Catenary optics for achromatic generation of perfect optical angular momentum

Manipulating terahertz guided wave excitation with Fabry-Perot cavity-assisted metasurfaces

Metasurfaces are emerging as powerful tools for manipulating complex light fields, offering enhanced control in free space and on-chip waveguide applications. Their ability to customize refractive indices and dispersion properties opens up new possibilities in light guiding, yet their efficiency in exciting guided waves, particularly through metallic structures, is not fully explored. Here, we present a new method for exciting terahertz (THz) guided waves using Fabry-Perot (FP) cavity-assisted metasurfaces that enable spin-selective directional coupling and mode selection. Our design uses a substrate-free ridge silicon THz waveguide with air cladding and a supporting slab, incorporating placed metallic metasurfaces to exploit their unique interaction with the guided waves. With the silicon thin layer and air serving as an FP cavity, THz waves enter from the bottom of the device, thereby intensifying the impact of the metasurfaces. The inverse-structured complementary metasurface could enhance excitation performance. We demonstrate selective excitation of TE00 and TE10 modes with directional control, confirmed through simulations and experimental validations using a THz vector network analyzer (VNA) system. This work broadens the potential of metasurfaces for advanced THz waveguide technologies.

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.

Highly efficient vortex generation at the nanoscale

Control of the angular momentum of light at the nanoscale is critical for many applications of subwavelength photonics, such as high-capacity optical communications devices, super-resolution imaging and optical trapping. However, conventional approaches to generate optical vortices suffer from either low efficiency or relatively large device footprints. Here we show a new strategy for vortex generation at the nanoscale that surpasses single-pixel phase control. We reveal that interaction between neighbouring nanopillars of a meta-quadrumer can tailor both the intensity and phase of the transmitted light. Consequently, a subwavelength nanopillar quadrumer is sufficient to cover a 2lπ phase change, thus efficiently converting incident light into high-purity optical vortices with different topological charges l. Benefiting from the nanoscale footprint of the meta-quadrumers, we demonstrate high-density vortex beam arrays and high-dimensional information encryption, bringing a new degree of freedom to many designs of meta-devices.

Independent Manipulations of Transmitting and Receiving Channels by Nonreciprocal Programmable Metasurface

The electromagnetic (EM) beam manipulations such as spatial scanning have always been the focus in information science and technology. Generally, the transmitting and receiving (T/R) beams of the same aperture should be coincident due to the reciprocal theory, and hence, more flexible controls of the spatial information are limited accordingly. Here, we propose a new approach to achieve independent controls of beam scanning in spatial T/R channels based on one aperture made by a nonreciprocal programmable metasurface. The meta-atom is designed to have independent propagation chains for T/R waves by introducing dual-direction power amplifiers (PAs) as the isolators for one-way transparency. A programmable phase shifter with a 360° coverage is loaded with the PA device in the transmitting or receiving chain to realize independent beam scanning in the T/R channels. A prototype of the proposed metasurface is fabricated, and independent beam scanning in the T/R channels is directly acquired with good performance in our measurements. In addition, a proof of concept of integrated sensing and auxiliary communications is accomplished to verify the validity of the presented method.

Wide-angle camouflage detectors by manipulating emissivity using a non-reciprocal metasurface array

Camouflage detectors that can detect incoming radiation from any angle without being detected are extremely important in stealth, guided missile, and heat-seeking missile industries. In order to accomplish this, the absorption and emission processes must be manipulated simultaneously. However, Kirchhoff's fundamental law suggests that absorption and emission are always in the same direction α(θ) = ε(θ), i.e., absorption and emission are reciprocal. This means that the emission from the detector always points back to the source, for example towards a laser source in a guided missile. Thus, detector emission serves as a complementary measure to hide an object. Here, we present a novel camouflage detector that uses a nonreciprocal metasurface array to independently detect the direction of the incoming radiation as well as manipulate its emissivity response. This is accomplished by using a magneto-optical material called indium arsenide (InAs), which breaks Lorentz reciprocity and Kirchhoff's fundamental law such that α(θ) ≠ ε(θ). This design results in the following absorption and emission: α(θ) = ε(-θ). Nine metasurfaces were designed, optimized, and operated at different incident angles from +50° to -50° at a wavelength of 13 μm. Furthermore, by keeping all metasurfaces in a pixilated array form, one could make a device that works over the full ±50° range. Potentially, this array of nonreciprocal metasurfaces can be used to fabricate thermal emitters or solar-harvesting systems.

