband range of UV light (250–400 nm) with outstanding focusing characteristics. The polarization conversion efficiency of the nano-rod unit and the focusing efficiency of the metasurface are optimized to be as high as 96% and 77%, respectively. The off-axis focusing characteristics at different incident angles are also investigated. The designed metalens that is composed of silicon nitride nanorods will significantly uphold the advancement of UV photonic devices and can provide opportunities for the miniaturization and integration of the UV nanophotonics and its applications.

Dual-functional dielectric metasurface doublets

Multi-functional metasurface devices are of great importance for miniaturization and integration. Here we propose a novel approach to design dual-functional dielectric metasurface doublets based on the principle of Pancharatnam-Berry phase. The functionalities for the two operating wavelengths can be independently designed by adjusting the orientation angle of the nanofins in the upper layer and that in the lower layer, respectively. The functionality can be switched by simply changing the incident wavelength. The simulated results of the examples demonstrate that the designed dual-functional doublet works well as expected.

Efficient light redirection via stretched field resonance in dielectric meta-resonator

Light redirection plays an important role in photonic integrated circuit system, which attracts much attention on account of thriving application prospects from microwave to visible frequency. By treating a two-dimensional photonic crystal array as a dielectric resonator with low effective index n_eff << 1, a new strategy of one-direction semi-enclosed meta-resonator is proposed for light redirection and splitting with a high efficiency beyond 90%. Instead of zero-index material, the phenomenon of significant collimating radiations with zero phase delay can also be achieved through a meta-resonator of low effective index to stretch the internal resonant field with a wavelength much longer than that in air. The geometrical dimensions and structural parameters of the meta-resonator offer a great design flexibility for modulating the operating frequency. The numerical simulation and experimental results perfectly coincide with the theoretical predictions. This strategy can also be extended to other artificial metamaterials and three-dimensional cases, which may offer fantastic possibilities to the development of integrated photonics.

Computational inverse design for cascaded systems of metasurface optics

Metasurfaces are an emerging technology that may supplant many of the conventional optics found in imaging devices, displays, and precision scientific instruments. Here, we develop a method for designing optical systems composed of multiple unique metasurfaces aligned in sequence and separated by distances much larger than the design wavelengths. Our approach is based on computational inverse design, also known as the adjoint-gradient method. This technique enables thousands or millions of independent design variables (e.g., the shapes of individual meta-atoms) to be optimized in parallel, with little or no intervention required by the user. The assumptions underlying our method are as follows: we use the local periodic approximation to determine the phase-response of a given meta-atom, we use the scalar wave approximation to propagate light fields between metasurface layers, and we do not consider multiple reflections between metasurface layers (analogous to a sequential-optics ray-tracer). To demonstrate the broad applicability of our method, we use it to design an achromatic doublet metasurface lens, a spectrally-multiplexed holographic element, and an ultra-compact optical neural network for classifying handwritten digits.

Asymmetric Spatial Power Dividers Using Phase-Amplitude Metasurfaces Driven by Huygens Principle

Recent years have witnessed an extraordinary spurt in attention toward the wave-manipulating strategies revealed by phase-amplitude metasurfaces. Recently, it has been shown that, when two different phase-encoded metasurfaces responsible for doing separate missions are added together based on the superposition theorem, the mixed digital phase distribution will realize both missions at the same time. In this paper, via a semi-analytical procedure, we demonstrate that such a theorem is not necessarily valid when using phase-only metasurfaces or ignoring the element pattern functions. We introduce the concept of asymmetric spatial power divider (ASPD) with arbitrary power ratio levels in which modulating both amplitude and phase of the meta-atoms is inevitable to fully control the power intensity pattern of a reflective metasurface. Numerical simulations illustrate that the proposed ASPD designed by proper phase and amplitude distribution over the surface can directly generate a desired number of beams with predetermined orientations and power budgets. The C-shaped Pancharatnam-Berry meta-atoms locally realize the optimal phase and amplitude distribution in each case, and the good conformity between simulations and theoretical predictions verifies the presented formalism. A prototype of our ASPD designs is also fabricated and measured, and the experimental results corroborate well our numerical and semi-analytical predictions. Our findings not only offer possibilities to realize arbitrary spatial power dividers over subwavelength scale but also reveal an economical and simple alternative for a beamforming array antenna.

