Metalenses at visible wavelengths: Diffraction-limited focusing and subwavelength resolution imaging

Recent development in surface/interface friction of two-dimensional black phosphorus: A review

In 2014, with the development of synthesis and modification methods of black phosphorus (BP), single or multiple layers of BP were stripped into two-dimensional (2D) layered materials, which had great prospects in transistors, batteries, optoelectronics, friction, and lubrication fields. From this point of view, we highlight recent advances in BP research, particularly its tribology and lubrication properties. This paper introduces mainly the research progress of BP in the solid-liquid lubrication fields, and systematically expounds its friction nature from the perspective of macroscopic, microscopic, and computational tribology. Under special conditions (high load, oxidation, etc.), a long-term superlubricity performance of BP could be obtained, which far exceeded other traditional 2D lubrication materials (Gr, MoS2, etc.). There were obvious deficiencies and misunderstandings about the macroscopic and microscopic superlubricity mechanism of BP lubricant, due to the complex and diversified frictional interfaces. The superlubricity mechanism of BP was roughly attributed to the multi-factor coupling or synergistic action in macroscopic, and it was still an open question whether there was secondary transition or contact area difference of the friction interface in microscopic. We believe that these deficiencies and misunderstandings are more ascribed to the lack of research on the interface transition behavior and mechanism during BP friction. We analyze and summarize the challenges and limitations in understanding BP's superlubricity mechanism based on macroscopic and microscopic experiments in the current BP friction research. Finally, we propose a computational tribology-based approach to reconcile discrepancies between macro- and micro-scale experiments.

Full-Color Quasi-Achromatic Imaging with a Dual-Functional Metasurface

Achieving broadband achromaticity in the visible spectrum is critical for enhancing the imaging performance of metalenses. However, many previous studies remain constrained by small device sizes or small numerical aperture. In this study, we propose a polarization-multiplexed metalens capable of generating zero- and high-order Bessel beams to achieve quasi-achromatic correction without size limitations. An image subtraction method with the two polarization channels is developed to mitigate the Bessel beam sidelobes to improve imaging quality. Our results demonstrate an effective quasi-achromatic focusing and imaging over a continuous wavelength range of 450-700 nm with long focus depth. The image subtraction method significantly enhances the image clarity and contrast, providing new insights for full-color imaging and detection.

Polarization-selective unidirectional and bidirectional diffractive neural networks for information security and sharing

Information security aims to protect confidentiality and prevent information leakage, which inherently conflicts with the goal of information sharing. Balancing these competing requirements is especially challenging in all-optical systems, where efficient data transmission and rigorous security are essential. Here we propose and experimentally demonstrate a metasurface-based approach that integrates phase manipulation, polarization conversion, as well as direction- and polarization-selective functionalities into all-optical diffractive neural networks (DNNs). This approach enables a polarization-controllable switch between unidirectional and bidirectional DNNs, thus simultaneously realizing information security and sharing. A cascaded terahertz metasurface comprising quarter-wave plates and metallic gratings is designed to function as a polarization-selective unidirectional-bidirectional classifier and imager. By introducing half-wave plates into a cascade metasurface, we achieve a polarization-controlled transition in unidirectional-bidirectional-unidirectional modes for classification and imaging. Furthermore, we demonstrate a high-security data exchange framework based on these polarization-selective DNNs. The proposed DNNs with polarization-switchable unidirectional/bidirectional capabilities offer significant potential for privacy protection, encryption, communications, and data exchange.

Reconstructing Three-Dimensional Optical Anisotropy with Tomographic Müller-Polarimetric Microscopy

Most visible light imaging methods using polarization to obtain ultrastructure information are limited to 2D analysis or require demanding phase measurements to be extended to 3D. A novel 3D polarized light imaging technique based on Müller-matrix formulations is introduced which numerically reconstructs 3D optical birefringence, that is anisotropic refractive indices and optical axis orientation, in each volumetric unit of sample. The new method is demonstrated, tomographic Müller-polarimetric microscopy, in simulation and using experimental data of 3D macroscopic sample of human trabecular bone sample, where the local main orientation of nanoscale collagen fibers is extracted with a resolution of ≈ 20 µm. Tomographic Müller-polarimetric microscopy offers a low-cost and experimentally simple imaging approach to access the ultrastructure which is not directly resolvable, in a wide range of biological and composite materials.

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.

Dynamics and Machine Learning Reveal the Link between Tripeptide Sequences and Evaporation-Driven Material Properties

Previous research showed that a peptide composed of three tyrosines (YYY) can turn into organic glass and cause strong adhesion between substrates via evaporation. However, the mechanisms of these processes remain unclear, and the exploration of applications of other peptide sequences is necessary. In this study, an optimized evaporation method was employed in molecular dynamics. It was found that YYY evaporation products possess abundant internal hydrogen bonds, which may facilitate the amorphous glass state formation. Moderate hydrophilicity of a peptide enhances molecular mobility and self-healing ability, while excessive hydrophilicity causes a water plasticizing effect. Stronger hydrophilicity also brings a larger curvature of evaporation products on polydimethylsiloxane (PDMS) substrate. A machine learning model was developed to predict the evaporation contact angle of peptide evaporation products and agrees well with the experiment. This research aims to improve understanding of peptide structure-function relationships and aid in designing custom organic optical devices based on peptide sequences.

