Metasurface-driven full-space structured light for three-dimensional imaging

Engineering Circular Dichroism in the Nonlinear Optical Region with a Refractory Plasmonic Molybdenum Metasurface

Chiral plasmonic metasurfaces exhibit circular dichroism (CD) and serve as a key tool for chiroptics. However, in the linear optical regime, the CD value remains fixed. Moreover, conventional chiral plasmonic metasurfaces suffer from poor photothermal stability, making it difficult to excite their nonlinear optical properties. Here, we present a refractory plasmonic molybdenum (Mo) metasurface that not only maintains strong chiroptical effects under extreme conditions, withstanding temperatures up to 1100 °C and laser intensities exceeding 12 GW/cm2, but also realizes intensity-dependent tunable CD in the nonlinear optical region due to chiral saturated absorption and chiral reverse saturable absorption. In addition, our metasurface exhibits giant third-harmonic generation. Finally, we demonstrate a proof-of-concept circularly polarized light limiter based on this refractory chiral Mo metasurface.

36-Channel Spin and Wavelength Co-Multiplexed Metasurface Holography by Phase-Gradient Inverse Design

Metasurface holography has emerged as a versatile tool for manipulating light at subwavelength scales, offering enhanced capabilities in multiplexing high-resolution holographic images. However, the scalability of channel multiplexing remains a significant challenge. In this paper, a high-capacity single-cell metasurface is presented capable of maximizing channels by multiplexing holographic images across both spin and wavelength using a single-phase map. The achievement of simultaneous multiplexing of left- and right-circular polarization states is detailed across a broad spectral range, from visible to near-infrared wavelengths, by using a single-cell metasurface, optimized through an inverse design to minimize loss between the target and output images by automatic differentiation. The phase profile is optimized to encode multiple holographic images without requiring complex meta-atoms, thereby reducing the fabrication complexity while maintaining high performance. Using this method, two metasurface implementations are demonstrated, an 8-channel hologram covering both the visible and near-infrared regions and a 36-channel hologram operating in the full-visible spectrum across 18 wavelengths separated by 20-nm intervals. Furthermore, noise-related loss functions are incorporated into the optimization process to suppress background noise and minimize inter-channel crosstalk, resulting in significantly improved image quality and fidelity. This approach offers a reliable solution for further photonic applications such as displays, optical data storage, and information encryption.

Experimental realization of orbital-angular-momentum-selective coherent perfect absorption

Coherent perfect absorption (CPA) dependent on orbital angular momentum (OAM) has been proposed theoretically but so far has lacked experimental validation. Here, we design an OAM-selective coherent perfect absorber and experimentally validate this intriguing phenomenon. By combining a spiral phase plate, a polarization-conversion metasurface, and a conical cavity, an incident plane wave of linear polarization is converted into cylindrical waves with different OAMs in two dimensions, which impinges on a dual-layer cylindrical absorber composed of a ceramic core and an indium tin oxide (ITO) coating. Experimental results, coinciding well with numerical simulations, confirm the OAM-selective CPA for different OAMs. Our findings offer a practical approach for electromagnetic energy harvesting and sensing at subwavelength dimensions.

Nanofabrication for Nanophotonics

Nanofabrication, a pivotal technology at the intersection of nanoscale engineering and high-resolution patterning, has substantially advanced over recent decades. This technology enables the creation of nanopatterns on substrates crucial for developing nanophotonic devices and other applications in diverse fields including electronics and biosciences. Here, this mega-review comprehensively explores various facets of nanofabrication focusing on its application in nanophotonics. It delves into high-resolution techniques like focused ion beam and electron beam lithography, methods for 3D complex structure fabrication, scalable manufacturing approaches, and material compatibility considerations. Special attention is given to emerging trends such as the utilization of two-photon lithography for 3D structures and advanced materials like phase change substances and 2D materials with excitonic properties. By highlighting these advancements, the review aims to provide insights into the ongoing evolution of nanofabrication, encouraging further research and application in creating functional nanostructures. This work encapsulates critical developments and future perspectives, offering a detailed narrative on the state-of-the-art in nanofabrication tailored for both new researchers and seasoned experts in the field.

Baseline-free structured light 3D imaging using a metasurface double-helix dot projector

Structured light is a widely used 3D imaging method with a drawback that it typically requires a long baseline length between the laser projector and the camera sensor, which hinders its utilization in space-constrained scenarios. On the other hand, the application of passive 3D imaging methods, such as depth from depth-dependent point spread functions (PSFs), is impeded by the challenge in measuring textureless scenes. Here, we combine the advantages of both structured light and depth-dependent PSFs and propose a baseline-free structured light 3D imaging system. A metasurface is designed to project a structured dot array and encode depth information in the double-helix pattern of each dot simultaneously. Combined with a straightforward and fast algorithm, we demonstrate accurate 3D point cloud acquisition for various real-world scenarios including multiple cardboard boxes and a living human face. Such a technique may find application in a broad range of areas including consumer electronics and precision metrology.

A perspective on plasmonic metasurfaces: unlocking new horizons for sensing applications

Metasurfaces (MSs), two-dimensional arrays of engineered nanostructures, have revolutionized optics by enabling precise manipulation of electromagnetic waves at subwavelength scales. These platforms offer unparalleled control over amplitude, phase, and polarization, unlocking advanced applications in imaging, communication, and sensing. Among them, plasmonic MSs stand out for their ability to exploit surface plasmon resonances (SPRs)-collective electron oscillations at metal-dielectric interfaces. This phenomenon enables extreme light confinement and field enhancement, leading to highly efficient light-matter interactions. The remarkable sensitivity of SPR to refractive index variations makes plasmonic MSs ideal for detecting minute biochemical and environmental changes with exceptional precision. Additionally, their tunable SPR characteristics enhance multifunctionality, enabling adaptive and real-time sensing. By leveraging these advantages, plasmonic MSs address critical challenges in modern sensing, driving breakthroughs in biomedical diagnostics, environmental monitoring, and chemical detection. This perspective explores recent advancements in plasmonic MSs, emphasizing flexible, multifunctional designs and the transformative role of artificial intelligence in optimizing performance and enabling real-time data analysis.

