Wavefront shaping with disorder-engineered metasurfaces

A High-Gain and Dual-Band Compact Metasurface Antenna for Wi-Fi/WLAN Applications

With the rapid development of Wi-Fi 6/6E and dual-band wireless systems, there is an increasing demand for compact antennas with balanced high-gain performance across both 2.4 GHz and 5 GHz bands. However, most existing dual-band metasurface antennas face challenges in uneven gain distribution between lower/higher-frequency bands and structural miniaturization. This paper proposes a high-gain dual-band metasurface antenna based on an artificial magnetic conductor (AMC) array, which has a significant advantage in miniaturization and improving antenna performance. Two types of dual-band AMC structures are applied to design the metasurface antenna. The optimal antenna with dual-slot AMC array operates in the 2.42-2.48 GHz and 5.16-5.53 GHz frequency bands, with a 25% size reduction compared to the reference dual-band U-slot antenna. Meanwhile, high gains of 7.65 dBi and 8 dBi are achieved at 2.4 GHz and 5 GHz frequency bands, respectively. Experimental results verify stable radiation gains across the operation bands, where the total efficiency remains above 90%, agreeing well with the simulation results. This research provides an effective strategy for high-gain and dual-band metasurface antennas, offering a promising solution for integrated modern wireless systems such as Wi-Fi 6, Bluetooth, and MIMO technology.

Transmission-reflection phase modulation for antenna gain enhancement and RCS reduction based on polarization-selective metamaterials

The manipulation of electromagnetic waves using metamaterials (MMs) has been extensively studied, yet research on simultaneously controlling both radiated and scattered electromagnetic waves within the same wideband remains limited. Building on previous work, this paper introduces transmission and reflection phase units based on polarization-selective metamaterials to regulate the radiation performance of a single-polarized source and the platform's scattering characteristics in a broadband range. The polarization-selective metamaterial consists of polarization-conversion transmission structures and polarization-selective reflection structures. Through phase design of these components, polarization-selective wavefront converter and chessboard metasurfaces were developed and fabricated. When integrated with the antenna platform, simulations and experimental results demonstrate an average radiation gain improvement of 7.3 dBi in the 8-12 GHz frequency range. Simultaneously, the monostatic radar cross section (RCS) reduction exceeds 10 dB on average within the same frequency range, and the bistatic RCS reduction averages over 10 dB for incident angles between 0° and 50°. In summary, this study presents an approach for the simultaneous control of both radiation and scattering performance over a broad frequency range.

Broadband perfect Littrow diffraction metasurface under large-angle incidence

Littrow diffraction devices are commonly used in the laser field (e.g., laser resonators and spectrometers), where system integration requires larger incidence angles and perfect broadband efficiency. Compared to traditional diffraction devices, which struggle to manipulate light paths under large-angle incidence, metasurfaces has the potential to enhance the broadband efficiency. Despite quasi three-dimensional metasurfaces effects, only perfect anomalous reflection under normal incidence at limited wavelengths was achieved due to energy flow mismatch in the broadband Littrow configuration. Here, we propose a supercell metasurface capable of regulating broadband non-local responses. The metasurface effectively suppresses non-local responses under Littrow mounting, while providing sufficient non-local responses through strong structural coupling effects when the incidence deviates from the Littrow mounting. A large-angle broadband Littrow diffraction metasurface in the mid-infrared spectrum (3.11 µm ∼ 3.52 µm) has been successfully realized, with 99 % efficiency at Littrow angle of 70°. Our results break through the bandwidth limitations of perfect diffraction, providing robust support for the practical applications of metasurfaces in Littrow diffraction devices.

Dielectric metasurfaces based on a phase singularity in the region of high reflectance

Metasurfaces, two-dimensional planar optical devices based on subwavelength-scale structures, have garnered significant attention for their potential to replace conventional optical components in various fields. These devices can manipulate the amplitude, phase, and polarization of light in versatile ways, offering complex functionalities within a single, space-efficient device. However, enhancing their functionality remains a challenge, requiring an expansion in the design flexibility of the structural elements, known as meta-atoms. In this study, we revealed that by varying the two independent lengths of the cross-shaped structure at a wavelength of 980 nm, a phase singularity exists in the region of high reflection. In addition, we found that the phase of transmitted light can be modulated from 0 to 2π by encircling this singularity. Based on the identified phase singularity, we designed and fabricated a polarization-independent metalens with varying numerical apertures to experimentally validate the feasibility of high-reflectivity transmissive wavefront engineering metasurfaces. The introduced meta-atoms based on a phase singularity are expected to open new avenues for applications, such as those requiring light attenuation and concentration simultaneously or the development of resonant cavity structures capable of beam modulation.

Self-Assembly Hybrid Manufacture of Nanoarrays for Metasurfaces

The development of metasurfaces necessitates the rapid fabrication of nanoarrays on diverse substrates at large scales, the preparation of patterned nanoarrays on both planar and curved surfaces, and even the creation of nanoarrays on prefabricated structures to form multiscale metastructures. However, conventional fabrication methods fall short of these rigorous requirements. In this work, a novel self-assembly hybrid manufacturing (SAHM) method is introduced for the rapid and scalable fabrication of shape-controllable nanoarrays on various rigid and flexible substrates. This method can be easily integrated with other fabrication techniques, such as lithography and screen printing, to produce patterned nanoarrays on both planar and non-developable surfaces. Utilizing the SAHM method, nanoarrays are fabricated on prefabricated micropillars to create multiscale pillar-nanoarray metastructures. Measurements indicate that these multiscale metastructures can manipulate electromagnetic waves across a range of wavelengths. Therefore, the SAHM method demonstrates the potential of multiscale structures as a new paradigm for the design and fabrication of metasurfaces.

2nd-Order Debye relaxation in electromagnetic metasurfaces for wideband dispersion engineering

In dielectric physics, electromagnetic (EM) properties of dielectrics arise from several important polarization mechanisms that can be described by Debye, Drude or Lorentz models. Metamaterials, as well as their 2D counterparts-metasurfaces, can exhibit bizarre EM parameters such as negative permittivity, whereas polarization mechanisms leading to such have long been discussed in dielectric physics. Drude and Lorentz's models are usually used in metamaterial design, whereas the Debye model is almost absent, though it is so important in dielectric physics. This leaves an unreconciled gap between the dielectric physics and metamaterials. In this paper, we explore Debye relaxations in metasurfaces for the sake of wideband dispersion engineering. By analyzing two fundamental resonance modes of a typical meta-atom, we first show that the reflection phase experiences 1st-order Debye relaxation under the two resonances, although they are typically Lorentzian. More importantly, the two resonances can be tailored to form a 2nd-order Debye relaxation process so as to achieve smooth phase variations in between them, which lays a solid foundation for wideband dispersion engineering. As proof of concept, we propose a quad-elliptical-arc (QEA) structure as the meta-atom, whose dispersion can be customized by tailoring the 2nd-order Debye relaxation. With this meta-atom, we demonstrated two metasurface prototypes that can achieve chromatic and achromatic focusing, respectively, in the entire X band (8.0-12.0 GHz), showcasing the powerful capacity of wideband dispersion engineering. This work digs out relaxation processes in metamaterials and opens up new territories for metamaterial research, which may find wide applications in wideband devices and systems.