3D plasmonic design approach for efficient transmissive Huygens metasurfaces

In this paper we present a design concept for 3D plasmonic scatterers as high- efficiency transmissive metasurface (MS) building blocks. A genetic algorithm (GA) routine partitions the faces of the walls inside an open cavity into a M x N grid of voxels which can be either covered with metal or left bare, and optimizes the distribution of metal coverage needed to generate electric and magnetic modes of equal strength with a targeted phase delay (Φt) at the design wavelength. Even though the electric and magnetic modes can be more complicated than typical low order modes, with their spectral overlap and equal strengths, they act as a Huygens source, with the accompanying low reflection magnitude. Square/hexagonal voxels inside square/rectangular cavities are thoroughly analyzed for operation at 8 µm, although the technique can be applied to different cavity geometries for operation across the electromagnetic spectrum. Results from full-wave simulations show the GA routine can repeatedly pinpoint scatterer geometries emitting at any Φt value across 2π phase space with transmittances of at least 60%, making these MS building blocks an attractive plasmonic alternative for practical optical applications. Full-scale metasurface devices are calculated from near-fields of the individual elements to validate the optical functionality.

High-efficiency Huygens' metasurface for terahertz wave manipulation

A fair amount of theoretical work has shown that Huygens' metasurfaces well modulate electromagnetic waves by properly designing electrical impedance Z_es and magnetic admittance Y_ms; however, the transmissive Huygens' metasurface is still challenging in the terahertz band. In this work, a transmission-type Huygens' metasurface with bilayer metallic patches has been proposed and theoretically demonstrated to show a reflectionless phase modulation for a linearly polarized terahertz wave. The simulation results show that the metasurface can achieve 2π phase coverage, and importantly the phase change can be simply achieved by changing a single geometric parameter of the metamolecule, along with a similar transmission effect. We design a high-efficiency beam deflector to realize an anomalous refraction with an angle of 19.8°. The proposed metasurface will provide a simple and direct way to realize efficient terahertz devices for wavefront manipulation.

High-performance meta-devices based on multilayer meta-atoms: interplay between the number of layers and phase coverage

Transmissive metasurfaces have provided an efficient platform to manipulate electromagnetic (EM) waves, but previously adopted multilayer meta-atoms are too thick and/or the design approach fully relies on brute-force simulations without physical understandings. Here, based on coupled-mode theory (CMT) analyses on multilayer meta-atoms of distinct types, it is found that meta-atoms of a specific type only allows the phase coverage over a particular range, thus suitable for polarization-control applications. However, combinations of meta-atoms with distinct types are necessary for building ultra-thin wavefront-control meta-devices requiring 360° phase coverage. Based on these physical understandings, high-efficiency meta-atoms are designed/fabricated, and used to construct three typical meta-devices, including quarter- and half-wave plates and a beam deflector. Our results elucidate the physics underlying the interplay between thicknesses and performances of transmissive metasurfaces, which can guide the realizations of miniaturized transmissive meta-devices in different frequency domains.

Efficient visible light modulation based on electrically tunable all dielectric metasurfaces embedded in thin-layer nematic liquid crystals

All-dielectric metasurfaces have attracted attention for highly efficient visible light manipulation. So far, however, they are mostly passive devices, while those allowing dynamic control remain a challenge. A highly efficient tuning mechanism is immersing the metasurface in a birefringent liquid crystal (LC), whose refractive index can be electrically controlled. Here, an all-dielectric tunable metasurface is demonstrated based on this concept, operating at visible frequencies and based on TiO₂ nanodisks embedded in a thin LC layer. Small driving voltages from 3~5 V are sufficient to tune the metasurface resonances, with an associated transmission modulation of more than 65%. The metasurface optical responses, including the observed electric and magnetic dipole resonance shifts as well as the interfacial anchoring effect of the LC induced by the presence of the nanostructures, are systematically discussed. The dynamic tuning observed in the transmission spectra can pave the way to dynamically tunable metasurface devices for efficient visible light modulation applications.