Tape-Assisted Residual Layer-Free One-Step Nanoimprinting of High-Index Hybrid Polymer for Optical Loss-Suppressed Metasurfaces

The commercialization of metasurfaces is crucial for real-world applications such as wearable sensors, pigment-free color pixels, and augmented and virtual reality devices. Nanoparticle-embedded resin-based nanoimprint lithography (PER-NIL) has shown itself to be a low-cost, high-throughput manufacturing method enabling the replication of high-index nanostructures. It has been extensively integrated into the fabrication of hologram metasurfaces, metalenses, and sensors due to its procedural simplicity. Most research on PER-NIL has been limited to exploring appropriate materials to enhance the efficiency of imprinted metasurfaces, but the intrinsic issue of PER-NIL lies in the high-index residual layer remaining on the substrate. This high-index residual layer generates undesired noise, limiting the efficiency and functionality of imprinted metasurfaces. Despite the need for the removal of the residual layer, it has never been experimentally achieved owing to the different etching rates between the nanoparticles and resin. Here, a new methodology named tape-assisted PER-NIL is proposed, achieving one-step removal of the residual layer using a tape. This novel method enables the replication of residual layer-free, high-index metasurfaces. As a result, imprinted residual layer-free metasurfaces prove their potential in high-purity dielectric structural colorations by achieving a sharp reflectance peak unattainable with conventional NIL, and in vivid hologram metasurfaces by covering a full 2π phase without unwanted scattering.

Curved geometric-phase optical element fabrication using top-down alignment

Advanced optical technologies, such as next-generation displays and holographic systems, demand high efficiency, lightweight designs, compact dimensions, and compatibility with curved and thin substrates. However, current optical devices for virtual and augmented reality displays, such as surface relief and holographic gratings, face challenges like light scattering, low optical efficiency, and limited scalability. Here, we present a geometric phase optical element (GPOE) fabricated using geometric-phase modulation. A pixelated nano grating system was employed, and reactive mesogens were aligned and transferred via a top-down imprinting process. The resulting GPOE exhibits a thin profile, compatibility with curved surfaces, and scalability for mass production. Integrating additional optical components, we realized a multi-focal GPOE and validated its optical performance. This innovation highlights the potential of GPOEs as compact, next-generation optical components for advanced curved-surface systems.

Polarization-controlled metasurface for simultaneous holographic display and three-dimensional depth perception

Simultaneous optical display and depth perception are crucial in many intelligent technologies but are usually realized by separate bulky systems unfriendly to integration. Metasurfaces, artificial two-dimensional optical surfaces with strong light-matter interaction capabilities at deep subwavelength scales, offer a promising approach for manufacturing highly integrated optical devices performing various complex functions. In this work, we report a polarization-multiplexed metasurface that can functionally switch between holographic display and Dammann gratings. By tailoring the incidence polarization, the metasurface can display high-quality holographic images in the Fresnel region or project a uniform spot cloud nearly covering the entire 180° × 180° transmissive space. For the latter, a projection and three-dimensional (3D) reconstruction experiment is conducted to elaborate the potential in retrieving 3D complex spatial information. The current results provide a prominent way to manufacture lightweight and highly-integrated comprehensive imaging systems especially vital for cutting-edge intelligent visual technologies.

The Road to Commercializing Optical Metasurfaces: Current Challenges and Future Directions

Optical metasurfaces, components composed of artificial nanostructures, are recognized for pushing boundaries of wavefront manipulation while maintaining a lightweight, compact design that surpasses conventional optics. Such advantages align with the current trends in optical systems, which demand compact communication devices and immersive holographic projectors, driving significant investment from the industry. Although interest in commercialization of optical metasurfaces has steadily grown since the initial breakthrough with diffraction-limited focusing, their practical applications have remained limited by challenges such as, massive-production yield, absence of standardized evaluation methods, and constrained design methodology. Here, this Perspective addresses the challenges in commercialization of optical metasurfaces, particularly focused on mass production, fabrication tolerance, performance evaluation, and integration into commercial systems. Additionally, we select the fields where metasurfaces may soon play significant roles and provide a perspective on their potentials. By addressing the challenges and exploring the solutions, this Perspective aims to foster discussions that will accelerate the utilization of optical metasurfaces and further build near-future metaphotonics platforms.

GaSb-based mid-wave computational multispectral infrared detectors based on photonic crystal plates

The mid-wave multispectral detector combines the traditional spectrometer and infrared detector technologies to provide image information and spectral information at the same time, which has an important role in both civil and military fields. To solve the working band limitation and low energy utilization, this paper presents an integrated superlattice mid-wave multispectral hypersurface detector that can be used for computational multispectroscopy for the first time, which consists of photonic crystal (PC) plates of GaSb material, and uses PC microstructures to modulate the incident spectra, which can be used to reconstruct incident signals with computational multispectroscopy methods. In this paper, the finite difference time domain method (FDTD) is used to simulate the structural parameters of different PCs, and finally calculate the correlation coefficients of the transmission spectra of the different structures as well as the energy utilization rate. The simulation results show that the optimized structures have unique response curves and rich spectral features, with the average value of Pearson correlation coefficients (PCCs) < 0.3 and energy utilization >50%. It can be utilized in various fields, including astronomical remote sensing, medical diagnostics, and military reconnaissance.