Optical Metasurfaces for Biomedical Imaging and Sensing

Optical metasurfaces, arrays of nanostructures engineered to manipulate light, have emerged as a transformative technology in both research and industry due to their compact design and exceptional light control capabilities. Their strong light-matter interactions enable precise wavefront modulation, polarization control, and significant near-field enhancements. These unique properties have recently driven their application in biomedical fields. In particular, metasurfaces have led to breakthroughs in biomedical imaging technologies, such as achromatic imaging, phase imaging, and extended depth-of-focus imaging. They have also advanced cutting-edge biosensing technologies, featuring high-quality factor resonators and near-field enhancements. As the demand for device miniaturization and system integration increases, metasurfaces are expected to play a pivotal role in the development of next-generation biomedical devices. In this review, we explore the latest advancements in the use of metasurfaces for biomedical applications, with a particular focus on imaging and sensing. Additionally, we discuss future directions aimed at transforming the biomedical field by leveraging the full potential of metasurfaces to provide compact, high-performance solutions for a wide range of applications.

A Review of Cascaded Metasurfaces for Advanced Integrated Devices

This paper reviews the field of cascaded metasurfaces, which are advanced optical devices formed by stacking or serially arranging multiple metasurface layers. These structures leverage near-field and far-field electromagnetic (EM) coupling mechanisms to enhance functionalities beyond single-layer metasurfaces. This review comprehensively discusses the physical principles, design methodologies, and applications of cascaded metasurfaces, focusing on both static and dynamic configurations. Near-field-coupled structures create new resonant modes through strong EM interactions, allowing for efficient control of light properties like phase, polarization, and wave propagation. Far-field coupling, achieved through greater interlayer spacing, enables traditional optical methods for design, expanding applications to aberration correction, spectrometers, and retroreflectors. Dynamic configurations include tunable devices that adjust their optical characteristics through mechanical motion, making them valuable for applications in beam steering, varifocal lenses, and holography. This paper concludes with insights into the potential of cascaded metasurfaces to create multifunctional, compact optical systems, setting the stage for future innovations in miniaturized and integrated optical devices.

3D alignment of distant patterns with deep-subwavelength precision using metasurfaces

Measurement of the relative positions of two objects in three dimensions with sub-nanometer precision is essential to fundamental physics experiments and applications such as aligning multi-layer patterns of semiconductor chips. Existing methods, which rely on microscopic imaging and registration of distant patterns, lack the required accuracy and precision for the next generation of three-dimensional (3D) chips. Here we show that 3D misalignment between two distant objects can be measured using metasurface alignment marks, a laser, and a camera with sub-nanometer precision. Through simulations, we demonstrate that the shot noise-limited precisions of the lateral and axial misalignments between the marks are λ0/50, 000 and λ0/6, 300 (λ0: laser's wavelength), respectively. With its high precision and simplicity, the technique enables the next generation of 3D-integrated optical and electronic chips and paves the way for developing cost-effective and compact sensors relying on sub-nanometer displacement measurements.

Harnessing the capabilities of VCSELs: unlocking the potential for advanced integrated photonic devices and systems

Vertical cavity surface emitting lasers (VCSELs) have emerged as a versatile and promising platform for developing advanced integrated photonic devices and systems due to their low power consumption, high modulation bandwidth, small footprint, excellent scalability, and compatibility with monolithic integration. By combining these unique capabilities of VCSELs with the functionalities offered by micro/nano optical structures (e.g. metasurfaces), it enables various versatile energy-efficient integrated photonic devices and systems with compact size, enhanced performance, and improved reliability and functionality. This review provides a comprehensive overview of the state-of-the-art versatile integrated photonic devices/systems based on VCSELs, including photonic neural networks, vortex beam emitters, holographic devices, beam deflectors, atomic sensors, and biosensors. By leveraging the capabilities of VCSELs, these integrated photonic devices/systems open up new opportunities in various fields, including artificial intelligence, large-capacity optical communication, imaging, biosensing, and so on. Through this comprehensive review, we aim to provide a detailed understanding of the pivotal role played by VCSELs in integrated photonics and highlight their significance in advancing the field towards efficient, compact, and versatile photonic solutions.

Continuous manipulation of electromagnetic radiation based on ultrathin flexible frequency coding metasurface

The physical characteristics of electromagnetic waves are combined with digital information in coding metasurfaces. Coding metasurfaces enable precise control of beams by flexibly designing coding sequences. However, achieving continuous multivariate modulation of electromagnetic waves on passive flexible coded metasurfaces remains a challenge. Previous passive coding metasurfaces have a fixed phase difference between adjacent coding units throughout the operating frequency band, and when the coding pattern is defined, the coded metasurface can only achieve a single electromagnetic function. Our proposed frequency coding metasurface units vary linearly in phase difference over the operating frequency band with different phase sensitivities. Frequency coding metarsurfaces enable a wide range of tunable and versatile electromagnetic energy radiation, without introducing any active devices and changing the coding pattern. As a demonstration of the concept, we have shown theoretically and numerically that frequency coding metasurface can achieve successive transformations of electromagnetic functions, including multi-beam generation, anomalous deflection and diffuse scattering. In addition, beam sweeping function is achieved by means of spatially non-periodically distributed frequency coding metasurface. When the frequency of the incident wave is changed, the deflection angle of the beam is also changed. In addition to the tunability of properties, research on coding metasurfaces has tended to be limited to rigid materials. Flexible coding metasurfaces have potential applications in microwave antennas, radar and aircraft. The passive flexible frequency coding metasurfaces provide a novel approach to manipulating electromagnetic waves with increased design flexibility. This promises applications in microwave antennas, radar, aircraft, and satellite communications.

Infrared color-sorting metasurfaces

The process of sorting light based on colors (photon energy) is a prerequisite in broadband optical systems, typically achieved in the form of guiding incoming signals through a sequence of spectral filters. The assembly of filters often leads to lengthy optical trains and consequently, large system footprints. In this work, we address this issue by proposing a flat color-sorting device comprising a diffraction grating and a dielectric Huygens' metasurface. Upon the incidence of a broadband beam, the grating disperses wavelengths to a continuous range of angles in accordance with the law of diffraction. The following metasurface with multiple paired Huygens' resonances corrects the dispersion and binds wavelengths to the corresponding waveband with a designated output angle. We demonstrate the sorting efficacy by designing a device with a color-sorting metasurface with two discrete dispersion-compensated outputs (10.8 ± 0.3 μm and 11.9 ± 0.3 μm), based on the proposed approach. The optimized metasurface possesses an overall transmittance exceeding 57% and reduces lateral dispersion by 90% at the output. The proposed color-sorting mechanism provides a solution that benefits the designing of metasurfaces for miniature multi-band systems.