Nonlinear Optical Information Encoding with Grayscale Lithography Enabled Metasurfaces

Optical information encoding is promising for many applications in sensing, data storage, and computing. Recently, various strategies have been suggested to encode optical information in planar devices. Among these, optical metasurfaces represent a flexible platform for manipulating multiple degrees of freedom of light with subwavelength scale meta-atoms. However, to realize both amplitude and phase control of light with metasurfaces, usually multiple meta-atoms per unit cell are required, so information density will be greatly reduced. Here, we develop a novel approach of nonlinear optical information encoding with grayscale lithography enabled hybrid metasurfaces composed of gold plasmonic meta-atoms deposited on an epsilon-near-zero material. By controlling the spacer layer thickness with electron beam grayscale lithography and varying orientation angles of the meta-atoms, we can control at the single-pixel level both the amplitude and phase of the generated second-harmonic waves. The proposed method opens new avenues for developing advanced nonlinear nanophotonic sources.

Nonlocal phase-change metaoptics for reconfigurable nonvolatile image processing

The next generation of smart imaging and vision systems will require compact and tunable optical computing hardware to perform high-speed and low-power image processing. These requirements are driving the development of computing metasurfaces to realize efficient front-end analog optical pre-processors, especially for edge detection capability. Yet, there is still a lack of reconfigurable or programmable schemes, which may drastically enhance the impact of these devices at the system level. Here, we propose and experimentally demonstrate a reconfigurable flat optical image processor using low-loss phase-change nonlocal metasurfaces. The metasurface is configured to realize different transfer functions in spatial frequency space, when transitioning the phase-change material between its amorphous and crystalline phases. This enables edge detection and bright field imaging modes on the same device. The metasurface is compatible with a large numerical aperture of ~0.5, making it suitable for high resolution coherent optical imaging microscopy. The concept of phase-change reconfigurable nonlocal metasurfaces may enable emerging applications of artificial intelligence-assisted imaging and vision devices with switchable multitasking.

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.

Multidimensional-Encrypted Meta-Optics Storage Empowered by Diffraction-Order Decoupling

Recent advancements in multidimensional multiplexing have paved the way for meta-optics encryption to be a viable solution to next-generation information storage and encryption security. However, challenges persist in increasing simultaneously modulated dimensions while minimizing structural complexity. Here, a novel single-cell order-decoupling method is proposed for the realization of a multidimensional encrypted meta-optics storage system. By analyzing the mathematic relationships between the phases of different diffraction orders, the detour phase structure is optimized to achieve independent encoding freedom for multiple orders. The proposed multidimensional encrypted meta-optics successfully realize the concurrent modulation of four optical dimensions: i) Wavelength, ii) Wavevector Direction, iii) Polarization, and iv) Diffraction Order. The system achieves up to sixteen-channel meta-holograms with low crosstalk and exponentially raises the threshold of brute-force decoding and thus remarkably enhances the information security in optical storage. It envisioned that the on-chip metasurface-based multidimensional encrypted strategy for augmented reality display functionalities presents promising applications in optical encryption/storage, anti-counterfeiting, and multifunctional photonics integrated circuits.

Multidimensional multiplexing metalens for STED microscopy

Stimulated emission depletion (STED) microscopy is a versatile super-resolution imaging technique for life sciences and data storage. Despite continuous breakthroughs, modern STED microscopes are still relatively bulky and limited to laboratory setups. Here, we exploit the multidimensional multiplexing properties of metalenses and experimentally demonstrate the realization of a compact STED lens with a single metasurface. A 635-nm right-handed circularly polarized excitation laser is focused by the metalens into a diffraction-limited Gaussian beam, while a 780-nm depletion beam with opposite chirality is converted into a high-quality donut-shaped focus on the same plane. As a consequence, STED super-resolution imaging based on the metalens has been obtained by recording the unpolarized photoluminescence using the same metalens. The experimentally demonstrated resolution reaches 0.7× of the diffraction limit and can be further improved. This study represents a critical step toward the miniaturization and integration of STED microscope.

Ultra narrowband geometric-phase resonant metasurfaces

The concept of a geometric phase has sparked a revolution in photonics. Conventional space-variant polarization manipulation in optical systems only results in broadband geometric phases. Recently emerged nonlocal metasurfaces show an ability to compress the operating bandwidth through modulations of wavelength-dependent amplitudes. However, their geometric phases are still broadband and not linear, posing severe challenges to realize ultra narrowband metadevices. Here, we propose and demonstrate the generation of ultra narrowband and spatially variable geometric phases in resonant metasurfaces. We find that an array of perturbed Mie resonators is able to simultaneously preserve its global symmetry and local transformation. Local transformation provides a pixel-level geometric phase, whereas global symmetry yields an ultranarrow operation bandwidth. We further reveal that this geometric phase can be well pinned to the resonant mode by introducing additional perturbations to individually define the phase at nonresonant wavelengths. Consequently, we realize experimentally pixelated phase-gradient metasurfaces and metalenses with record-breaking Q factors and high confidentiality. We believe that our general approach and demonstrated results will open a paradigm of multiplexed metasurfaces and information encryption.