GaAs gradient metasurface for vortex beam emission with high deflection angles

This study experimentally demonstrates metasurfaces capable of producing output emissions with deflection angles greater than 75 degrees. These metasurfaces are composed of high-aspect-ratio gallium arsenide (GaAs) nano-resonators on double-sided polished GaAs substrates, operating reflectively at a wavelength of 650 nm. Beyond deflecting the incident light beam, the metasurfaces are successfully shown to generate high-deflection-angle, doughnut-shaped emissions by incorporating vortex beam (VB) structured light with a topological charge of up to 8. The broadband capabilities of these metasurfaces are also explored in this work. This work significantly advances the manipulation of structured light. Achieving high deflection angles opens new pathways for developing compact optical systems that leverage the unique properties of vortex beams in a range of applications.

Anti-aliased metasurfaces beyond the Nyquist limit

Sampling is a pivotal element in the design of metasurfaces, enabling a broad spectrum of applications. Despite its flexibility, sampling can result in reduced efficiency and unintended diffractions, which are more pronounced at high numerical aperture or shorter wavelengths, e.g. ultraviolet spectrum. Prevailing metasurface research has often relied on the conventional Nyquist sampling theorem to assess sampling appropriateness, however, our findings reveal that the Nyquist criterion is insufficient guidance for sampling in metasurface. Specifically, we find that the performance of a metasurface is significantly correlated to the geometric relationship between the spectrum morphology and sampling lattice. Based on lattice-based diffraction analysis, we demonstrate several anti-aliasing strategies from visible to ultraviolet regimes. These approaches significantly reduce aliasing phenomena occurring in high numerical aperture metasurfaces. Our findings not only deepen the understanding in phase gradient metasurface but also pave the way for high numerical aperture operation down to the ultraviolet spectrum.

Inverse design of metalenses with polarization and chromatic dispersion modulation via transfer learning

Polarization and wavelength multiplexed metalenses address the bulkiness of traditional imaging systems. However, despite progress with numerical simulations and parameter scanning, the engineering complexity of classical methods highlights the urgent need for efficient deep learning approaches. This paper introduces a deep learning-driven inverse design model for polarization-multiplexed metalenses, employing propagation phase theory alongside spectral transfer learning to address chromatic dispersion challenges. The model facilitates the rapid design of metalenses with off-axis and dual-focus capabilities within a single wavelength. Numerical simulations reveal a focal length deviation of less than 5% and an average focusing efficiency of 43.3%. The integration of spectral transfer learning streamlines the design process, enabling multifunctional metalenses with enhanced full-color imaging and displacement measurement, thus advancing the field of metasurfaces.

Amorphous to Crystalline Transition in Nanoimprinted Sol-Gel Titanium Oxide Metasurfaces

Crystalline titanium dioxide (TiO2) (anatase and rutile) possesses a higher refractive index than amorphous TiO2 and near-zero absorption in the visible region, making them an ideal material for visible metasurfaces. However, fabrication limitations hinder their implementation into flat optics. In this work, a wafer-scale manufacturing platform is proposed for crystalline TiO2 metasurfaces. Sol-gel TiO2 is developed as a printable material in which its material phase can be precisely controlled to produce amorphous, anatase, or rutile, depending on the crystallization temperature. Therefore, anatase or rutile metalenses can be fabricated on a wafer scale using thermal nanoimprint lithography and sintering process. The high refractive index of the crystalline TiO2 contributes to the enhanced conversion efficiency of the fabricated metalenses. The fabricated metalenses exhibit diffraction-limited focusing and imaging capabilities, comparable to the theoretically ideal lenses.

Fluid-responsive tunable metasurfaces for high-fidelity optical wireless communication

Optical wireless communication (OWC), with its blazing data transfer speed and unparalleled security, is a futuristic technology for wireless connectivity. Despite the significant advancements in OWC, the realization of tunable devices for on-demand and versatile connectivity still needs to be explored. This presents a considerable limitation in utilizing adaptive technologies to improve signal directivity and optimize data transfer. This study proposes a unique platform that utilizes tunable, fluid-responsive multifunctional metasurfaces offering dynamic and unprecedented control over electromagnetic wave manipulation to enhance the performance of OWC networks. We have achieved real-time, on-demand beam steering with vary-focusing capability by integrating the fabricated metasurfaces with different isotropic fluids. Furthermore, the designed metasurfaces are capable of polarization-based switching of the diffracted light beams to enhance overall productivity. Our research has showcased the potential of fluid-responsive tunable metasurfaces in revolutionizing OWC networks by significantly improving transmission reliability and signal quality through real-time adjustments. The proposed methodology is verified by designing and fabricating an all-dielectric metasurface measuring 500 μm × 500 μm and experimentally investigating its fluid-responsive vary-focal capability. By incorporating fluid-responsive properties into spin-decoupled metasurfaces, we aim to develop advanced high-tech optical devices and systems to simplify beam-steering and improve performance, adaptability, and functionality, making the devices suitable for various practical applications.

Optical skyrmions from metafibers with subwavelength features

Optical skyrmions are an emerging class of structured light with sophisticated particle-like topologies with great potential for revolutionizing modern informatics. However, the current generation of optical skyrmions involves complex or bulky systems, hindering the development of practical applications. Here, exploiting the emergent "lab-on-fiber" technology, we demonstrate the design of a metafiber-integrated photonic skyrmion generator. We not only successfully generate high-quality optical skyrmions from metafibers, but also verify their remarkable properties, such as topology switchability and topology stability with subwavelength polarization features beyond the diffraction limits. Our flexible fiber-integrated optical skyrmions platform paves the avenue for future applications of topologically-enhanced remote super-resolution microscopy and robust information transfer.