Adaptive meshing strategies for nanophotonics using a posteriori error estimation

As nanophotonic devices become increasingly complex, computer simulations of such devices are becoming ever more important. Unfortunately, computer simulations of nanophotonic devices are computationally expensive, especially if many simulations are necessary, e.g., when optimizing or inverse designing a device. Here we study adaptive mesh refinement for finite-element method simulations using an a posteriori error estimation method. We demonstrate that the use of adaptive meshing leads to faster convergence with lower memory footprint for complex three-dimensional nanophotonic structures. Nevertheless, one needs to be careful to avoid a mesh propagation effect for adaptive mesh refinement to be a successful strategy.

Anisotropic plates with architected tendon network

We synthesize geometrically tailorable anisotropic plates by combining button shaped fish-scale like features on soft substrates, then lacing them with high-stiffness strings. This creates a new type of biomimetic architectured structure with multiple broken symmetries. First, the tendons and substrate together break the symmetry of the bending response between the concave and convex curvature. Next, the weave pattern of the tendons further breaks symmetry along the two directors of plates. The anisotropy is clearly evident in 3-point bending experiments. Motivated by these experiments and the need for design, we formulate an analytical energy-based model to quantify the anisotropic elasticity. The derived architecture-property relationships can be used to design architected tendon plates with desirable properties.

Design of Far-Infrared High-Efficiency Polarization-Independent Retroreflective Metasurfaces

Retroreflective gratings serve as fundamental optical elements in nanophotonics, with polarization-independent diffraction efficiency being one of the critical parameters for assessing their performance. In the far-infrared spectral range, traditional retroreflective gratings typically refer to metal echelette gratings, but their diffraction efficiency cannot approach 100% due to metal absorption. In the visible and near-infrared spectral ranges, metal echelette gratings have gradually been replaced by all-dielectric metasurfaces because dielectric materials exhibit negligible absorption at specific wavelengths. However, there is still a lack of relevant research in the far-infrared range, mainly due to the weak control capability of the existing devices over the polarization-independent phase. Here, we propose a kind of all-dielectric retroreflective metasurface composed of asymmetric pillars and freely tunable aperiodic multilayer films. The pillar structure can achieve polarization insensitivity, and the insufficient modulation capability of the dielectric materials can be compensated for by aperiodic Ge/ZnS films. The designed metasurface achieves the diffraction efficiency by RCWA, with the maximum larger than 99% and the overall reaching 95% (9.3-9.6 µm). We have provided detailed explanations of the design methodology and fabrication process. Our work lays the groundwork for further exploration and application of far-infrared lasers.

Augmented reality system based on the integration of polarization-independent metalens and micro-LEDs

Augmented reality (AR), a technology that superimposes virtual information onto a user's direct view of real-world scenes, is considered one of the next-generation display technologies and has been attracting considerable attention. Here, we propose a flat optic AR system that synergistically integrates a polarization-independent metalens with micro light-emitting diodes (LEDs). A key component is a meticulously designed metalens with a numerical aperture of 0.25, providing a simulated focusing efficiency of approximately 76.5% at a wavelength of 532 nm. Furthermore, the laser measurement system substantiates that the fabricated metalens achieves a focusing efficiency of 70.8%. By exploiting the reversibility of light characteristics, the metalens transforms the divergent light from green micro-LEDs into a collimated beam that passes through the pupil and images on the retina. Monochromatic pixels with a size of 5×5 µm2 and a pitch of 10 µm can be distinctly resolved with a power efficiency of 50%. This work illustrates the feasibility of integrating the metalens with microdisplays, realizing a high-efficiency AR device without the need for additional optical components and showcasing great potential for the development of near-eye display applications.

Broadband angular spectrum differentiation using dielectric metasurfaces

Signal processing is of critical importance for various science and technology fields. Analog optical processing can provide an effective solution to perform large-scale and real-time data processing, superior to its digital counterparts, which have the disadvantages of low operation speed and large energy consumption. As an important branch of modern optics, Fourier optics exhibits great potential for analog optical image processing, for instance for edge detection. While these operations have been commonly explored to manipulate the spatial content of an image, mathematical operations that act directly over the angular spectrum of an image have not been pursued. Here, we demonstrate manipulation of the angular spectrum of an image, and in particular its differentiation, using dielectric metasurfaces operating across the whole visible spectrum. We experimentally show that this technique can be used to enhance desired portions of the angular spectrum of an image. Our approach can be extended to develop more general angular spectrum analog meta-processors, and may open opportunities for optical analog data processing and biological imaging.

Optical computing metasurfaces: applications and advances

Integrated photonic devices and artificial intelligence have presented a significant opportunity for the advancement of optical computing in practical applications. Optical computing technology is a unique computing system based on optical devices and computing functions, which significantly differs from the traditional electronic computing technology. On the other hand, optical computing technology offers the advantages such as fast speed, low energy consumption, and high parallelism. Yet there are still challenges such as device integration and portability. In the burgeoning development of micro-nano optics technology, especially the deeply ingrained concept of metasurface technique, it provides an advanced platform for optical computing applications, including edge detection, image or motion recognition, logic computation, and on-chip optical computing. With the aim of providing a comprehensive introduction and perspective for optical computing metasurface applications, we review the recent research advances of optical computing, from nanostructure and computing methods to practical applications. In this work, we review the challenges and analysis of optical computing metasurfaces in engineering field and look forward to the future development trends of optical computing.

High-Efficiency Multi-Channel Orbital Angular Momentum Multiplexing Enabled by the Angle-Dispersive Metasurface

Orbital angular momentum (OAM) multiplexing of electromagnetic (EM) waves is of great significance for high-speed wireless communication and remote sensing. To achieve high-efficiency OAM multiplexing for multi-channel incident EM waves, this paper presents a novel angle-dispersive meta-atom structure, which can introduce the required anti-symmetric phase dispersion as well as high transmission efficiency for OAM multiplexing. These meta-atoms are then arranged delicately to form an angle-dispersive metasurface working at the X band, which enables three-channel OAM multiplexing by converting highly directional transverse-magnetic (TM) waves incident from 0 and ±45° to coaxial OAM beams with l = 0 and ±2 modes, respectively. The simulation and experimental results reveal that the proposed metasurface can convert a higher proportion of energy to the required OAM modes compared to the conventional OAM multiplexing metasurfaces, which can significantly improve the coaxial transmission efficiency of multi-channel OAM multiplexing.