Mechanically tunable focusing metamirror in the visible

A compact, flat lens with dynamically tunable focal length will be an essential component in advanced reconfigurable optical systems. One approach to realize a flat tunable lens is by utilizing metasurfaces, which are two-dimensional nanostructures capable of tailoring the wavefront of incident light. When a metasurface with a hyperboloidal phase profile, i.e., a metalens, is fabricated on a substrate that can be actuated, its focal length can be adjusted dynamically. Here, we design and realize the first reflection type, tunable metalens (i.e., metamirror) operating in the visible regime (670 nm). It is shown that the focal length can be continuously adjusted by up to 45% with a 0% to 20% lateral stretching of the substrate, while maintaining diffraction-limited focusing and high focusing efficiency. Our design as a flat optics element has potential in widespread applications, such as wearable mixed reality systems, biomedical instruments and integrated optics devices.

Compound Metaoptics for Amplitude and Phase Control of Wave Fronts

Metasurfaces allow tailored control of electromagnetic wave fronts. However, due to local conservation of power flow, passive, lossless, and reflectionless metasurfaces have been limited to imparting phase discontinuities-and not power density discontinuities-onto a wave front. Here, we show how the phase and amplitude profiles of a wave front can be independently controlled using two closely spaced phase-discontinuous metasurfaces. The two metasurfaces, each designed to exhibit spatially varying refractive properties, are separated by a wavelength-scale distance and together form a compound metaoptic. A method of designing the compound metaoptic is presented, which enables transformation between arbitrary complex-valued field distributions without reflection, absorption, polarization loss, or active components. Such compound metaoptics may find applications in the optical trapping of particles, displaying three-dimensional holographic images, shrinking the size of optical systems, or producing custom (shaped and steered) far-field radiation patterns.

Nonlinear Metasurface for Structured Light with Tunable Orbital Angular Momentum

Orbital angular momentum (OAM) beams may create a new paradigm for the future classical and quantum communication systems. A majority of existing OAM beam converters are bulky, slow, and cannot withstand high powers. Here, we design and experimentally demonstrate an ultra-fast, compact chalcogenide-based all-dielectric metasurface beam converter which has the ability to transform a Hermite–Gaussian (HG) beam into a beam carrying an OAM at near infrared wavelength. Depending on the input beam intensity, the topological charge carried by the output OAM beam can be switched between positive and negative. The device provides high transmission efficiency and is fabricated by a standard electron beam lithography. Arsenic trisulfide (As 2 S 3 ) chalcogenide glass (ChG) offers ultra-fast and large third-order nonlinearity as well as a low two-photon absorption coefficient in the near infrared spectral range.

Plasmonic metasurfaces with 42.3% transmission efficiency in the visible

Metasurfaces are two-dimensional nanoantenna arrays that can control the propagation of light at will. In particular, plasmonic metasurfaces feature ultrathin thicknesses, ease of fabrication, field confinement beyond the diffraction limit, superior nonlinear properties, and ultrafast performances. However, the technological relevance of plasmonic metasurfaces operating in the transmission mode at optical frequencies is questionable due to their limited efficiency. The state-of-the-art efficiency of geometric plasmonic metasurfaces at visible and near-infrared frequencies, for example, is ≤10%. Here, we report a multipole-interference-based transmission-type geometric plasmonic metasurface with a polarization conversion efficiency that reaches 42.3% at 744 nm, over 400% increase over the state of the art. The efficiency is augmented by breaking the scattering symmetry due to simultaneously approaching the generalized Kerker condition for two orthogonal polarizations. In addition, the design of the metasurface proposed in this study introduces an air gap between the antennas and the surrounding media that confines the field within the gap, which mitigates the crosstalk between meta-atoms and minimizes metallic absorption. The proposed metasurface is broadband, versatile, easy to fabricate, and highly tolerant to fabrication errors. We highlight the technological relevance of our plasmonic metasurface by demonstrating a transmission-type beam deflector and hologram with record efficiencies.

A high-efficiency dual-wavelength achromatic metalens based on Pancharatnam-Berry phase manipulation

The application of metasurfaces requires the reduction or elimination of their chromatic aberration while maintaining a high efficiency. We propose a method for the design of dual-wavelength operating achromatic metalenses, which can focus two different wavelengths at the same position. Phase manipulation was achieved by crossing two crystalline Si nanorods on each pixel carrying phase information for wavelengths of 780 and 660 nm based on the principal of Pancharatnam-Berry (PB) phase. At 660 nm, chromatic aberration was reduced from 1.28 to 0.46 μm in numerical experiments. The focusing efficiency of the circularly polarized light reached 90.2% for 780 nm and 49.7% for 660 nm. This method can be extended to other wavelengths.