Advances in LiDAR Hardware Technology: Focus on Elastic LiDAR for Solid Target Scanning

This paper explores the development of elastic LiDAR technology, focusing specifically on key components relevant to solid target scanning applications. By analyzing its fundamentals and working mechanisms, the advantages of elastic LiDAR for precise measurement and environmental sensing are demonstrated. This paper emphasizes innovative advances in emitters and scanning systems, and examines the impact of optical design on performance and cost. Various ranging methods are discussed. Practical application cases of elastic LiDAR are presented, and future trends and challenges are explored. The purpose of this paper is to provide a comprehensive perspective on the technical details of elastic LiDAR, the current state of application, and future directions. All instances of "LiDAR" in this paper specifically refer to elastic LiDAR.

Metasurface-enabled 3D imaging via local bright spot gray scale matching using the structured light dot array

Three-dimensional (3D) imaging is widely utilized in various applications, such as light detection, autonomous vehicles, and machine vision. However, conventional 3D imaging systems often rely on bulky optical components. Metasurfaces, as next-generation optical devices, possess flexible wavefront modulation capabilities and excellent combination with computer vision algorithms. Here, we propose a large field-of-view (FOV) structured light dot array projection device based on a metasurface, covering a 2π-FOV, for projecting coded point clouds in Fourier space. We explore a local bright spot gray scale matching algorithm for depth extraction, enabling 3D imaging. This algorithm simplifies the data processing flow and optimizes depth extraction and feature matching processes through a customized region gray scale comparison. As a result, it effectively reduces computational complexity and enhances tolerance to image quality fluctuations. The proposed approach provides new possibilities for developing compact and high-performance planar 3D optical imaging devices, which will drive the advancement of fields such as computer vision and artificial intelligence.

Recording and Revealing 2.5D Nanopatterned Hidden Information on Silk Protein Bioresists

Nanopatterning on biomaterials has attracted significant attention as it can lead to the development of biomedical devices capable of performing diagnostic and therapeutic functions while being biocompatible. Among various nanopatterning techniques, electron-beam lithography (EBL) enables precise and versatile nanopatterning in desired shapes. Various biomaterials are successfully nanopatterned as bioresists by using EBL. However, the use of high-energy electron beams (e-beams) for high-resolutive patterning has incorporated functional materials and has caused adverse effects on biomaterials. Moreover, the scattering of electrons not absorbed by the bioresist leads to proximity effects, thus deteriorating pattern quality. Herein, EBL-based nanopatterning is reported by inducing molecular degradation of amorphous silk fibroin, followed by selectively inducing secondary structures. High-resolution EBL nanopatterning is achievable, even at low-energy e-beam (5 keV) and low doses, as it minimizes the proximity effect and enables precise 2.5D nanopatterning via grayscale lithography. Additionally, integrating nanophotonic structures into fluorescent material-containing silk allows for fluorescence amplification. Furthermore, this post-exposure cross-linking way indicates that the silk bioresist can maintain nanopatterned information stored in silk molecules in the amorphous state, utilizing for the secure storage of nanopatterned information as a security patch. Based on the fabrication technique, versatile biomaterial-based nanodevices for biomedical applications can be envisioned.

Integrated optical probing scheme enabled by localized-interference metasurface for chip-scale atomic magnetometer

Emerging miniaturized atomic sensors such as optically pumped magnetometers (OPMs) have attracted widespread interest due to their application in high-spatial-resolution biomagnetism imaging. While optical probing systems in conventional OPMs require bulk optical devices including linear polarizers and lenses for polarization conversion and wavefront shaping, which are challenging for chip-scale integration. In this study, an integrated optical probing scheme based on localized-interference metasurface for chip-scale OPM is developed. Our monolithic metasurface allows tailorable linear polarization conversion and wavefront manipulation. Two silicon-based metasurfaces namely meta-polarizer and meta-polarizer-lens are fabricated and characterized, with maximum transmission efficiency and extinction ratio (ER) of 86.29 % and 14.2 dB for the meta-polarizer as well as focusing efficiency and ER of 72.79 % and 6.4 dB for the meta-polarizer-lens, respectively. A miniaturized vapor cell with 4 × 4 × 4 mm3 dimension containing 87Rb and N2 is combined with the meta-polarizer to construct a compact zero-field resonance OPM for proof of concept. The sensitivity of this sensor reaches approximately 9 fT/Hz1/2 with a dynamic range near zero magnetic field of about ±2.3 nT. This study provides a promising solution for chip-scale optical probing, which holds potential for the development of chip-integrated OPMs as well as other advanced atomic devices where the integration of optical probing system is expected.

Metasurface-Embedded Contact Lenses for Holographic Light Projection

Contact lenses have been instrumental in vision correction and are expected to be utilized in augmented reality (AR) displays through the integration of electronic and optical components. In optics, metasurfaces, an array of sub-wavelength nanostructures, have offered optical multifunctionality in an ultra-compact form factor, facilitating integration into various imaging, and display systems. However, transferring metasurfaces onto contact lenses remains challenging due to the non-biocompatible materials of extant imprinting methods and the structural instability caused by the swelling and shrinking of the wetted surface. Here, a biocompatible method is presented to transfer metasurfaces onto contact lenses using hyaluronic acid (HA) as a soft mold and to allow for holographic light projection. A high-efficiency metahologram is obtained with an all-metallic 3D meta-atom enhanced by the anisotropy of a rectangular structure, and a reflective background metal layer. A corrugated metal layer on the HA mold is supported with a SiO2 capping layer, to avoid unwanted wrinkles and to ensure structural stability when transferred to the surface of pliable and wettable contact lenses. Biocompatible method of transferring metasurfaces onto contact lenses promises the integration of diverse optical components, including holograms, lenses, gratings and more, to advance the visual experience for AR displays and human-computer interfaces.