Off-axis metasurfaces for folded flat optics

The overall size of an optical system is limited by the volume of the components and the internal optical path length. To reach the limits of miniaturization, it is possible to reduce both component volume and path length by combining the concepts of metasurface flat optics and folded optics. In addition to their subwavelength component thickness, metasurfaces enable bending conventional folded geometries off axis beyond the law of reflection. However, designing metasurfaces for highly off-axis illumination with visible light in combination with a high numerical aperture is non-trivial. In this case, traditional designs with gradient metasurfaces exhibit low diffraction efficiencies and require the use of deep-subwavelength, high-index, and high-aspect-ratio semiconductor nanostructures that preclude inexpensive, large-area nanofabrication. Here, we describe a design approach that enables the use of low-index (n ≈ 1.5), low-aspect ratio structures for off-axis metagratings that can redirect and focus visible light (λ = 532 nm) with near-unity efficiency. We show that fabricated optical elements offer a very large angle-of-view (110°) and lend themselves to scalable fabrication by nano-imprint lithography.

Highly Tunable Cascaded Metasurfaces for Continuous Two-Dimensional Beam Steering

Cascaded metasurfaces can exhibit powerful dynamic light manipulation by mechanically tuning the far-field interactions in the layers. However, in most current designs, the metasurfaces are separated by gaps smaller than a wavelength to form a total phase profile, representing the direct accumulation of the phase profiles of each layer. Such small gap sizes may not only conflict with the far-field conditions but also pose great difficulties for practical implementations. To overcome this limitation, a design paradigm taking advantage of a ray-tracing scheme that allows the cascaded metasurfaces to operate optimally at easily achievable gap sizes is proposed. Enabled by the relative lateral translation of two cascaded metasurfaces, a continuous two-dimensional (2D) beam-steering device for 1064 nm light is designed as a proof of concept. Simulation results demonstrate tuning ranges of ±45° for biaxial deflection angles within ±3.5 mm biaxial translations, while keeping the divergence of deflected light less than 0.007°. The experimental results agree well with theoretical predictions, and a uniform optical efficiency is observed. The  generializeddesign paradigm can pave a way towards myriad tunable cascaded metasurface devices for various applications, including but not limited to light detection and ranging (LiDAR) and free space optical communication.

Recent advances in metasurface design and quantum optics applications with machine learning, physics-informed neural networks, and topology optimization methods

As a two-dimensional planar material with low depth profile, a metasurface can generate non-classical phase distributions for the transmitted and reflected electromagnetic waves at its interface. Thus, it offers more flexibility to control the wave front. A traditional metasurface design process mainly adopts the forward prediction algorithm, such as Finite Difference Time Domain, combined with manual parameter optimization. However, such methods are time-consuming, and it is difficult to keep the practical meta-atom spectrum being consistent with the ideal one. In addition, since the periodic boundary condition is used in the meta-atom design process, while the aperiodic condition is used in the array simulation, the coupling between neighboring meta-atoms leads to inevitable inaccuracy. In this review, representative intelligent methods for metasurface design are introduced and discussed, including machine learning, physics-information neural network, and topology optimization method. We elaborate on the principle of each approach, analyze their advantages and limitations, and discuss their potential applications. We also summarize recent advances in enabled metasurfaces for quantum optics applications. In short, this paper highlights a promising direction for intelligent metasurface designs and applications for future quantum optics research and serves as an up-to-date reference for researchers in the metasurface and metamaterial fields.

Eye accommodation-inspired neuro-metasurface focusing

The human eye, which relies on a flexible and controllable lens to focus light onto the retina, has inspired many scientific researchers to understand better and imitate the biological vision system. However, real-time environmental adaptability presents an enormous challenge for artificial eye-like focusing systems. Inspired by the mechanism of eye accommodation, we propose a supervised-evolving learning algorithm and design a neuro-metasurface focusing system. Driven by on-site learning, the system exhibits a rapid response to ever-changing incident waves and surrounding environments without any human intervention. Adaptive focusing is achieved in several scenarios with multiple incident wave sources and scattering obstacles. Our work demonstrates the unprecedented potential for real-time, fast, and complex electromagnetic (EM) wave manipulation for various purposes, such as achromatic, beam shaping, 6 G communication, and intelligent imaging.

Recent advanced applications of metasurfaces in multi-dimensions

Unlike traditional optical components, which rely on the gradual accumulation of light along the optical path over a distance much larger than the wavelength to form a wavefront, metasurfaces manipulate light field properties on the wavelength thickness by specially arranging various meta-atoms. Due to the ease of integration and compact planar structure, metasurfaces play a key role in the light field manipulations. Here, we review the recent advances of metasurfaces in multi-dimensions, including light wavelength, polarization, orbital angular momentum(OAM), and angular response. Progress in these fields has brought new applications in areas such as imaging, display, communication, and information encryption, etc. Finally, we also discuss the challenges and prospects of metasurfaces applications.

Photonic integrated beam delivery for a rubidium 3D magneto-optical trap

Cold atoms are important for precision atomic applications including timekeeping and sensing. The 3D magneto-optical trap (3D-MOT), used to produce cold atoms, will benefit from photonic integration to improve reliability and reduce size, weight, and cost. These traps require the delivery of multiple, large area, collimated laser beams to an atomic vacuum cell. Yet, to date, beam delivery using an integrated waveguide approach has remained elusive. Here we report the demonstration of a ⁸⁷Rb 3D-MOT using a fiber-coupled photonic integrated circuit to deliver all beams to cool and trap > 1 ×10⁶ atoms to near 200 μK. The silicon nitride photonic circuit transforms fiber-coupled 780 nm cooling and repump light via waveguides to three mm-width non-diverging free-space cooling and repump beams directly to the rubidium cell. This planar, CMOS foundry-compatible integrated beam delivery is compatible with other components, such as lasers and modulators, promising system-on-chip solutions for cold atom applications.

Plasmonic metasurface enhanced by nanobumps for label-free biosensing of lung tumor markers in serum

Plasmonic metasurface biosensing has excellent potential in label-free detection of tumor biomarkers. In general, a variety of plasmonic metasurface nanofabrication leads to various degree of metallic surface roughness. However, the metasurface roughness effects on plasmonic sensing of tumor markers have been barely reported. Here we fabricate high-roughness (HR) gold nanohole metasurfaces with nanobumps and investigate their biosensing in comparison with the low-roughness (LR) counterparts. The HR metasurfaces demonstrate the surface sensitivity of multilayer polyelectrolyte molecules, which is 57.0% higher than the LR ones. The HR metasurfaces also illuminate higher immunoassay sensitivity to multiple lung cancer biomarkers, including carcinoembryonic antigen, neuron-specific enolase and cytokeratin fragment 21-1. The highest increasement of tumor marker sensitivity is up to 71.4%. The biosensing enhancement is attributed to the introduction of gold nanobumps on metasurfaces, which provides more hot-spot regions, higher localized near-field intensity and better optical impedance matching. Furthermore, the biosensing of HR metasurfaces effectively covers the threshold values of tumor markers for early lung cancer diagnosis, and is used for the detection of clinical serum samples. The testing deviation is less than 4% compared with commercial immunoassay, which implies promising applications on medical examinations. Our research provides a scientific guide to surface roughness engineering for plasmonic metasensing in the future point-of-care testing.