Extreme Huygens' Metasurfaces Based on Quasi-Bound States in the Continuum

We introduce the concept of and a generic approach to realizing extreme Huygens' metasurfaces by bridging the concepts of Huygens' conditions and optical bound states in the continuum. This novel paradigm allows the creation of Huygens' metasurfaces with quality factors that can be tuned over orders of magnitude, generating extremely dispersive phase modulation. We validate this concept with a proof-of-concept experiment at the near-infrared wavelengths, demonstrating all-dielectric Huygens' metasurfaces with different quality factors. Our study points out a practical route for controlling the radiative decay rate while maintaining the Huygens' condition, complementing existing Huygens' metasurfaces whose bandwidths are relatively broad and complicated to tune. This novel feature can provide new insight for various applications, including optical sensing, dispersion engineering and pulse shaping, tunable metasurfaces, metadevices with high spectral selectivity, and nonlinear meta-optics.

Sideways adiabaticity: beyond ray optics for slowly varying metasurfaces

Optical metasurfaces (subwavelength-patterned surfaces typically described by variable effective surface impedances) are typically modeled by an approximation akin to ray optics: the reflection or transmission of an incident wave at each point of the surface is computed as if the surface were "locally uniform," and then the total field is obtained by summing all of these local scattered fields via a Huygens principle. (Similar approximations are found in scalar diffraction theory and in ray optics for curved surfaces.) In this paper, we develop a precise theory of such approximations for variable-impedance surfaces. Not only do we obtain a type of adiabatic theorem showing that the "zeroth-order" locally uniform approximation converges in the limit as the surface varies more and more slowly, including a way to quantify the rate of convergence, but we also obtain an infinite series of higher-order corrections. These corrections, which can be computed to any desired order by performing integral operations on the surface fields, allow rapidly varying surfaces to be modeled with arbitrary accuracy, and also allow one to validate designs based on the zeroth-order approximation (which is often surprisingly accurate) without resorting to expensive brute-force Maxwell solvers. We show that our formulation works arbitrarily close to the surface, and can even compute coupling to guided modes, whereas in the far-field limit our zeroth-order result simplifies to an expression similar to what has been used by other authors.

Reconfiguring structured light beams using nonlinear metasurfaces

Ultra-compact, low-loss, fast, and reconfigurable optical components, enabling manipulation of light by light, could open numerous opportunities for controlling light on the nanoscale. Nanostructured all-dielectric metasurfaces have been shown to enable extensive control of amplitude and phase of light in the linear optical regime. Among other functionalities, they offer unique opportunities for shaping the wave front of light to introduce the orbital angular momentum (OAM) to a beam. Such structured light beams bring a new degree of freedom for applications ranging from spectroscopy and micromanipulation to classical and quantum optical communications. To date, reconfigurability or tuning of the optical properties of all-dielectric metasurfaces have been achieved mechanically, thermally, electrically or optically, using phase-change or nonlinear optical materials. However, a majority of demonstrated tuning approaches are either slow or require high optical powers. Arsenic trisulfide (As₂S₃) chalcogenide glass offering ultra-fast and large χ⁽³⁾nonlinearity as well as a low two-photon absorption coefficient in the near and mid-wave infrared spectral range, could provide a new platform for the realization of fast and relatively low intensity reconfigurable metasurfaces. Here, we design and experimentally demonstrate an As₂S₃ chalcogenide glass based metasurface that enables reshaping of a conventional Hermite-Gaussian beam with no OAM into an OAM beam at low intensity levels, while preserves the original beam's amplitude and phase characteristics at high intensity levels. The proposed metasurface could find applications for a new generation of optical communication systems and optical signal processing.