Ultracompact Single-Photon Sources of Linearly Polarized Vortex Beams

Ultracompact chip-integrated single-photon sources of collimated beams with polarization-encoded states are crucial for integrated quantum technologies. However, most of currently available single-photon sources rely on external bulky optical components to shape the polarization and phase front of emitted photon beams. Efficient integration of quantum emitters with beam shaping and polarization encoding functionalities remains so far elusive. Here, ultracompact single-photon sources of linearly polarized vortex beams based on chip-integrated quantum emitter-coupled metasurfaces are presented, which are meticulously designed by fully exploiting the potential of nanobrick-arrayed metasurfaces. The authors first demonstrate on-chip single-photon generation of high-purity linearly polarized vortex beams with prescribed topological charges of 0, - 1, and +1. The multiplexing of single-photon emission channels with orthogonal linear polarizations carrying different topological charges are further realized and their entanglement is demonstarated. The work illustrates the potential and feasibility of ultracompact quantum emitter-coupled metasurfaces as a new quantum optics platform for realizing chip-integrated high-dimensional single-photon sources.

Metasurface Holography with Multiplexing and Reconfigurability

Metasurface holography offers significant advantages, including a broad field of view, minimal noise, and high imaging quality, making it valuable across various optical domains such as 3D displays, VR, and color displays. However, most passive pure-structured metasurface holographic devices face a limitation: once fabricated, as their functionality remains fixed. In recent developments, the introduction of multiplexed and reconfigurable metasurfaces breaks this limitation. Here, the comprehensive progress in holography from single metasurfaces to multiplexed and reconfigurable metasurfaces is reviewed. First, single metasurface holography is briefly introduced. Second, the latest progress in angular momentum multiplexed metasurface holography, including basic characteristics, design strategies, and diverse applications, is discussed. Next, a detailed overview of wavelength-sensitive, angle-sensitive, and polarization-controlled holograms is considered. The recent progress in reconfigurable metasurface holography based on lumped elements is highlighted. Its instant on-site programmability combined with machine learning provides the possibility of realizing movie-like dynamic holographic displays. Finally, we briefly summarize this rapidly growing area of research, proposing future directions and potential applications.

All-silicon polarization-independent broadband achromatic metalens designed for the mid-wave and long-wave infrared

Metasurfaces demonstrate excellent capabilities in manipulating the phase, amplitude and polarization of light. Metalens, as a typical kind of metasurface devices, shows great prospect in simplifying imaging systems. However, like diffractive optical elements, intrinsic dispersion of metasurfaces is high. Thus, significant chromatic aberration is present in common metalenses, deteriorating imaging quality under broadband illumination condition and limiting their applications. To tackle this problem, broadband achromatic metalenses have been proposed and demonstrated in the visible and near-infrared wavelength regions so far. However, broadband achromatic metalens working in the mid-wave and long-wave infrared is still rare. In this paper, thanks to the ingenious design of meta-units that provide the required local phase and phase dispersion, several all-silicon broadband achromatic metalenses working in the mid-wave infrared (3-5 µm) or long-wave infrared (8-14 µm) wavelengths are proposed. Numerical simulation results demonstrate that the designed broadband achromatic metalenses can provide a near-constant focal length with small deviations and an average focusing efficiency of about 70% over the whole operation bandwidths. In addition, these metalenses hold near diffraction-limited focusing capability and polarization-independent focusing features. The achromatic metalenses proposed here are beneficial for improving imaging quality under broadband illumination and increasing detection efficiency of mid-wave and long-wave infrared detection systems.