Eagle-Eye Inspired Meta-Device for Phase Imaging

The dual-focus vision observed in eagles' eyes is an intriguing phenomenon captivates scientists since a long time. Inspired by this natural occurrence, the authors' research introduces a novel bifocal meta-device incorporating a polarized camera capable of simultaneously capturing images for two different polarizations with slightly different focal distances. This innovative approach facilitates the concurrent acquisition of underfocused and overfocused images in a single snapshot, enabling the effective extraction of quantitative phase information from the object using the transport of intensity equation. Experimental demonstrations showcase the application of quantitative phase imaging to artificial objects and human embryonic kidney cells, particularly emphasizing the meta-device's relevance in dynamic scenarios such as laser-induced ablation in human embryonic kidney cells. Moreover, it provides a solution for the quantification during the dynamic process at the cellular level. Notably, the proposed eagle-eye inspired meta-device for phase imaging (EIMPI), due to its simplicity and compact nature, holds promise for significant applications in fields such as endoscopy and headsets, where a lightweight and compact setup is essential.

Monocular Metasurface for Structured Light Generation and 3D Imaging with a Large Field-of-View

Structured light three-dimensional (3D) imaging technology captures the geometric information on 3D objects by recording waves reflected from the objects' surface. The projection angle and point number of the laser dots directly determine the field-of-view (FOV) and the resolution of the reconstructed image. Conventionally, diffractive optical elements with micrometer-scale pixel size have been used to generate laser dot arrays, leading to limited FOV and point number within the projection optical path. Here, we theoretically put forward and experimentally demonstrate a monocular geometric phase metasurface composed of deep subwavelength meta-atoms to generate a 10 798 dot array within an FOV of 163°. Attributed to the vast number and high-density point cloud generated by the metasurface, the 3D reconstructed results showcase a maximum relative error in depth of 5.3 mm and a reconstruction error of 6.07%. Additionally, we propose a spin-multiplexed metasurface design method capable of doubling the number of lattice points. We demonstrate its application in the field of 3D imaging through experiments, where the 3D reconstructed results show a maximum relative depth error of 0.44 cm and a reconstruction error of 2.78%. Our proposed metasurface featuring advanced point cloud generation holds substantial potential for various applications such as facial recognition, autonomous driving, virtual reality, and beyond.

Ultra-robust informational metasurfaces based on spatial coherence structures engineering

Optical information transmission is vital in modern optics and photonics due to its concurrent and multi-dimensional nature, leading to tremendous applications such as optical microscopy, holography, and optical sensing. Conventional optical information transmission technologies suffer from bulky optical setup and information loss/crosstalk when meeting scatterers or obstacles in the light path. Here, we theoretically propose and experimentally realize the simultaneous manipulation of the coherence lengths and coherence structures of the light beams with the disordered metasurfaces. The ultra-robust optical information transmission and self-reconstruction can be realized by the generated partially coherent beam with modulated coherence structure even 93% of light is recklessly obstructed during light transmission, which brings new light to robust optical information transmission with a single metasurface. Our method provides a generic principle for the generalized coherence manipulation on the photonic platform and displays a variety of functionalities advancing capabilities in optical information transmission such as meta-holography and imaging in disordered and perturbative media.

Structured-Light 3D Imaging Based on Vector Iterative Fourier Transform Algorithm

Quasi-continuous-phase metasurfaces overcome the side effects imposed by high-order diffraction on imaging and can impart optical parameters such as amplitude, phase, polarization, and frequency to incident light at sub-wavelength scales with high efficiency. Structured-light three-dimensional (3D) imaging is a hot topic in the field of 3D imaging because of its advantages of low computation cost, high imaging accuracy, fast imaging speed, and cost-effectiveness. Structured-light 3D imaging requires uniform diffractive optical elements (DOEs), which could be realized by quasi-continuous-phase metasurfaces. In this paper, we design a quasi-continuous-phase metasurface beam splitter through a vector iterative Fourier transform algorithm and utilize this device to realize structured-light 3D imaging of a target object with subsequent target reconstruction. A structured-light 3D imaging system is then experimentally implemented by combining the fabricated quasi-continuous-phase metasurface illuminated by the vertical-cavity surface-emitting laser and a binocular recognition system, which eventually provides a new technological path for the 3D imaging field.

Multi-Dimensional Multiplexed Metasurface Holography by Inverse Design

Multi-dimensional multiplexed metasurface holography extends holographic information capacity and promises revolutionary advancements for vivid imaging, information storage, and encryption. However, achieving multifunctional metasurface holography by forward design method is still difficult because it relies heavily on Jones matrix engineering, which places high demands on physical knowledge and processing technology. To break these limitations and simplify the design process, here, an end-to-end inverse design framework is proposed. By directly linking the metasurface to the reconstructed images and employing a loss function to guide the update of metasurface, the calculation of hologram can be omitted; thus, greatly simplifying the design process. In addition, the requirements on the completeness of meta-library can also be significantly reduced, allowing multi-channel hologram to be achieved using meta-atoms with only two degrees of freedom, which is very friendly to processing. By exploiting the proposed method, metasurface hologram containing up to 12 channels of multi-wavelength, multi-plane, and multi-polarization is designed and experimentally demonstrated, which exhibits the state-of-the-art information multiplexing capacity of the metasurface composed of simple meta-atoms. This method is conducive to promoting the intelligent design of multifunctional meta-devices, and it is expected to eventually accelerate the application of meta-devices in colorful display, imaging, storage and other fields.

Visible Meta-Displays for Anti-Counterfeiting with Printable Dielectric Metasurfaces

Metasurfaces, 2D arrays of nanostructures, have gained significant attention in recent years due to their ability to manipulate light at the subwavelength scale. Their diverse applications range from advanced optical devices to sensing and imaging technologies. However, the mass production of dielectric metasurfaces with tailored properties for visible light has remained a challenge. Therefore, the demand for efficient and cost-effective fabrication methods for metasurfaces has driven the continuing development of various techniques. In this research article, a high-throughput production method is presented for multifunctional dielectric metasurfaces in the visible light range using one-step high-index TiO2-polymer composite (TPC) printing, which is a variant of nanoprinting lithography (NIL) for the direct replication of patterned multifunctional dielectric metasurfaces using a TPC material as the printing ink. The batch fabrication of dielectric metasurfaces is demonstrated with controlled geometry and excellent optical response, enabling high-performance light-matter interactions for potential applications of visible meta-displays.