Ligand-Engineered Direct Optical Lithography of Nanocrystals with Industrially Compatible Solvents

Nondestructive and precise patterning of colloidal semiconductor nanocrystals (NCs) is critical in the fabrication of solution-processable optoelectronic devices. Direct optical lithography of functional inorganic nanomaterials (DOLFIN) is widely used for the high-resolution patterning of NCs. However, conventional DOLFIN chemistry relies on solvents incompatible with mainstream industrial lithography processes, which impedes DOLFN's widespread adoption as a universal technology for real-world additive manufacturing. In this work, we proposed specific criteria for ligand design and designed a series of multifunctional ligands combining methacrylate and carboxyl groups. Such ligands allowed us to colloidally stabilize and optically pattern NCs with i-line and h-line light sources by using industrially friendly solvents. We showed that single-color and multicolor patterns with a spatial resolution of 1 μm can be achieved without compromising the optical and optoelectronic properties. The patterned NC films showed photoluminescence (PL) and electroluminescence (EL) on par with those of unpatterned films. The red-emitting QLEDs showed a peak external quantum efficiency (EQE) of 22.0%. The ability to reliably pattern bright NCs from PGMEA will facilitate the adoption of DOLFIN as an industrialized system-level integration platform and will significantly impact the production of high-resolution, full-color QLED devices.

Metasurface-integrated silicon nitride waveguides for holography with full polarization control

Here, a silicon-based metasurface integrated on a silicon nitride (SiNx) waveguide is proposed, enabling continuous control of light's phase and polarization. By utilizing resonant phase, geometric phase, and detour phase with fixed inter-unit spacing, the metasurface achieves full phase coverage under arbitrary polarization states while reducing design complexity. We demonstrate hologram images of letters under four polarization states: x-, y-, right circular, and left elliptical polarizations, achieving phase control with the same amplitude in corresponding polarizations. The results provide more possibilities for generating complex free space light fields via on-chip devices.

Broadband and polarization-independent complex amplitude modulation using a single layer dielectric metasurface

Precise control over amplitude and phase across the entire space is crucial for generating user-defined wavefronts and has significant value for designing flexible optical systems. Metasurfaces have emerged as compact and effective platforms for such control, offering high spatial resolution and continuity. However, traditional methods only work at specific wavelengths or polarization states, and the demonstration of full space complex amplitude control for broadband and unpolarized light remains limited. In this study, we leverage the principle of dual meta-atom interference to simultaneously modulate amplitude and phase using a single layer metasurface. Using a randomly polarized light source, nanoprinting and Fourier holography displays of complex patterns are achieved within the wavelength range of 480-640 nm, and the results are consistent with simulations. This approach presents several key advantages: continuous, precise and robust modulation of complex amplitude as well as polarization-independent and broadband response, which significantly reduce constraints on the light source's property and fabrication and make it well-suited for a variety of practical applications, including holographic displays, high-capacity communications, computational imaging, and laser beam processing.

Perfect anomalous refraction metasurfaces empowered half-space optical beam scanning

Metasurface-based optical beam scanning devices are gaining attention in optics and photonics for their potential to revolutionize light detection and ranging systems. However, achieving anomalous refraction with perfect efficiency (>99%) remains challenging, limiting the efficiency and field of view (FOV) of metasurface-based optical beam scanning devices. Here, we introduce a paradigm for achieving perfect anomalous refraction by augmenting longitudinal degrees of freedom arousing a multiple scattering process to optimize symmetry breaking. An all-dielectric quasi-three-dimensional subwavelength structure (Q3D-SWS), composed of a purposely designed multilayer film and a dielectric metasurface separated by a spacer, is proposed to eliminate reflection loss and spurious diffraction, achieving >99% anomalous refraction efficiency. By independently rotating two cascaded Q3D-SWSs, we experimentally showcase half-space optical beam scanning, achieving a FOV of 144° × 144°, with a maximum efficiency exceeding 86%. Our results open new avenues for high-efficiency metasurfaces and advances applications in light detection and ranging systems.

Neural Network-Assisted End-to-End Design for Full Light Field Control of Meta-Optics

Meta-optics, with unique light-matter interactions and extensive design space, underpins versatile and compact optical devices through flexible multi-parameter light field control. However, conventional designs struggle with the intricate interdependencies of nano-structural complex responses across wavelengths and polarizations at a system level, hindering high-performance full-light field control. Here, a neural network-assisted end-to-end design framework that facilitates global, gradient-based optimization of multifunctional meta-optics layouts for full light field control is proposed. Its superiority over separated design is showcased by utilizing the limited design space for multi-wavelength-polarization holography with enhanced performance (e.g., ≈6 × structural similarity index experimentally). By harnessing the dispersive full-parameter Jones matrix, orthogonal tri-polarization multi-wavelength-depth holography is further demonstrated, breaking conventional channel limitations. To highlight its versatility, non-orthogonal polarizations (>3) are showcased for arbitrary polarized-spectral multi-information processing applications in display, imaging, and computing. The comprehensive framework elevates light field control in meta-optics, delivering superior performance, enhanced functionality, and improved reliability, thereby paving the way for next-generation intelligent optical technologies.

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.

Chalcogenide Metasurfaces Enabling Ultra-Wideband Detectors From Visible to Mid-infrared

Thermoelectric materials can be designed to support optical resonances across multiple spectral ranges to enable ultra-wideband photodetection. For instance, antimony telluride (Sb2Te3) chalcogenide exhibits interband plasmonic resonances in the visible range and Mie resonances in the mid-infrared (mid-IR) range, while simultaneously possessing large thermoelectric Seebeck coefficients of 178 µV K-1. However, chalcogenide metasurfaces for achieving miniaturized and wavelength-sensitive ultra-wideband detectors have not been explored so far, especially with a single material platform. In this paper, Sb2Te3 metasurface devices are designed and fabricated to achieve ≈97% resonant absorption for enabling photodetectors operating across an ultra-wideband spectrum, from visible to mid-IR. Furthermore, relying on linear polarization-sensitive Sb2Te3 metasurface, the thermoelectric photodetectors with linear polarization-selectivity are demonstrated. This work provides a potential platform toward the portable ultrawide band spectrometers without requiring cryogenic cooling, for environmental sensing applications.