Flat metasurfaces to collimate electromagnetic waves with high efficiency

Directional beaming of electromagnetic waves passing through a subwavelength aperture has attracted considerable interests in photonics, but the traditional approach of utilizing gratings to directionally couple surface waves (SWs) to a desired far-field direction faces the low-efficiency issue owing to high-order diffractions. Here we experimentally demonstrate that directional beaming of light can be realized with very high efficiencies, in which two specifically designed metasurfaces (MTSs) are placed at two sides of the aperture to serve as SW to propagating-wave meta-couplers. Different from the grating couplers, the well-designed phase-gradient meta-couplers can freely select the desired diffraction orders by suppressing the undesired diffraction orders. We design and fabricate MTSs with different phase gradients, and perform both far-field and near-field measurements to verify the predicted high-efficiency on/off-axis directional beaming effects. Experimental results are in good agreement with full wave simulations and theoretical analyses.

Synchrotron radiation from an accelerating light pulse

Synchrotron radiation-namely, electromagnetic radiation produced by charges moving in a curved path-is regularly generated at large-scale facilities where giga-electron volt electrons move along kilometer-long circular paths. We use a metasurface to bend light and demonstrate synchrotron radiation produced by a subpicosecond pulse, which moves along a circular arc of radius 100 micrometers inside a nonlinear crystal. The emitted radiation, in the terahertz frequency range, results from the nonlinear polarization induced by the pulse. The generation of synchrotron radiation from a pulse revolving about a circular trajectory holds promise for the development of on-chip terahertz sources.

Flexible control of transmitting terahertz beams based on multilayer encoding metasurfaces

The concept of encoding metasurfaces makes a connection between the physical metamaterial particles and the digital code so the digital coding can be performed on the coding surface for signal processing to achieve unusual physical phenomena. Here, a complete 2π transmission phase with high efficiency at the target frequency can be manipulated by a multilayer metasurface structure. Different period sequence codes are designed to obtain the deflection angles, and then a new sequence is obtained through a four-bit system operation sequence according to the Fourier convolution principle. This study shows that Fourier calculation provides an efficient way to optimize the coding to achieve a pre-designed transmitted beam. The advantage of this method over previous solutions in generating an anomalous single transmission beam is its flexibility and continuous control of arbitrary directions, and it is possible to transmit a normally incident terahertz beam to an abnormal arbitrary direction with cross-polarization. This work opens up a new digital perspective on the new approach to metamaterials, predicting the possibility of combining electromagnetic wave operations with the convolution theorems in digital signal processing and the transcoding surface.

Vertically integrated visible and near-infrared metasurfaces enabling an ultra-broadband and highly angle-resolved anomalous reflection

An optical device with minimized dimensions, which is capable of efficiently resolving an ultra-broad spectrum into a wide splitting angle but incurring no spectrum overlap, is of importance in advancing the development of spectroscopy. Unfortunately, this challenging task cannot be easily addressed through conventional geometrical or diffractive optical elements. Herein, we propose and demonstrate vertically integrated visible and near-infrared metasurfaces which render an ultra-broadband and highly angle-resolved anomalous reflection. The proposed metasurface capitalizes on a supercell that comprises two vertically concatenated trapezoid-shaped aluminum antennae, which are paired with a metallic ground plane via a dielectric layer. Under normal incidence, reflected light within a spectral bandwidth of 1000 nm ranging from λ = 456 nm to 1456 nm is efficiently angle-resolved to a single diffraction order with no spectrum overlap via the anomalous reflection, exhibiting an average reflection efficiency over 70% and a substantial angular splitting of 58°. In light of a supercell pitch of 1500 nm, to the best of our knowledge, the micron-scale bandwidth is the largest ever reported. It is noted that the substantially wide bandwidth has been accomplished by taking advantage of spectral selective vertical coupling effects between antennae and ground plane. In the visible regime, the upper antenna primarily renders an anomalous reflection by cooperating with the lower antenna, which in turn cooperates with the ground plane and produces phase variations leading to an anomalous reflection in the near-infrared regime. Misalignments between the two antennae have been particularly inspected to not adversely affect the anomalous reflection, thus guaranteeing enhanced structural tolerance of the proposed metasurface.