Super-reflector enabled by non-interleaved spin-momentum-multiplexed metasurface

Electromagnetic wave multiplexing, especially for that occurring at different incidences (spatial-frequency multiplexing), is pivotal for ultrathin multifunctional interfaces and high-capacity information processing and communication. It is yet extremely challenging based on passive and compact wave elements, since the wave excitation and scattering channels are exclusively coupled through gradient phases and hence momentum matching condition at the interface. Here, we propose a spin-momentum multiplexed paradigm called a super-reflector enabling on-demand control of both retroreflections and anomalous reflections using a non-interleaved single-celled metasurface. By multiplexing four channels connecting two spin states excited onto each input of three spatial frequencies, a total of twelve channels are engineered, among which three are retroreflected channels and the residual are anomalous reflection ones. Our compound multiplexed super-reflector allows five degrees of freedom in circular polarization Jones' matrix, approaching the intrinsic upper limit of such planar metasurface. The concept has been experimentally verified by a proof-of-concept super-reflector at microwave frequency, showcasing twelve reflected beams and a high efficiency exceeding 90.6% defined as the ratio of reflected power to incidence for each channel beam. Our strategy opens a new avenue for angle multiplexing and angle-resolved metadevices toward the capacity limit of 2D planar Jones' matrix.

Simple route for high-throughput fabrication of metasurfaces using one-step UV-curable resin printing

Phase-gradient metasurfaces are two-dimensional (2D) optical elements that can manipulate light by imposing local, space-variant phase changes on an incident electromagnetic wave. These metasurfaces hold the potential and the promise to revolutionize photonics by providing ultrathin alternatives for a wide range of common optical elements such as bulky refractive optics, waveplates, polarizers, and axicons. However, the fabrication of state-of-the-art metasurfaces typically requires some time-consuming, expensive, and possibly hazardous processing steps. To overcome these limitations on conventional metasurface fabrication, a facile methodology to produce phase-gradient metasurfaces through one-step UV-curable resin printing is developed by our research group. The method dramatically reduces the required processing time and cost, as well as eliminates safety hazards. As a proof-of-concept, the advantages of the method are clearly demonstrated via a rapid reproduction of high-performance metalenses based on the Pancharatnam-Berry phase gradient concept in the visible spectrum.

Observation of full-parameter Jones matrix in bilayer metasurface

Metasurfaces, artificial 2D structures, have been widely used for the design of various functionalities in optics. Jones matrix, a 2×2 matrix with eight parameters, provides the most complete characterization of the metasurface structures in linear optics, and the number of free parameters (i.e., degrees of freedom, DOFs) in the Jones matrix determines the limit to what functionalities we can realize. Great efforts have been made to continuously expand the number of DOFs, and a maximal number of six has been achieved recently. However, the realization of the ultimate goal with eight DOFs (full free parameters) has been proven as a great challenge so far. Here, we show that by cascading two layer metasurfaces and utilizing the gradient descent optimization algorithm, a spatially varying Jones matrix with eight DOFs is constructed and verified numerically and experimentally in optical frequencies. Such ultimate control unlocks opportunities to design optical functionalities that are unattainable with previously known methodologies and may find wide potential applications in optical fields.

Thickness bound for nonlocal wide-field-of-view metalenses

Metalenses-flat lenses made with optical metasurfaces-promise to enable thinner, cheaper, and better imaging systems. Achieving a sufficient angular field of view (FOV) is crucial toward that goal and requires a tailored incident-angle-dependent response. Here, we show that there is an intrinsic trade-off between achieving a desired broad-angle response and reducing the thickness of the device. Like the memory effect in disordered media, this thickness bound originates from the Fourier transform duality between space and angle. One can write down the transmission matrix describing the desired angle-dependent response, convert it to the spatial basis where its degree of nonlocality can be quantified through a lateral spreading, and determine the minimal device thickness based on such a required lateral spreading. This approach is general. When applied to wide-FOV lenses, it predicts the minimal thickness as a function of the FOV, lens diameter, and numerical aperture. The bound is tight, as some inverse-designed multi-layer metasurfaces can approach the minimal thickness we found. This work offers guidance for the design of nonlocal metasurfaces, proposes a new framework for establishing bounds, and reveals the relation between angular diversity and spatial footprint in multi-channel systems.

Recent Progress in Reconfigurable and Intelligent Metasurfaces: A Comprehensive Review of Tuning Mechanisms, Hardware Designs, and Applications

Intelligent metasurfaces have gained significant importance in recent years due to their ability to dynamically manipulate electromagnetic (EM) waves. Their multifunctional characteristics, realized by incorporating active elements into the metasurface designs, have huge potential in numerous novel devices and exciting applications. In this article, recent progress in the field of intelligent metasurfaces are reviewed, focusing particularly on tuning mechanisms, hardware designs, and applications. Reconfigurable and programmable metasurfaces, classified as space gradient, time modulated, and space-time modulated metasurfaces, are discussed. Then, reconfigurable intelligent surfaces (RISs) that can alter their wireless environments, and are considered as a promising technology for sixth-generation communication networks, are explored. Next, the recent progress made in simultaneously transmitting and reflecting reconfigurable intelligent surfaces (STAR-RISs) that can achieve full-space EM wave control are summarized. Finally, the perspective on the challenges and future directions of intelligent metasurfaces are presented.

Mid-Infrared Broadband Achromatic Metalens with Wide Field of View

Metasurfaces have the ability to flexibly control the light wavefront, and they are expected to fill the gaps of traditional optics. However, various aberrations pose challenges for the application of metasurfaces in the wide angle and wide spectral ranges. The previous multi-aberration simultaneous optimization works had shortcomings such as large computational load, complex structure, and low generality. Here, we propose a metalens design method that corrects both monochromatic and chromatic aberrations simultaneously. The monochromatic aberration-corrected phase distribution is obtained by the optical design, and the chromatic aberration is reduced by using the original search algorithm combined with dispersion engineering. The designed single-layered wide-angle achromatic metalens has a balanced and efficient focusing effect in the mid-infrared band from 3.7 μm to 5 μm and a wide angle of ±30°. The design method proposed has the advantages of low computational load, wide application range, and easy experimental fabrication, which provides new inspiration for the development of generalized software for the design and optimization of metasurfaces.

Large-Scale Functionalized Metasurface-Based SARS-CoV-2 Detection and Quantification

Plasmonic metasurfaces consist of metal-dielectric interfaces that are excitable at background and leakage resonant modes. The sharp and plasmonic excitation profile of metal-free electrons on metasurfaces at the nanoscale can be used for practical applications in diverse fields, including optoelectronics, energy harvesting, and biosensing. Currently, Fano resonant metasurface fabrication processes for biosensor applications are costly, need clean room access, and involve limited small-scale surface areas that are not easy for accurate sample placement. Here, we leverage the large-scale active area with uniform surface patterns present on optical disc-based metasurfaces as a cost-effective method to excite asymmetric plasmonic modes, enabling tunable optical Fano resonance interfacing with a microfluidic channel for multiple target detection in the visible wavelength range. We engineered plasmonic metasurfaces for biosensing through efficient layer-by-layer surface functionalization toward real-time measurement of target binding at the molecular scale. Further, we demonstrated the quantitative detection of antibodies, proteins, and the whole viral particles of SARS-CoV-2 with a high sensitivity and specificity, even distinguishing it from similar RNA viruses such as influenza and MERS. This cost-effective plasmonic metasurface platform offers a small-scale light-manipulation system, presenting considerable potential for fast, real-time detection of SARS-CoV-2 and pathogens in resource-limited settings.