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.

Metasurface enabled high-order differentiator

Metasurface-enabled optical analog differentiation has garnered significant attention due to its inherent capacity of parallel operation, compactness, and low power consumption. Most previous works focused on the first- and second-order operations, while several significant works have also achieved higher-order differentiation in both real space and k-space. However, how to construct the desired optical transfer function in a practical system to realize scalable and multi-order-parallel high-order differentiation of images in real space, and particularly how to leverage it to tackle practical problems, have not been fully explored. Here, drawing on the basic mathematical feature of the Fourier transform, we theoretically propose universal phase-gradient functions of the Pancharatnam-Berry-phase-based meta-device for performing arbitrary order differentiation. The fifth-order optical differentiations for both intensity and phase images are realized in the experiment. More importantly, by exploring this elaborately designed spatial differentiator, we construct another scheme for optical super-resolution and achieve the estimation of the distance between two incoherent point sources within 0.015 of the Rayleigh distance, which thereby provides a potential toolkit for optical alignment in high-precision semiconductor nano-fabrication. Our findings hold promise for image processing, microscopy imaging, and optical super-resolution imaging.

High-performance achromatic flat lens by multiplexing meta-atoms on a stepwise phase dispersion compensation layer

Flat optics have attracted interest for decades due to their flexibility in manipulating optical wave properties, which allows the miniaturization of bulky optical assemblies into integrated planar components. Recent advances in achromatic flat lenses have shown promising applications in various fields. However, it is a significant challenge for achromatic flat lenses with a high numerical aperture to simultaneously achieve broad bandwidth and expand the aperture sizes. Here, we present the zone division multiplex of the meta-atoms on a stepwise phase dispersion compensation (SPDC) layer to address the above challenge. In principle, the aperture size can be freely enlarged by increasing the optical thickness difference between the central and marginal zones of the SPDC layer, without the limit of the achromatic bandwidth. The SPDC layer also serves as the substrate, making the device thinner. Two achromatic flat lenses of 500 nm thickness with a bandwidth of 650-1000 nm are experimentally achieved: one with a numerical aperture of 0.9 and a radius of 20.1 µm, and another with a numerical aperture of 0.7 and a radius of 30.0 µm. To the best of our knowledge, they are the broadband achromatic flat lenses with highest numerical apertures, the largest aperture sizes and thinnest thickness reported so far. Microscopic imaging with a 1.10 µm resolution has also been demonstrated by white light illumination, surpassing any previously reported resolution attained by achromatic metalenses and multi-level diffractive lenses. These unprecedented performances mark a substantial step toward practical applications of flat lenses.

Non-invasive and fully two-dimensional quantitative visualization of transparent flow fields enabled by photonic spin-decoupled metasurfaces

Transparent flow field visualization techniques play a critical role in engineering and scientific applications. They provide a clear and intuitive means to understand fluid dynamics and its complex phenomena, such as laminar flow, turbulence, and vortices. However, achieving fully two-dimensional quantitative visualization of transparent flow fields under non-invasive conditions remains a significant challenge. Here, we present an approach for achieving flow field visualization by harnessing the synergistic effects of a dielectric metasurface array endowed with photonic spin-decoupled capability. This approach enables the simultaneous acquisition of light-field images containing flow field information in two orthogonal dimensions, which allows for the real-time and quantitative derivation of multiple physical parameters. As a proof-of-concept, we experimentally demonstrate the applicability of the proposed visualization technique to various scenarios, including temperature field mapping, gas leak detection, visualization of various fluid physical phenomena, and 3D morphological reconstruction of transparent phase objects. This technique not only establishes an exceptional platform for advancing research in fluid physics, but also exhibits significant potential for broad applications in industrial design and vision.

A Single Sub-Millimetric Metasurface-Based Optical Element for Lattice Bessel Beam Excitation Enabling Brain Activity Recordings In Vivo

Bessel beams (BBs) are propagation-invariant optical fields that retain a narrow central intensity profile over longer propagation lengths than Gaussian beams (GBs). Due to this property, they have been adopted in fluorescence-based light sheet microscopy (LSM) to obtain 2D longitudinally-extended light-sheets. Yet, current approaches for generating BB lattices in LSM focus on regular excitation patterns and involve complex and bulky optics, limiting integration capability and versatility. Here, a flexible method is presented to obtain BB-arrays with arbitrary geometries by encoding on a single sub-millimetric surface all the optical transformations required. This method is applied using a single metasurface to encode the generation of a linear array of BBs, avoiding the use of conjugation and focusing optics. With respect to the current strategies, this approach, allowing for the independent design of each beamlet of the array, increases the degrees of freedom while making optimal use of the available light with no rejection, thus facilitating its integration into optical systems. According to this method, we fabricated a metasurface-based optical element for generating a linear BB-array of excitation in an LSM configuration and recorded neuronal activity at cellular resolution from the zebrafish larva brain. Thus, the proposed approach greatly extends the BB-array versatility and the application scenarios.