Nanoscale Artificial Plasmonic Lattice in Self-Assembled Vertically Aligned Nitride-Metal Hybrid Metamaterials

Nanoscale metamaterials exhibit extraordinary optical properties and are proposed for various technological applications. Here, a new class of novel nanoscale two-phase hybrid metamaterials is achieved by combining two major classes of traditional plasmonic materials, metals (e.g., Au) and transition metal nitrides (e.g., TaN, TiN, and ZrN) in an epitaxial thin film form via the vertically aligned nanocomposite platform. By properly controlling the nucleation of the two phases, the nanoscale artificial plasmonic lattices (APLs) consisting of highly ordered hexagonal close packed Au nanopillars in a TaN matrix are demonstrated. More specifically, uniform Au nanopillars with an average diameter of 3 nm are embedded in epitaxial TaN platform and thus form highly 3D ordered APL nanoscale metamaterials. Novel optical properties include highly anisotropic reflectance, obvious nonlinear optical properties indicating inversion symmetry breaking of the hybrid material, large permittivity tuning and negative permittivity response over a broad wavelength regime, and superior mechanical strength and ductility. The study demonstrates the novelty of the new hybrid plasmonic scheme with great potentials in versatile material selection, and, tunable APL spacing and pillar dimension, all important steps toward future designable hybrid plasmonic materials.

Manipulation of visible-light polarization with dendritic cell-cluster metasurfaces

Cross-polarization conversion plays an important role in visible light manipulation. Metasurface with asymmetric structure can be used to achieve polarization conversion of linearly polarized light. Based on this, we design a quasi-periodic dendritic metasurface model composed of asymmetric dendritic cells. The simulation indicates that the asymmetric dendritic structure can vertically rotate the polarization direction of the linear polarization wave in visible light. Silver dendritic cell-cluster metasurface samples were prepared by the bottom-up electrochemical deposition. It experimentally proved that they could realize the cross - polarization conversion in visible light. Cross-polarized propagating light is deflected into anomalous refraction channels. Dendritic cell-cluster metasurface with asymmetric quasi-periodic structure conveys significance in cross-polarization conversion research and features extensive practical application prospect and development potential.

Broadband birefringent metamaterial lens with bi-functional high-gain radiation and deflection properties

Birefringent metamaterial shows fantastic properties in controlling the propagation of electromagnetic wave. Based on the birefringent theory, a Ku-band birefringent metamaterial lens is proposed, which can radiate high gain TE wave with a TE wave feed and deflect incident TM wave. To realize bi-functional high gain radiation or deflection, the required permittivity distribution is analyzed first. Then, the unit cells of metamaterial lens are carefully designed based on effective permittivity equations to achieve the desired permittivity distribution. To demonstrate the present design, a cylindrical birefringent lens prototype is fabricated using 3D printing techniques. The experiments verify that, with a rectangular waveguide feeding on the lens surface or a horn antenna illuminated in the far field, the Ku-band birefringent metamaterial lens performs as a high-gain broadband radiator for the TE wave feed or a deflector for the incident TM wave.

Advances in optical metasurfaces: fabrication and applications [Invited]

The research and development of optical metasurfaces has been primarily driven by the curiosity for novel optical phenomena that are unattainable from materials that exist in nature and by the desire for miniaturization of optical devices. Metasurfaces constructed of artificial patterns of subwavelength depth make it possible to achieve flat, ultrathin optical devices of high performance. A wide variety of fabrication techniques have been developed to explore their unconventional functionalities which in many ways have revolutionized the means with which we control and manipulate electromagnetic waves. The relevant research community could benefit from an overview on recent progress in the fabrication and applications of the metasurfaces. This review article is intended to serve that purpose by reviewing the state-of-the-art fabrication methods and surveying their cutting-edge applications.

Broadband full-color multichannel hologram with geometric metasurface

Due to the abilities of manipulating the wavefront of light with well-controlled amplitude, and phase and polarization, optical metasurfaces are very suitable for optical holography, enabling applications with multiple functionalities and high data capacity. Here, we demonstrate encoding two- and three-dimensional full-color holographic images by an ultrathin metasurface hologram whose unit cells are subwavelength nanoslits with spatially varying orientations. We further show that it is possible to achieve full-color holographic multiplexing with such kind of geometric metasurfaces, realized by a synthetic spectrum holographic algorithm. Our results provide an efficient way to design multi-color optical display elements that are ready for fabrication.