Metasurface on integrated photonic platform: from mode converters to machine learning

Integrated photonic circuits are created as a stable and small form factor analogue of fiber-based optical systems, from wavelength-division multiplication transceivers to more recent mode-division multiplexing components. Silicon nanowire waveguides guide the light in a way that single and few mode fibers define the direction of signal flow. Beyond communication tasks, on-chip cascaded interferometers and photonic meshes are also sought for optical computing and advanced signal processing technology. Here we review an alternative way of defining the light flow in the integrated photonic platform, using arrays of subwavelength meta-atoms or metalines for guiding the diffraction and interference of light. The integrated metasurface system mimics free-space optics, where on-chip analogues of basic optical components are developed with foundry compatible geometry, such as low-loss lens, spatial-light modulator, and other wavefront shapers. We discuss the role of metasurface in integrated photonic signal processing systems, introduce the design principles of such metasurface systems for low loss compact mode conversion, mathematical operation, diffractive optical systems for hyperspectral imaging, and tuning schemes of metasurface systems. Then we perceive reconfigurability schemes for metasurface framework, toward optical neural networks and analog photonic accelerators.

Nanostructured In3SbTe2 antennas enable switching from sharp dielectric to broad plasmonic resonances

Phase-change materials (PCMs) allow for non-volatile resonance tuning of nanophotonic components. Upon switching, they offer a large dielectric contrast between their amorphous and crystalline phases. The recently introduced "plasmonic PCM" In3SbTe2 (IST) additionally features in its crystalline phase a sign change of its permittivity over a broad infrared spectral range. While optical resonance switching in unpatterned IST thin films has been investigated before, nanostructured IST antennas have not been studied, yet. Here, we present numerical and experimental investigations of nanostructured IST rod and disk antennas. By crystallizing the IST with microsecond laser pulses, we switched individual antennas from narrow dielectric to broad plasmonic resonances. For the rod antennas, we demonstrated a resonance shift of up to 1.2 µm (twice the resonance width), allowing on/off switching of plasmonic resonances with a contrast ratio of 2.7. With the disk antennas, we realized an increase of the resonance width by more than 800% from 0.24 µm to 1.98 µm while keeping the resonance wavelength constant. Further, we demonstrated intermediate switching states by tuning the crystallization depth within the resonators. Our work empowers future design concepts for nanophotonic applications like active spectral filters, tunable absorbers, and switchable flat optics.

Coplanar Meta-Surface-Based Substrate-Integrated Waveguide Antennas with Broadband and Low Reflections for K-Band Beam Scanning

Four novel substrate-integrated waveguide (SIW) antennas are proposed, in order to obtain K-band beam scanning through the coplanar meta-surfaces of properly devised complementary split-ring resonators. More specifically, coplanar rhombus- and hexagon-shaped meta-surfaces replace the metallized via holes in the traditional SIW structure, achieving low reflection and wide bandwidth, respectively. Another trapezoid-shaped meta-surface is introduced, in order to realize good leaky-wave radiation performance with high-gain beam scanning in both rhombus- and hexagon-shaped SIW components. These designs are further extended to two different mixed types of two-row meta-surfaces, with the rhombus and hexagon structures combined in different orders to enhance the complex SIW transmission lines and antennas, which can simultaneously obtain good reflection and bandwidth with different priority, depending on the arrangement. We explain the performance differences with rhombus and hexagon meta-surfaces through the analysis of relevant equivalent circuit models and extracting the effective medium parameters, and we verify the bandwidths and radiations of four SIW antennas both numerically and experimentally. The maximum gains of the four antennas are 18.1 dBi, 17.0 dBi, 18.8 dBi and 17.1 dBi, where the corresponding relative bandwidths are 10.74%, 19.42%, 14.13% and 18.38%. The maximum simulated radiation efficiency and aperture efficiency of the proposed antennas are 91.20% and 61.12%, respectively. Our approach for generating flexible and selectable tuned electromagnetic fields from SIWs is applicable for the development of mm-Wave antennas or sensors on PCB-integrated platforms for highly directive scanning radiation.

Planar metal-only omnidirectional retroreflector using transmitarray and blazed grating for TE and TM polarizations

Retroreflector is a device that reflects light along its incident direction over a continuous range of incident angles. The retroreflectors proposed so far are so bulky and uneven that they cannot be mounted on large structures. Recently, two-layer planar retroreflectors have been proposed in the field of optics and acoustics, but at microwave frequencies only one retroreflector with non-planar overall structure has been presented that doesn't cover omnidirectional incidence and only supports TM polarization. The term omnidirectional means the capability of retroreflectivity in all azimuth angles (φ_i) and a wide elevation angle (θ_i) range. In this paper, a novel planar and metal-only retroreflector is proposed at microwave frequencies that efficiently reflects the electromagnetic wave along its incident direction in omnidirectional angle range and supports both TE and TM polarization. The proposed structure consists of two separate cascaded layers. The top layer is a transmitarray structure in which incident waves with different angles are focused at different locations on the bottom layer. The bottom layer reflects back focused waves along its focus direction and two distinct structures are proposed to implement it. The first type is a novel spatially varying blazed grating and the second type is a spatially varying phase gradient metasurface, as used in the optics and aucostics. The first type offers omnidirectional property for both TE and TM polarization while the second type offers higher monostatic RCS for only TE polarization. Simulated and measured results of the proposed retroreflector with the first type bottom layer show an omnidirectional retroreflectivity with half-power (3-dB RCS level) elevation field of view of 60^° (-30^°to 30^°) at frequency of 10 GHz with a normal incidence efficiency of 63%.

Wide-angle and high-efficiency flat retroreflector

We propose a flat retroreflector that can efficiently reflect the electromagnetic waves back along its incident direction in a wide continuous range of angles. This retroreflector consists of a quadratic metalens and a flat metallic reflector at the focal plane of the former. The quadratic metalens is a dielectric pillar array encoded with a quadratic phase profile and it is embedded in the top side of the substrate. The flat reflector is on the bottom side of the substrate. The designed retroreflector has a diameter of 40 mm, a thickness of 15 mm, and a working frequency of 77 GHz. Through meta-units optimization, a retroreflection efficiency of 38.51% at ± 60° incidence and an average retroreflection efficiency of 46.39% for the incident angles from 0° to 60° can be numerically demonstrated. This flat retroreflector is easy for integration, which is promising for potential applications in the miniature wireless communication systems.