Broadband achromatic and wide field of view metalens-doublet by inverse design

Metalenses, composed of patterned meta-atoms in various dimensions, offer tailored modulation of phase, amplitude, and polarization for diverse imaging applications across the visible and near-infrared spectra. However, simultaneously achieving achromatic and wide field of view (WFOV) imaging remains a significant challenge. In this paper, we propose a general inverse design framework for metalens-doublets that simultaneously enables broadband achromatic and WFOV imaging. The broadband achromatic and WFOV (BA&WFOV) metalens-doublet comprises a propagation phase metalens and a geometric phase metalens positioned on opposite sides of the substrate. This framework requires only once polarization conversion and mitigates aperture size constraints imposed by the limited group delay (GD) range of meta-atoms. We present a BA&WFOV metalens-doublet with an f-number of 3.9, a full field of view (FOV) of 68°, and a wavelength range from 640nm to 820nm. This metalens-doublet exhibits diffraction-limited focusing with an average absolute focusing efficiency of 16% and an average relative focusing efficiency of 60%. This innovative framework holds significant promise for applications in fields such as phone cameras, VR/AR, and endoscopes.

Wide field-of-hearing metalens for aberration-free sound capture

Metalenses are instruments that manipulate waves and have exhibited remarkable capabilities to date. However, an important hurdle arises due to the severe hampering of the angular response originating from coma and field curvature aberrations, which result in a loss of focusing ability. Herein, we provide a blueprint by introducing the notion of a wide field-of-hearing (FOH) metalens, designed particularly for capturing and focusing sound with decreased aberrations. Employing an aberration-free planar-thin metalens that leverages perfect acoustic symmetry conversion, we experimentally realize a robust wide FOH capability of approximately 140∘ in angular range. Moreover, our metalens features a relatively short focal length, enabling compact implementation by reducing the aperture-to-hearing plane distance. This is beneficial for space-efficient source-tracking sound sensing. Our strategy can be used across various platforms, potentially including energy harvesting, monitoring, imaging, and communication in auditory, ultrasonic, and submerged environments.

Ultra-thin, zoom capable, flexible metalenses with high focusing efficiency and large numerical aperture

The ever-growing demand for miniaturized optical systems presents a significant challenge in revolutionizing their core element - the varifocal lens. Recent advancements in ultra-thin, tunable metasurface optics have introduced new approaches to achieving zoom imaging. However, current varifocal metalens have faced challenges such as low focusing efficiency, limited tunability, and complicated designs. Here, we employ the high-contrast transmit arrays (HCTA) structures to design and fabricate a polarization-independent, single-layer flexible metalens that operates at a wavelength of 940 nm. Using a uniform stretching system, we characterized its optical performance to achieve over 60 % focusing efficiency within a 0 %-25 % stretch range, while the focal length changes align with theoretical predictions. Furthermore, our research also successfully demonstrated the capacity of a metalens with a numerical aperture (NA) of 0.5 to efficiently adjust imaging magnification within a 2× range, achieving imaging results that approach the diffraction limit. This research offers promising prospects for the practical use of compact and miniaturized optoelectronic devices in fields like photography, mixed reality, microscopy, and biomedical imaging.

Electrochromic nanopixels with optical duality for optical encryption applications

Advances in nanophotonics have created numerous pathways for light-matter interactions in nanometer scale, enriched by physical and chemical mechanisms. Over the avenue, electrically tunable photonic response is highly desired for optical encryption, optical switch, and structural color display. However, the perceived obstacle, which lies in the energy-efficient tuning mechanism and/or its weak light-matter interaction, is treated as a barrier. Here, we introduce electrochromic nanopixels made of hybrid nanowires integrated with polyaniline (PANI). The device shows optical duality between two resonators: (i) surface plasmon polariton (SPP)-induced waveguide (wavelength-selective absorber) and (ii) ultrathin resonator (broadband absorber). With switching effect of between resonant modes, we achieve enhanced chromatic variation spanning from red to green and blue while operating at a sub-1-volt level, ensuring compatibility with the CMOS voltage range. This modulation is achieved by improving the light-matter interaction, effectively harnessing the intrinsic optical property transition of PANI from lossy to dielectric in response to the redox states. In our experimental approach, we successfully scaled up device fabrication to an 8-inch wafer, tailoring the nanowire array to different dimensions for optical information encryption. Demonstrating distinct chromaticity modulation, we achieve optical encryption of multiple data bits, up to 8 bits per unit cell. By capitalizing on the remarkable sensitivity to the angular dependence of the waveguiding mode, we further enhance the information capacity to an impressive 10 bits per unit cell.

Metasurface- and PCSEL-Based Structured Light for Monocular Depth Perception and Facial Recognition

The novel depth-sensing system presented here revolutionizes structured light (SL) technology by employing metasurfaces and photonic crystal surface-emitting lasers (PCSELs) for efficient facial recognition in monocular depth-sensing. Unlike conventional dot projectors relying on diffractive optical elements (DOEs) and collimators, our system projects approximately 45,700 infrared dots from a compact 297-μm-dimention metasurface, drastically more spots (1.43 times) and smaller (233 times) than the DOE-based dot projector in an iPhone. With a measured field-of-view (FOV) of 158° and a 0.611° dot sampling angle, the system is lens-free and lightweight and boasts lower power consumption than vertical-cavity surface-emitting laser (VCSEL) arrays, resulting in a 5-10 times reduction in power. Utilizing a GaAs-based metasurface and a simplified optical architecture, this innovation not only addresses the drawbacks of traditional SL depth-sensing but also opens avenues for compact integration into wearable devices, offering remarkable advantages in size, power efficiency, and potential for widespread adoption.