Centimeter-size achromatic metalens in long-wave infrared

Metalens has shown its significantly ultra-light and ultra-thin features. However, large-aperture achromatic metalens is constrained by both maximum dispersion range and computational memory. Here, we propose a fully device optimizing framework that engineers phase dispersion and amplitude transmittance to create centimeter-size achromatic metalens operating in long-wave infrared regime (8-12 μm). Via wrapping group delay within a defined range and optimizing dispersion phase of desired wavelengths, chromatic aberrations can be effectively corrected. We verify our design by characterizing all-silicon 3.18-cm-diameter and 6.36-cm-diameter LWIR achromatic metalenses. Diffraction-limited tight-focusing can be achieved, and the normalized focal length shift is less than 3.3 × 10-4. Thermal imaging performance is verified on targets of holes or letters with a diameter or line width exceeding 2 mm. These findings facilitate the development of large-aperture achromatic metalenses and open up possibilities for lightweight imaging systems in long-wave infrared.

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.

Experimental Realization of On-Chip Surface Acoustic Wave Metasurfaces at Sub-GHz

Metasurfaces, consisting of subwavelength-thickness units with different wave responses, provide an innovative possible method to manipulate elastic and acoustic waves efficiently. The application of metasurfaces to manipulate on-chip surface acoustic wave (SAW) at sub-GHz frequencies requires further exploration since their wave functions are highly demanded in nanoelectromechanical systems (NEMS), sensing, communications, microfluid control and quantum processing. Here, the experimental realization of on-chip SAW metasurfaces is reported, consisting of gradient submicron niobium (Nb) rectangular pillars positioned on a 128°Y-cut lithium niobate (LiNbO3) substrate that operate at hundreds of megahertz. The proposed SAW metasurfaces are able to manipulate transmitted SAW wavefront functions by designing on-demand pillar's profile distributions. Broadband subwavelength focusing effects as the typical functions of SAW metasurfaces are experimentally demonstrated. This study opens a door for realizing on-chip SAW metasurfaces for diverse potential applications at micro- and nanoscale.

Nonreciprocal Metasurfaces with Epsilon-Near-Zero Materials

Nonreciprocal optics enables the asymmetric transmission of light when its sources and detectors are exchanged. A canonical example─optical isolator─enables light propagation in only one direction, similar to how electrical diodes enable unidirectional flow of electric current. Nonreciprocal optics today, unlike nonreciprocal electronics, remains bulky. Recently, nonlinear metasurfaces opened a pathway to strong optical nonreciprocity on the nanoscale. However, demonstrations to date were based on optically slow nonlinearities involving thermal effects or phase transition materials. In this work, we demonstrate a nonreciprocal metasurface with an ultrafast optical response based on indium tin oxide in its epsilon-near-zero regime. It operates in the spectral range of 1200-1300 nm with incident power densities of 40-70 GW/cm2. Furthermore, the nonreciprocity of the metasurface extends to both amplitude and phase of the forward/backward transmission, opening a pathway to nonreciprocal wavefront control at the nanoscale.

Three-Photon Direct Laser Writing of the QD-Polymer Metasurface for Large Field-of-View Optical Holography

Conventional metasurface holography based on special structural designs is extremely sensitive to the angle of the incident light. Without complex angle optimization for metasurface units, even a small increase in the angle may lead to a rapid decrease in the diffraction efficiency and loss of imaging information. Moreover, the response spectral range of most metasurface holographies cannot be freely adjusted from ultraviolet to infrared. In this study, we prepare a quantum dot (QD)-polymer material system and introduce 1035 nm three-photon direct laser writing (DLW) technology to fabricate the QD-polymer metasurface for large field-of-view optical holography. Based on the stable light absorption characteristics and insensitivity to the angle of incident light of QDs, we achieve a binary amplitude-only holography with a large field of view of ±70°. Moreover, based on the quantum confinement effect of the QDs, the tunable broadband characteristic of the QD-polymer metasurface holography from the ultraviolet to near-infrared is demonstrated, and the binary amplitude-only holography also shows polarization independence. In addition, based on the QD-polymer material system, we can realize a Pancharatnam-Berry phase holography. DLW-processed QD-polymer metasurfaces have the potential to maintain a long-term stability. This study provides a material system and a versatile and flexible technology for realizing various nanoparticle-polymer metasurface holography with a large field of view and tunable broadband characteristics.

a-SiC heteromorphic immersion nanocavities enabling wide-field real-time single-molecule detection

Real-time single-molecule detection via fluorescence exhibits advantages of non-contact and specificity, especially in illustrating the dynamic heterogeneity of living substances. However, wide-field view and signal-to-noise ratio (SNR) are always contradictory in real-time single-molecule detection with fluorescence labels, owing to the limitation of the omnidirectional radiation characteristics of fluorophores. Herein, we propose a nano optical sensing device based on a-SiC heteromorphic immersion nanocavities (aHINCs), enabling wide-field real-time single-molecule imaging without sacrificing SNR. The characteristics of architectural aHINCs for far-field imaging are investigated, and the designed sensing device help adjust the emission direction of fluorescence in gold hot spots of nanocavities, allowing the fluorescence divergence angle to be adjusted to ±10°. The experimental results show the SNR reached 22, markedly strengthening the fluorescence collection efficiency. The simultaneous observation of nanocavity sites, within the same field of view of the chip, is 488 times the number of sites observed with classic detection method. Furthermore, the wide-field real-time detection of the single molecule-specific binding process of the oligo DNA complementary chain is successfully realized. The nano optical sensing device based on the aHINC shows potential for parallel real-time single-molecule detection applications.