Reflection Mechanism of Dielectric Corner Reflectors: The Role of the Diffraction of Evanescent Waves and the Goos-Hänchen Shift

Nano- and microstructures have been developed for asymmetric light transmission (ALT) filters operating in a wide wavelength range. One of the most straightforward structures with ALT properties is a dielectric corner reflector (DCR) comprising a one-dimensional grating of a triangular shape on one surface. The DCR possesses strong reflection only for one-way light illumination due to multiple total internal reflections (TIRs) inside the triangular grating. For triangular structures being much larger than the wavelength of light, the reflection properties are expected to be fully described by geometrical optics. However, geometrical optics do not account for the Goos-Hänchen (GH) shift, which is caused by the evanescent wave of the TIR. In this work, the reflection mechanism of DCRs is elucidated using the finite element method and a quantitative model built by considering the GH shift. The reduction in reflection of the DCR is dominated by diffraction of the evanescent wave at the corner of the triangular structure. Our model is based on simple mathematics and can optimize the DCR geometry for applications addressing a wide wavelength range such as radiative cooling.

Planar, wide-band omnidirectional retroreflector using metal-only transmitarray structure for TE and TM polarizations

In this paper, a novel planar and wideband metal-only retroreflector is proposed that efficiently reflects the obliquely incident electromagnetic wave along its incident direction in omnidirectional angle range. The means of omnidirectional is the capability of retroreflectivity in all azimuth angles \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$(\varphi_{i} )$$\end{document}(φi) and in a wide elevation angle \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$(\theta_{i} )$$\end{document}(θi) range. The proposed structure consists of a symmetrical transmitarray structure with beam scanning capability in which a metal plate is placed instead of the feed. The transmitarray is designed by using the generalized multifocal approach in which the beam scanning capability is possible via feed displacement and the phase of the elements has azimuthal symmetry that causes omnidirectional property. Now by placing a metal plate at the location of feed displacement and provided that the focused waves are nearly perpendicular to the metal plate, the plane wave is reflected back along its incident direction i.e. the retroreflectivity property is obtained. The proposed retroreflector simulated by CST STUDIO software and fabricated with laser cut technology. The results of measurement show an omnidirectional retroreflectivity with half-power (3-dB RCS level) elevation field of view of 60° (− 30° to 30°) in the frequency range of 8.5–10 GHz (approximately 17%) for both TE and TM polarization.

Constant polarization generation metasurface for arbitrarily polarized light

State of polarization (SoP) of light is one of the fundamental characteristics of light and has great significance to optical communication, imaging, quantum optics and medical facilities. The generation and maintenance of polarized light have always been research concerns in polarization optics. Polarization-maintaining fibers are frequently used to transmit polarized light without changing its polarization in optical systems, but the high cost and coupling efficiency problems hinder their usage in large-scale light paths. Polarization controllers, which operate arbitrary polarization generation and conversion at the expense of utilizing at least two optical elements such as a half-wave plate and quarter-wave plate, are too bulky for some special applications. Meanwhile, they can only generate desired output polarization of light by transcendentally determining the input polarization, which means that the existing polarization controllers cannot respond in real time. Metasurfaces composed of subwavelength nanoscatterers offer fruitful functionalities to manipulate the amplitude, phase and polarization of light. Here, we propose and experimentally demonstrate a real-time polarization controller realized by combining a depolarizer and polarizer into one monolithic metasurface. Arbitrary polarization states can be transferred to the required polarization with no requirement to determine the incident polarization in advance. Through combining with ordinary optical fibers, the proposed metasurface may also replace polarization-maintaining fibers and optical fiber polarizers in some polarization-dependent applications. This versatile concept may settle the problems of arbitrary polarization conversion once and for all.

Recent Advancement in Optical Metasurface: Fundament to Application

Metasurfaces have gained growing interest in recent years due to their simplicity in manufacturing and lower insertion losses. Meanwhile, they can provide unprecedented control over the spatial distribution of transmitted and reflected optical fields in a compact form. The metasurfaces are a kind of planar array of resonant subwavelength components that, depending on the intended optical wavefronts to be sculpted, can be strictly periodic or quasi-periodic, or even aperiodic. For instance, gradient metasurfaces, a subtype of metasurfaces, are designed to exhibit spatially changing optical responses, which result in spatially varying amplitudes of scattered fields and the associated polarization of these fields. This paper starts off by presenting concepts of anomalous reflection and refraction, followed by a brief discussion on the Pancharatanm-Berry Phase (PB) and Huygens' metasurfaces. As an introduction to wavefront manipulation, we next present their key applications. These include planar metalens, cascaded meta-systems, tunable metasurfaces, spectrometer retroreflectors, vortex beams, and holography. The review concludes with a summary, preceded by a perspective outlining our expectations for potential future research work and applications.

Pixel-level Bayer-type colour router based on metasurfaces

The three primary colour model, i.e., red, green, and blue model, based on the colour perception of the human eye, has been widely used in colour imaging. The most common approach for obtaining colour information is to use a Bayer colour filter, which filters colour light with four pixels of an imaging sensor to form an effective colour pixel. However, its energy utilization efficiency and colour collection efficiency are limited to a low level due to the three-channel filtering nature. Here, by employing an inverse-design method, we demonstrate a pixel-level metasurface-based Bayer-type colour router that presents peak colour collection efficiencies of 58%, 59%, and 49% for red, green and blue light, and an average energy utilization efficiency as high as 84% over the visible region (400 nm–700 nm), which is twice as high as that of a commercial Bayer colour filter. Furthermore, by using a 200 µm × 200 µm metasurface-based colour router sample working with a monochromatic imaging sensor, colour imaging is further realized, obtaining an image intensity twice that achieved by a commercial Bayer colour filter. Our work innovates the mechanism of high-efficiency spectrum information acquisition, which is expected to have promising applications in the development of next-generation imaging systems.

Though metasurface-based 3D colour routers are attractive for next-generation colour imaging systems, their complex fabrication and poor performance remain a bottleneck. Here, the authors use the inverse-design method to realize high performance metasurface-based Bayer-type colour routers.

Dual-polarization multi-angle retroreflective metasurface with bilateral transmission windows

Metasurfaces have provided unprecedented degrees of freedom in manipulating electromagnetic (EM) waves and also granted high possibility of integrating multiple functions into one single meta-device. In this paper, we propose to incorporate the retroreflection function with transmission function by means of metasurface design and then demonstrate a dual-polarization multi-angle retroreflective metasurface (DMRM) with bilateral transmission bands. To achieve high-efficiency retroreflections, the compact bend structures (CBSs), which exhibit high reflections around 10.0 GHz in X band, are added onto the substrate of the DMRM. Two selected metasurface elements are periodically arranged so as to form 0-π-0 phase profile. By delicately adjusting the periodicity, high-efficiency retroreflections can be produced for both TE and TM-polarized waves under both vertical incidence and oblique incident angles ±50.0°, with an average efficiency of 90.2% at the designed frequency. Meanwhile, the two metasurface elements exhibit high transmission properties and minor phase disparities in S, C and Ku bands, resulting in bilateral transmission windows. Prototypes were designed and fabricated. Both simulated and measured results verified our design. This work provides an effective means of integrating retroreflection functions with other functions and may find applications in target tracking, radomes and other sensor integrated devices in higher frequency or even optical frequency bands.