High-efficiency dual-modes vortex beam generator with polarization-dependent transmission and reflection properties

Vortex beam is believed to be an effective way to extend communication capacity, but available efforts suffer from the issues of complex configurations, fixed operation mode as well as low efficiency. Here, we propose a general strategy to design dual-modes vortex beam generator by using metasurfaces with polarization-dependent transmission and reflection properties. Combining the focusing and vortex functionalities, we design/fabricate a type of compact dual-modes vortex beam generator operating at both reflection/transmission sides of the system. Experimental results demonstrate that the designed metadevice can switch freely and independently between the reflective vortex with topological charge m₁ = 2 and transmissive vortex with m₂ = 1. Moreover, the metadevice exhibits very high efficiencies of 91% and 85% for the reflective and transmissive case respectively. Our findings open a door for multifunctional metadevices with high performances, which indicate wide applications in modern integration-optics and wireless communication systems.

Ultra-thin high-efficiency mid-infrared transmissive Huygens meta-optics

The mid-infrared (mid-IR) is a strategically important band for numerous applications ranging from night vision to biochemical sensing. Here we theoretically analyzed and experimentally realized a Huygens metasurface platform capable of fulfilling a diverse cross-section of optical functions in the mid-IR. The meta-optical elements were constructed using high-index chalcogenide films deposited on fluoride substrates: the choices of wide-band transparent materials allow the design to be scaled across a broad infrared spectrum. Capitalizing on a two-component Huygens’ meta-atom design, the meta-optical devices feature an ultra-thin profile (λ₀/8 in thickness) and measured optical efficiencies up to 75% in transmissive mode for linearly polarized light, representing major improvements over state-of-the-art. We have also demonstrated mid-IR transmissive meta-lenses with diffraction-limited focusing and imaging performance. The projected size, weight and power advantages, coupled with the manufacturing scalability leveraging standard microfabrication technologies, make the Huygens meta-optical devices promising for next-generation mid-IR system applications.

Mid-IR optics can require exotic materials or complicated processing, which can result in high cost and inferior quality. Here the authors report the demonstration of high-efficiency mid-IR transmissive lenses based on dielectric Huygens metasurface, showing diffraction limited focusing and imaging performance.

Multifunctional Metasurfaces Based on the “Merging” Concept and Anisotropic Single-Structure Meta-Atoms

Metasurfaces offer great opportunities to control electromagnetic (EM) waves, attracting intensive attention in science and engineering communities. Recently, many efforts were devoted to multifunctional metasurfaces integrating different functionalities into single flat devices. In this article, we present a concise review on the development of multifunctional metasurfaces, focusing on the design strategies proposed and functional devices realized. We first briefly review the early efforts on designing such systems, which simply combine multiple meta-structures with distinct functionalities to form multifunctional devices. To overcome the low-efficiency and functionality cross-talking issues, a new strategy was proposed, in which the meta-atoms are carefully designed single structures exhibiting polarization-controlled transmission/reflection amplitude/phase responses. Based on this new scheme, various types of multifunctional devices were realized in different frequency domains, which exhibit diversified functionalities (e.g., focusing, deflection, surface wave conversion, multi-beam emissions, etc.), for both pure-reflection and pure-transmission geometries or even in the full EM space. We conclude this review by presenting our perspectives on this fast-developing new sub-field, hoping to stimulate new research outputs that are useful in future applications.

Reconfigurable meta-mirror for wavefronts control: applications to microwave antennas

A planar metasurface composed of electronically tunable meta-atoms incorporating voltage-controlled varactor diodes is proposed as a reconfigurable meta-mirror for wavefronts control in microwave antenna applications. The dispersion responses of the cells are individually tailored in the reconfigurable metasurface so as to overcome the bandwidth limitations of passive metasurfaces and also to control the phase characteristics. By controlling the bias voltage of the varactor diodes on the planar metasurface, the phase characteristics of reflectors can be engineered. The reconfigurable meta-mirror is utilized to implement three different types of reflectors. As such, a reflectarray, a cylindrical parabolic reflector and a dihedral reflector are numerically verified in microwave regime through finite element method. Moreover, experimental measurements are performed on a fabricated prototype to validate the proposed device. Frequency agility, beam deflection and beam focusing are the main functionalities demonstrated from the proposed reconfigurable meta-mirror.