Metalens formed by structured arrays of atomic emitters

Arrays of atomic emitters have proven to be a promising platform to manipulate and engineer optical properties, due to their efficient cooperative response to near-resonant light. Here, we theoretically investigate their use as an efficient metalens. We show that, by spatially tailoring the (subwavelength) lattice constants of three consecutive two-dimensional arrays of identical atomic emitters, one can realize a large transmission coefficient with arbitrary position-dependent phase shift, whose robustness against losses is enhanced by the collective response. To characterize the efficiency of this atomic metalens, we perform large-scale numerical simulations involving a substantial number of atoms (N ∼ 5 × 105) that is considerably larger than comparable works. Our results suggest that low-loss, robust optical devices with complex functionalities, ranging from metasurfaces to computer-generated holograms, could be potentially assembled from properly engineered arrays of atomic emitters.

Four-dimensional imaging based on a binocular chiral metalens

In this Letter, we present a binocular chiral metalens (BCM) device designed for four-dimensional (4D) imaging, which integrates both three-dimensional spatial perception and polarization detection. The BCM consists of two identical monocular metalenses that spatially separate left- and right-handed circularly polarized (LCP and RCP) light. When integrated with a commercial camera, the metalenses enable simultaneous measurement of the depth and polarization information. Numerical simulations and experimental results demonstrate that the BCM can achieve a circular polarization extinction ratio (CPER) of 29.2 dB and an average 3D reconstruction error of 4.09%. The proposed system paves a pathway for multi-dimensional imaging, with significant potential in applications in security, surveillance, and future advancements in more complex imaging tasks across other electromagnetic bands, including terahertz and infrared regimes.

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.

Polarization-independent full-color holographic movie with a single metasurface free from crosstalk

Metasurface holograms offer advantages, such as a wide viewing angle, compact size, and high resolution. However, projecting a full-color movie using a single hologram without polarization dependence has remained challenging. Here, we report a full-color dielectric metasurface holographic movie with a resolution of 512 × 512. Eight frames were multiplexed across blue (445 nm), green (532 nm), and red (633 nm) color channels, achieving a maximum reconstruction rate of 5.6 frames per second. The superposition of the three wavelengths was achieved by adjusting the resolution and position of each target image while maintaining a constant pitch of the meta-atoms. Additionally, we identified the positions of crosstalk images generated that occur due to fabrication errors and proposed and demonstrated conditions and corrections to ensure they do not overlap with the intended images. The superimposition of phase distributions for each wavelength was achieved using the least squares error method, based on a library of over 20,000 types of meta-atoms. These results are anticipated to advance the future development of three-dimensional metasurface holographic movies.

Tandem achromatic metasurface for waveguide coupling in full-color AR displays

Waveguide coupling design is one of the most challenging topics in augmented reality (AR) near-eye displays (NED). The primary challenge stems from the necessity to simultaneously address two competing factors: the overall volume of the AR system and the occurrence of chromatic aberration. To address this issue, what we believe to be a novel tandem trilayer achromatic metasurface is specifically designed for waveguide coupling in AR NEDs, capable of achieving an achromatic effect in a nanometer-thin layer. By analyzing the influence of unit structure parameters on the phase delay of input electromagnetic waves, the optimal parameters are determined and the tandem trilayer achromatic metasurface structure is established. Simulation results show that the incident light can be deflected by 45°, 46°, and 45° at wavelengths of 440 nm ∼ 470 nm, 520 nm ∼ 550 nm, and 620 nm ∼ 660 nm, respectively. The angular deviation error of the three primary colors is maintained lower than 1° in the AR waveguide, ensuring a satisfactory achromatic effect. This design provides a new solution for developing ultra-thin and compact optical systems for full-color AR NEDs.

Optical Fresnel zone plate flat lenses made entirely of colored photoresist through an i-line stepper

Light manipulation and control are essential in various contemporary technologies, and as these technologies evolve, the demand for miniaturized optical components increases. Planar-lens technologies, such as metasurfaces and diffractive optical elements, have gained attention in recent years for their potential to dramatically reduce the thickness of traditional refractive optical systems. However, their fabrication, particularly for visible wavelengths, involves complex and costly processes, such as high-resolution lithography and dry-etching, which has limited their availability. In this study, we present a simplified method for fabricating visible Fresnel zone plate (FZP) planar lenses, a type of diffractive optical element, using an i-line stepper and a special photoresist (color resist) that only necessitates coating, exposure, and development, eliminating the need for etching or other post-processing steps. We fabricated visible FZP lens patterns using conventional photolithography equipment on 8-inch silica glass wafers, and demonstrated focusing of 550 nm light to a diameter of 1.1 μm with a focusing efficiency of 7.2%. Numerical simulations showed excellent agreement with experimental results, confirming the high precision and designability of our method. Our lenses were also able to image objects with features down to 1.1 μm, showcasing their potential for practical applications in imaging. Our method is a cost-effective, simple, and scalable solution for mass production of planar lenses and other optical components operating in the visible region. It enables the development of advanced, miniaturized optical systems to meet modern technology demand, making it a valuable contribution to optical component manufacturing.