Ultrathin All-Angle Hyperbolic Metasurface Retroreflectors Based on Directed Routing of Canalized Plasmonics

Retroreflectors that can accurately redirect the incident waves in free space back along their original channels provide unprecedented opportunities for light manipulation. However, to the best of our knowledge, they suffer from either the bulky size, narrow angular bandwidths, or time-consuming postprocessing, which essentially limits their further applications. Here, a scheme for designing ultrathin all-angle real-time retroreflectors based on hyperbolic plasmonic metasurfaces is proposed and experimentally demonstrated. The physical mechanism underlying the scheme is the orthogonality between the traveling waves in free space and the canalized spoof surface plasmon on the hyperbolic plasmonic metasurfaces, which guarantee their high-efficiency and all-angle mutual conversion. In this case, the strong confinement characteristic that benefited from the enhanced light-matter interaction enables us to route and retroreflect the canalized spoof surface plasmon with extremely thin structures. As proof of the scheme, a retroreflector prototype with a thickness approximately equal to the central wavelength is designed and fabricated. Further experimental investigation obtains a half-power field of view up to 53° and a maximum efficiency of 83.2%. This scheme can find promising applications in target detection, remote sensing, and diverse on-chip light control devices.

Enhanced lighting on the road surface with TIR-PA optical element on tunnel sidewall

Both the lighting quality and driving safety inside the tunnel have been quite a concern around the world. In this study, a plate with prism array of total internal reflection (TIR-PA) has been designed to enhance the lighting on the road surface inside the tunnel in adherence to traffic safety as well as energy savings. Such optical elements, above the tunnel sidewall, can redirect the rays emitted from both the headlight and lighting devices to the road surface, which supports higher flux on the road. Simultaneously, the simulation is conducted with consideration of practical factors based on the mathematical model and edge-ray principle to attain the effect of the TIR-PA plates. On the one hand, for the rays from the lighting devices, the enhancement rate of illuminance has reached 12.60%-16.58%, which is attributed to TIR-PA plates above the different sidewalls of the Lambertian scattering property. TIR-PA can be suitable for various sidewall materials due to the small effect of illumination reduction. On the other hand, 30.2%-32.2% rays of the headlight source received by the sidewall, due to the effect of TIR-PA plates, are redirected to the driving distance for 27.3-71.9 m, which makes 15.9-31.1m² of road surface brighter. It makes contributions to the improvement of illuminance distribution and drivers' pre-judgment. In the future, broader applications regarding the TIR-PA plates will be explored widely in tunnel lighting systems due to the low production cost.

Millimeter-scale focal length tuning with MEMS-integrated meta-optics employing high-throughput fabrication

Miniature varifocal lenses are crucial for many applications requiring compact optical systems. Here, utilizing electro-mechanically actuated 0.5-mm aperture infrared Alvarez meta-optics, we demonstrate 3.1 mm (200 diopters) focal length tuning with an actuation voltage below 40 V. This constitutes the largest focal length tuning in any low-power electro-mechanically actuated meta-optic, enabled by the high energy density in comb-drive actuators producing large displacements at relatively low voltage. The demonstrated device is produced by a novel nanofabrication process that accommodates meta-optics with a larger aperture and has improved alignment between meta-optics via flip-chip bonding. The whole fabrication process is CMOS compatible and amenable to high-throughput manufacturing.

Perfect anomalous reflectors at optical frequencies

Reflecting light to a predetermined nonspecular direction is an important ability of metasurfaces, which is the basis for a wide range of applications (e.g., beam steering/splitting and imaging). However, anomalous reflection with 100% efficiency has not been achieved at optical frequencies yet, because of losses and/or insufficient nonlocal control of light waves. Here, we propose an all-dielectric quasi-three-dimensional subwavelength structure, consisting of multilayer films and metagratings, to achieve perfect anomalous reflections at optical frequencies. A complex multiple scattering process was stimulated by effectively coupling different Bloch waves and propagating waves, thus offering the metasystem the desired nonlocal control on light waves required by perfect anomalous reflections. Two perfect anomalous reflectors were demonstrated to reflect normally incident 1550-nm light to the 40°/75° directions with absolute efficiencies of 99%/99% in design (98%/88% in experiment). Our results pave the way toward realizing optical metadevices with desired high efficiencies in realistic applications.

Active optical metasurfaces: comprehensive review on physics, mechanisms, and prospective applications

Optical metasurfaces with subwavelength thickness hold considerable promise for future advances in fundamental optics and novel optical applications due to their unprecedented ability to control the phase, amplitude, and polarization of transmitted, reflected, and diffracted light. Introducing active functionalities to optical metasurfaces is an essential step to the development of next-generation flat optical components and devices. During the last few years, many attempts have been made to develop tunable optical metasurfaces with dynamic control of optical properties (e.g., amplitude, phase, polarization, spatial/spectral/temporal responses) and early-stage device functions (e.g., beam steering, tunable focusing, tunable color filters/absorber, dynamic hologram, etc) based on a variety of novel active materials and tunable mechanisms. These recently-developed active metasurfaces show significant promise for practical applications, but significant challenges still remain. In this review, a comprehensive overview of recently-reported tunable metasurfaces is provided which focuses on the ten major tunable metasurface mechanisms. For each type of mechanism, the performance metrics on the reported tunable metasurface are outlined, and the capabilities/limitations of each mechanism and its potential for various photonic applications are compared and summarized. This review concludes with discussion of several prospective applications, emerging technologies, and research directions based on the use of tunable optical metasurfaces. We anticipate significant new advances when the tunable mechanisms are further developed in the coming years.

Chemical Vapor Deposition of Spherical Amorphous Selenium Mie Resonators for Infrared Meta-Optics

Applying direct growth and deposition of optical surfaces holds great promise for the advancement of future nanophotonic technologies. Here, we report on a chemical vapor deposition (CVD) technique for depositing amorphous selenium (a-Se) spheres by desorption of selenium from Bi₂Se₃ and re-adsorption on the substrate. We utilize this process to grow scalable, large area Se spheres on several substrates and characterize their Mie-resonant response in the mid-infrared (MIR) spectral range. We demonstrate size-tunable Mie resonances spanning the 2-16 μm spectral range for single isolated resonators and large area ensembles. We further demonstrate strong absorption dips of up to 90% in ensembles of particles in a broad MIR range. Finally, we show that ultra-high-Q resonances arise in the case where Se Mie-resonators are coupled to low-loss epsilon-near-zero (ENZ) substrates. These findings demonstrate the enabling potential of amorphous Selenium as a versatile and tunable nanophotonic material that may open up avenues for on-chip MIR spectroscopy, chemical sensing, spectral imaging, and large area metasurface fabrication.