Multi-wavelength structured light based on metasurfaces for 3D imaging

Structured light projection provides a promising approach to achieving fast and non-contact three-dimensional (3D) imaging. The resolution is a crucial index that represents security and accuracy in applications such as face recognition and robot vision. It depends on the density of dots in the projection. However, further improving the density of dots in the current system must be at the cost of speed or volume. Here, an all-dielectric ultra-thin metasurface is designed and fabricated to project a multi-wavelength dot array. The density of dots is improved because projected dots with different wavelengths fill the gaps with each other. The experimental results demonstrate that the multi-wavelength projection improves the resolution of 3D imaging. Furthermore, the multi-wavelength system is beneficial to measuring a surface with varying colors. The approach has the potential to achieve a new generation of high-resolution systems for tiny fluctuations and colorful 3D imaging in dark environments.

Photonic Bound States in the Continuum in Nanostructures

Bound states in the continuum (BIC) have garnered considerable attention recently for their unique capacity to confine electromagnetic waves within an open or non-Hermitian system. Utilizing a variety of light confinement mechanisms, nanostructures can achieve ultra-high quality factors and intense field localization with BIC, offering advantages such as long-living resonance modes, adaptable light control, and enhanced light-matter interactions, paving the way for innovative developments in photonics. This review outlines novel functionality and performance enhancements by synergizing optical BIC with diverse nanostructures, delivering an in-depth analysis of BIC designs in gratings, photonic crystals, waveguides, and metasurfaces. Additionally, we showcase the latest advancements of BIC in 2D material platforms and suggest potential trajectories for future research.

Freestanding, Freeform Metamolecule Fibers Tailoring Artificial Optical Magnetism

Metamolecule clusters support various unique types of artificial electromagnetism at optical frequencies. However, the technological challenges regarding the freeform fabrication of freestanding metamolecule clusters with programmed geometries and multiple compositions remain unresolved. Here, the freeform, freestanding raspberry-like metamolecule (RMM) fibers based on the directional guidance of a femtoliter meniscus are presented, resulting in the evaporative co-assembly of silica nanoparticles and gold nanoparticles with the aid of 3D nanoprinting. This method offers a facile and universal pathway to shape RMM fibers in 3D, enabling versatile manipulation of near- and far-field characteristics. In particular, the authors demonstrate the ability to decrease the scattering of the millimeter-scale RMM fiber in visible spectrum. In addition, the influence of electric and magnetic dipole modes on the directional scattering of RMM fibers is investigated. These experiments show that the magnetic response of an individual RMM can be controlled by adjusting the filling factor of gold nanoparticles. The authors anticipate that this method will allow for unrestricted design and realization of nanophotonic structures, surpassing the limitations of conventional fabrication processes.

Periodic diffractive optical element for high-density and large-scale spot array structured light projection

Structured light projection has been widely used for depth sensing in computer vision. Diffractive optical elements (DOEs) play a crucial role in generating structured light projected onto objects, and spot array is a common projection pattern. However, the primary metrics of the spot array, including density and field of view, are restricted by the principle of diffraction and its calculation. In this paper, a novel, to the best of our knowledge, method is proposed to achieve high-density periodic spot array on a large scale. Further, periodic DOEs, for the first time, are optimized to increase the density of the spot array without decreasing the periods of the DOE. Simulation and experimental results of high-density and large-scale spot array structured light projection are presented, demonstrating the effectiveness of the proposed method.

Bright-Field and Edge-Enhanced Imaging Using an Electrically Tunable Dual-Mode Metalens

The imaging of microscopic biological samples faces numerous difficulties due to their small feature sizes and low-amplitude contrast. Metalenses have shown great promise in bioimaging as they have access to the complete complex information, which, alongside their extremely small and compact footprint and potential to integrate multiple functionalities into a single device, allow for miniaturized microscopy with exceptional features. Here, we design and experimentally realize a dual-mode metalens integrated with a liquid crystal cell that can be electrically switched between bright-field and edge-enhanced imaging on the millisecond scale. We combine the concepts of geometric and propagation phase to design the dual-mode metalens and physically encode the required phase profiles using hydrogenated amorphous silicon for operation at visible wavelengths. The two distinct metalens phase profiles include (1) a conventional hyperbolic metalens for bright-field imaging and (2) a spiral metalens with a topological charge of +1 for edge-enhanced imaging. We demonstrate the focusing and vortex generation ability of the metalens under different states of circular polarization and prove its use for biological imaging. This work proves a method for in vivo observation and monitoring of the cell response and drug screening within a compact form factor.

Design of high-efficiency and large-angle homo-metagratings for light source integration

Meta-optics integrated with light sources has gained significant attention. However, most focused on the efficiency of metasurfaces themselves, rather than the efficiency of integration. To design highly efficient beam deflection, we develop a scheme of homo-metagrating, involving the same material for meta-atoms, substrate, and top layer of the laser, to achieve near-unity power from light-emitting to metasurfaces. We utilize three degrees of freedom: overall add-on phase, parameters of meta-atoms in a period, and lattice arrangement. The overall efficiency of homo-metagratings is higher than that of hetero-metagratings. We believe our approach is capable of being implemented in various ultracompact optic systems.

Monocular depth sensing using metalens

3-D depth sensing is essential for many applications ranging from consumer electronics to robotics. Passive depth sensing techniques based on a double-helix (DH) point-spread-function (PSF) feature high depth estimation precision, minimal power consumption, and reduced system complexity compared to active sensing methods. Here, we propose and experimentally implemented a polarization-multiplexed DH metalens designed using an autonomous direct search algorithm, which utilizes two contra-rotating DH PSFs encoded in orthogonal polarization states to enable monocular depth perception. Using a reconstruction algorithm that we developed, concurrent depth calculation and scene reconstruction with minimum distortion and high resolution in all three dimensions were demonstrated.