Raman spectroscopy with a microfluidic device embedded with plasmonic metasurface

Metasurfaces offer a powerful tool to realize label-free and highly sensitive Raman spectroscopy. Embedding metasurfaces into microfluidic channels is promising to establish a new characterizing platform for microfluids. In this Letter, we present a highly stable method for improving the Raman scattering intensity of biological microfluids by using a microfluidic chip embedded with a plasmonic metasurface. The embedded metasurface consists of a nanosphere array coated with a silver layer, where the diameter of the nanosphere is ∼100 nm. The Langmuir-Blodgett method and a chemical spraying method were adopted to prepare the nanosphere-array metasurface. In the case of red blood cell measurement, a giant enhancement of Raman spectra intensity is achieved with a metasurface compared to that without a metasurface. Moreover, a two-time enhancement of Raman spectra intensity is obtained with a metasurface under radially polarized beam illumination compared to linearly polarized beam illumination. Furthermore, a microfluidic device embedded with a plasmonic metasurface was applied to monitor the environmental variation of rat red blood cells. Peaks in the range from 2143 cm-1 to 2303 cm-1 arise with the addition of glucose and are still obviously distinguishable when the additive concentration is down to 10-3 M. This indicates high sensitivity to the concentration of glucose mixed with rat red blood cells, which could be further applied to monitor biological cell environments such as glucose concentration, pH, and sodium salt concentration.

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.

3D-printed aberration-free terahertz metalens for ultra-broadband achromatic super-resolution wide-angle imaging with high numerical aperture

Terahertz (THz) lens constitutes a vital component in the THz system. Metasurfaces-based THz metalenses and classical bulky lenses are severely constrained by chromatic/ spherical aberration and the diffraction limit. Consequently, achromatic super-resolution THz lenses are urgently needed. In this study, through translating the required phase distribution into a refractive index (RI) profile with a specific thickness, an innovative approach to designing THz metalenses is proposed and achieved by dielectric gradient metamaterials. The samples fabricated by 3D printing can realize achromatic super focusing with a numerical aperture (NA) of 0.555 from 0.2 to 0.9 THz. Submillimeter features separated by approximately 0.2 mm can be resolved with high precision, such as glass fabric patterns within FR4 panels and fibrous tissue on leaves, with a field of view (FOV) of 90°. Our approach offers a feasible and cost-effective means to implement THz super-resolution imaging, which holds great potential in non-destructive testing and biomedical imaging.

Heterogeneous-Gradient Supercell Metasurfaces for Independent Complex Amplitude Control over Multiple Diffraction Channels

The ability to achieve independent complex amplitude control across multiple channels can significantly increase the information capacity of photonic devices. Diffraction inherently holds numerous channels, which are good candidates for dense light manipulation in angular space. However, no convenient method is currently available for attaining this. Here, a flexible interference approach utilizing silicon-based transmission-type heterogeneous-gradient supercell metasurfaces is proposed. By simply designing the phases of the meta-atoms' radiations within a supercell, the complex amplitude of each diffraction channel can be individually and analytically controlled. Crucially, the complex amplitudes of multiple diffraction channels can be simultaneously controlled in a non-interleaved manner, where the number of channels is determined by the number of effective adjusting degrees of freedom (DoF). As a proof-of-concept validation, several meta-devices are experimentally demonstrated in the terahertz regime, which can generate multiple vortex beams, focal points, and splitting beams in different desired diffraction angles. This advancement heralds a new pathway for the development of multifunctional photonic devices with enhanced channel capacity, offering significant potential for both research and practical applications in photonics.

Portable astronomical observation system based on large-aperture concentric-ring metalens

The core advantage of metalenses over traditional bulky lenses lies in their thin volume and lightweight. Nevertheless, as the application scenarios of metalenses extend to the macro-scale optical imaging field, a contradiction arises between the increasing demand for large-aperture metalenses and the synchronous rise in design and processing costs. In response to the application requirements of metalens with diameter reaching the order of 104λ or even 105λ, this paper proposes a novel design method for fixed-height concentric-ring metalenses, wherein, under the constraints of the processing technology, a subwavelength 2D building unit library is constructed based on different topological structures, and the overall cross-section of the metalens is assembled. Compared to global structural optimization, this approach reduces computational resources and time consumption by several orders of magnitude while maintaining nearly identical focusing efficiency. As a result, a concentric-ring metalens with a designed wavelength of 632.8 nm and a diameter of 46.8 mm was developed, and a quasi-telecentric telescope system composed of aperture stop and metalens was constructed, achieving high-resolution detection within a 20° field of view. In the subsequent experiments, the unique weak polarization dependence and narrowband adaptability of the meta-camera are quantitatively analyzed and tested, and excellent imaging results were finally obtained. Our work not only ensures the narrowband optical performance but also promotes the simplicity and light weight of the metalens based telescopic system, which further advances the deep application of large-diameter metalenses in the field of astronomical observation.

High dioptric power micro-lenses fabricated by two-photon polymerization

Specimen-induced aberrations limit the penetration depth of standard optical imaging techniques in vivo, mainly due to the propagation of high NA beams in a non-homogenous medium. Overcoming these limitations requires complex optical imaging systems and techniques. Implantable high NA micro-optics can be a solution to tissue induced spherical aberrations, but in order to be implanted, they need to have reduced complexity, offering a lower surface to the host immune reaction. Here, we design, fabricate, and test a single micro-optical element with high dioptric power and high NA (up to 1.25 in water). The sag function is inspired by the classical metalens phase and improved to reduce the spherical aberrations arising from the refractive origin of the phase delay at the lens periphery. We successfully fabricated these high-NA quasi-parabolic aspheric microlenses with varying focal lengths by two-photon polymerization in biocompatible photoresist SZ2080. The entire process is optimized to minimize fabrication time while maintaining the structures' robustness: the smoothness reaches optical (λ20) quality. The dioptric power and magnification of the microlenses were quantified over a 200 × 200 µm aberration-free field of view. Our results indicate that these microlenses can be used for wide-field imaging under linear excitation and have the optical quality to be utilized for nonlinear excitation imaging. Moreover, being made of biocompatible photoresist, they can be implanted close to the observation volume and help to reduce the spherical aberration of laser beams penetrating living tissues.

Optical power-handling capabilities and temporal dynamics of reconfigurable phase-change metasurfaces

Metasurfaces based on chalcogenide phase-change materials offer a highly promising route towards the realization of non-volatile reconfigurable metasurfaces. However, since their switching mechanism between amorphous and crystalline states is based on thermal stimuli, phase-change metasurfaces should be treated carefully when operating under high power laser sources, since optically induced heating could trigger unwanted state changes during their operation. In this work, therefore, we develop a thermodynamic model capable of tracking the crystallization, melting and reamorphization dynamics of phase-change optical metadevices, and so too their optical performance, when operating under (i.e., aiming to control) high power laser sources. Our model is used, by way of example, to ascertain the optical power-handling capabilties of two typical phase-change metasurface architectures, one for beam steering and one for active lensing.

Tunable Beam Steering Metasurface Based on a PMN-PT Crystal with a High Electro-Optic Coefficient

Existing tunable optical metasurfaces based on the electro-optic effect are either complex in structure or have a limited phase modulation range. In this paper, a simple rectangular metasurface structure based on a Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT) crystal with high electro-optic coefficient of 120 pm/V was designed to demonstrate its electrically tunable performance in the optical communication band through simulations. By optimizing the structure parameters, a tunable metasurface was generated that can induce a complete 2π phase shift for beam deflection while maintaining relatively uniform transmittance. Simulations further demonstrated the electrical tunability of the beam deflection direction and operating wavelength of the metasurface. This tunable optical metasurface, with its simple and easily fabricated structure, can promote the development and application of multifunctional and controllable metasurfaces. Its adjustable beam deflection direction and operating wavelength may find applications in fields such as optical communication systems and imaging.

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.

Dual-Wavelength On-Chip Integrated Metalens for Epi-Fluorescence Single-Molecule Sensing

Single-molecule fluorescence spectroscopy offers unique capabilities for the low-concentration sensing and probing of molecular dynmics. However, employing such a methodology for versatile sensing and diagnostics under point-of-care demands device miniaturization to lab-on-a-chip size. In this study, we numerically design metalenses with high numerical aperture (NA = 1.1), which are composed of silicon nitride nanostructures deposited on a waveguide and can selectively focus guided light into an aqueous solution at two wavelengths of interest in the spectral range of 500-780 nm. Despite the severe chromatic focal shift in the lateral directions owing to the wavelength-dependent propagation constant in a waveguide, segmented on-chip metalenses provide perfectly overlapping focal volumes that meet the requirements for epi-fluorescence light collection. We demonstrate that the molecule detection efficiencies of metalenses designed for the excitation and emission wavelengths of ATTO 490LS, Alexa 555, and APC-Cy7 tandem fluorophores are sufficient to collect several thousand photons per second per molecule at modest excitation rate constants. Such sensitivity provides reliable diffusion fluorescence correlation spectroscopy analysis of single molecules on a chip to extract their concentration and diffusion properties in the nanomolar range. Achromatic on-chip metalenses open new avenues for developing ultra-compact and sensitive devices for precision medicine and environmental monitoring.

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.

Inverse-designed polarization-insensitive metasurface holography fabricated by nanoimprint lithography

Metasurface holography, capable of fully engineering the wavefronts of light in an ultra-compact manner, has emerged as a promising route for vivid imaging, data storage, and information encryption. However, the primary manufacturing method for visible metasurface holography remains limited to the expensive and low-productivity electron-beam lithography (EBL). Here, we experimentally demonstrate the polarization-insensitive visible metasurface holography fabricated by high-throughput and low-cost nanoimprint lithography (NIL). The high-index titanium dioxide (TiO2) film is thinly deposited on the imprinted meta-atoms via atomic layer deposition (ALD) to achieve sufficient phase coverage. The calculated high-fidelity holograms are obtained by an inverse design method based on gradient-descent (GD) optimization. Under the various polarized light incidence, the correlation coefficients between the experimental reconstructed images and the target images all exceed 0.7 and the measured efficiencies are approximately 20%. The results demonstrate the high-precision, high-throughput, and cost-effective production of visible metaholograms, paving the way for the commercialization of meta-optics.

Quantitative phase imaging endoscopy with a metalens

Quantitative phase imaging (QPI) recovers the exact wavefront of light from intensity measurements. Topographical and optical density maps of translucent microscopic bodies can be extracted from these quantified phase shifts. We demonstrate quantitative phase imaging at the tip of a coherent fiber bundle using chromatic aberrations inherent in a silicon nitride hyperboloid metalens. Our method leverages spectral multiplexing to recover phase from multiple defocus planes in a single capture using a color camera. Our 0.5 mm aperture metalens shows robust quantitative phase imaging capability with a28 ∘ field of view and 0. 2 π phase resolution ( ~ 0. 1 λ in air) for experiments with an endoscopic fiber bundle. Since the spectral functionality is encoded directly in the imaging lens, the metalens acts both as a focusing element and a spectral filter. The use of a simple computational backend will enable real-time operation. Key limitations in the adoption of phase imaging methods for endoscopy such as multiple acquisition, interferometric alignment or mechanical scanning are completely mitigated in the reported metalens based QPI.