Integrated metasurfaces for re-envisioning a near-future disruptive optical platform

Metasurfaces have been continuously garnering attention in both scientific and industrial fields, owing to their unprecedented wavefront manipulation capabilities using arranged subwavelength artificial structures. To date, research has mainly focused on the full control of electromagnetic characteristics, including polarization, phase, amplitude, and even frequencies. Consequently, versatile possibilities of electromagnetic wave control have been achieved, yielding practical optical components such as metalenses, beam-steerers, metaholograms, and sensors. Current research is now focused on integrating the aforementioned metasurfaces with other standard optical components (e.g., light-emitting diodes, charged-coupled devices, micro-electro-mechanical systems, liquid crystals, heaters, refractive optical elements, planar waveguides, optical fibers, etc.) for commercialization with miniaturization trends of optical devices. Herein, this review describes and classifies metasurface-integrated optical components, and subsequently discusses their promising applications with metasurface-integrated optical platforms including those of augmented/virtual reality, light detection and ranging, and sensors. In conclusion, this review presents several challenges and prospects that are prevalent in the field in order to accelerate the commercialization of metasurfaces-integrated optical platforms.

Compact structured light generation based on meta-hologram PCSEL integration

Metasurfaces, a catalog of optical components, offer numerous novel functions on demand. They have been integrated with vertical cavity surface-emitting lasers (VCSELs) in previous studies. However, the performance has been limited by the features of the VCSELs such as low output power and large divergence angle. Although the solution of the module of VCSEL array could solve these issues, the practical application is limited by extra lens and large size. In this study, we experimentally demonstrate reconstruction of a holographic images using a compact integration of a photonic crystal surface-emitting laser and metasurface holograms designed for structured light generation. This research showcases the flexible design capabilities of metasurfaces, high output power (on the order of milliwatts), and the ability to produce well-uniformed images with a wide field of view without the need for a collection lens, making it suitable for 3D imaging and sensing.

Mid-infrared single-photon 3D imaging

Active mid-infrared (MIR) imagers capable of retrieving three-dimensional (3D) structure and reflectivity information are highly attractive in a wide range of biomedical and industrial applications. However, infrared 3D imaging at low-light levels is still challenging due to the deficiency of sensitive and fast MIR sensors. Here we propose and implement a MIR time-of-flight imaging system that operates at single-photon sensitivity and femtosecond timing resolution. Specifically, back-scattered infrared photons from a scene are optically gated by delay-controlled ultrashort pump pulses through nonlinear frequency upconversion. The upconverted images with time stamps are then recorded by a silicon camera to facilitate the 3D reconstruction with high lateral and depth resolutions. Moreover, an effective numerical denoiser based on spatiotemporal correlation allows us to reveal the object profile and reflectivity under photon-starving conditions with a detected flux below 0.05 photons/pixel/second. The presented MIR 3D imager features high detection sensitivity, precise timing resolution, and wide-field operation, which may open new possibilities in life and material sciences.

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.

Active 3D positioning and imaging modulated by single fringe projection with compact metasurface device

Three-dimensional (3D) information is vital for providing detailed features of the physical world, which is used in numerous applications such as industrial inspection, automatic navigation and identity authentication. However, the implementations of 3D imagers always rely on bulky optics. Metasurfaces, as the next-generation optics, shows flexible modulation abilities and excellent performance combined with computer vision algorithm. Here, we demonstrate an active 3D positioning and imaging method with large field of view (FOV) by single fringe projection based on metasurface and solve the accurate and robust calibration problem with the depth uncertainty of 4 μm. With a compact metasurface projector, the demonstrated method can achieve submillimeter positioning accuracy under the FOV of 88°, offering robust and fast 3D reconstruction of the texture-less scene due to the modulation characteristic of the fringe. Such scheme may accelerate prosperous engineering applications with the continued growth of flat-optics manufacturing process by using metadevices.

Advances in Meta-Optics and Metasurfaces: Fundamentals and Applications

Meta-optics based on metasurfaces that interact strongly with light has been an active area of research in recent years. The development of meta-optics has always been driven by human's pursuits of the ultimate miniaturization of optical elements, on-demand design and control of light beams, and processing hidden modalities of light. Underpinned by meta-optical physics, meta-optical devices have produced potentially disruptive applications in light manipulation and ultra-light optics. Among them, optical metalens are most fundamental and prominent meta-devices, owing to their powerful abilities in advanced imaging and image processing, and their novel functionalities in light manipulation. This review focuses on recent advances in the fundamentals and applications of the field defined by excavating new optical physics and breaking the limitations of light manipulation. In addition, we have deeply explored the metalenses and metalens-based devices with novel functionalities, and their applications in computational imaging and image processing. We also provide an outlook on this active field in the end.

Recent Development in Metasurfaces: A Focus on Sensing Applications

One of the fastest-expanding study areas in optics over the past decade has been metasurfaces (MSs). These subwavelength meta-atom-based ultrathin arrays have been developed for a broad range of functions, including lenses, polarization control, holography, coloring, spectroscopy, sensors, and many more. They allow exact control of the many properties of electromagnetic waves. The performance of MSs has dramatically improved because of recent developments in nanofabrication methods, and this concept has developed to the point that it may be used in commercial applications. In this review, a vital topic of sensing has been considered and an up-to-date study has been carried out. Three different kinds of MS absorber sensor formations, all-dielectric, all-metallic, and hybrid configurations, are presented for biochemical sensing applications. We believe that this review paper will provide current knowledge on state-of-the-art sensing devices based on MSs.