Optical fiber-integrated achromatic metalens based on catenary metasurfaces

A challenge in all-fiber-integrated metasurface devices is to efficiently control dispersion in the limited fiber end area to build metasurfaces, therefore, the design of metasurfaces with a special structure becomes crucial to meet the demands of dispersion control. A unique phase response of circularly polarized light in catenary metasurfaces can offer new opportunities for polarization-sensitive arbitrary chromatic dispersion control. Herein, we proposed an optical achromatic metalens based on equal width catenary metasurfaces integrated on the large-mode optical fiber (LMF) end. To reduce phase distortions, the LMF is designed to generate quasi-plane waves (QPW), and then QPW converts from catenary metasurfaces to realize achromatic focusing. A notable feature of this device is its axial focal length shift as low as 0.09% across the working wavelength range from 1.33 µm to 1.55 µm, commonly used in optical fiber communication, demonstrating its excellent dispersion control capability. Furthermore, the device exhibits exceptional capabilities to break through the diffraction limit of the output field. This research has potential applications in the fields of achromatic devices, chromatic aberration correction, fiber lasers, and optical communication and modulation.

Varifocal Metalenses: Harnessing Polarization-Dependent Superposition for Continuous Focal Length Control

Traditional varifocal lenses are bulky and mechanically complex. Emerging active metalenses promise compactness and design flexibility but face issues like mechanical tuning reliability and nonlinear focal length tuning due to additional medium requirements. In this work, we propose a varifocal metalens design based on superimposing light intensity distributions from two orthogonal polarization states. This approach enables continuous and precise focal length control within the visible spectrum, while maintaining relatively high focusing efficiencies (∼41% in simulation and ∼28% in measurement) and quality. In experimental validation, the metalens exhibited flexible tunability, with the focal length continuously adjustable between two spatial positions upon variation of the incident polarization angle. The MTF results showed high contrast reproduction and sharp imaging, with a Strehl ratio of >0.7 for all polarization angles. With compactness, design flexibility, and high focusing quality, the proposed varifocal metalens holds potential for diverse applications, advancing adaptive and versatile optical devices.

Full-space wavefront control enabled by a bilayer metasurface sandwiching 1D photonic crystal

Metasurfaces, composed of sub-wavelength structures, have a powerful capability to manipulate light propagations. However, metasurfaces usually work either in pure reflection mode or pure transmission mode. Achieving full-space manipulation of light at will in the optical region is still challenging. Here we propose a design method of full-space meta-device containing a bilayer metasurface sandwiching 1D photonic crystal to manipulate the transmitted and reflected wave independently. To provide a proof-of-concept demonstration, a device is proposed to show the light focusing in transmission and a vortex beam in reflection. Meanwhile, a device focusing the reflected light with oblique 45° incidence and the transmitted light with normal incidence is designed to indicate its application potential in augmented reality (AR) application. Our design provides a promising way to enrich the multifunctional meta-devices for potential applications.

Disordered metasurface enabled single-shot full-Stokes polarization imaging leveraging weak dichroism

Polarization, one of the fundamental properties of light, is critical for certain imaging applications because it captures information from the scene that cannot directly be recorded by traditional intensity cameras. Currently, mainstream approaches for polarization imaging rely on strong dichroism of birefringent crystals or artificially fabricated structures that exhibit a high diattenuation typically exceeding 99%, which corresponds to a polarization extinction ratio (PER) >~100. This not only limits the transmission efficiency of light, but also makes them either offer narrow operational bandwidth or be non-responsive to the circular polarization. Here, we demonstrate a single-shot full-Stokes polarization camera incorporating a disordered metasurface array with weak dichroism. The diattenuation of the metasurface array is ~65%, which corresponds to a PER of ~2. Within the framework of compressed sensing, the proposed disordered metasurface array serves as an efficient sensing matrix. By incorporating a mask-aware reconstruction algorithm, the signal can be accurately recovered with a high probability. In our experiments, the proposed approach exhibits high-accuracy full-Stokes polarimetry and high-resolution real-time polarization imaging. Our demonstration highlights the potential of combining meta-optics with reconstruction algorithms as a promising approach for advanced imaging applications.

Geometric Phase in Twisted Topological Complementary Pair

Geometric phase enabled by spin-orbit coupling has attracted enormous interest in optics over the past few decades. However, it is only applicable to circularly-polarized light and encounters substantial challenges when applied to wave fields lacking the intrinsic spin degree of freedom. Here, a new paradigm is presented for achieving geometric phase by elucidating the concept of topological complementary pair (TCP), which arises from the combination of two compact phase elements possessing opposite intrinsic topological charge. Twisting the TCP leads to the generation of a linearly-varying geometric phase of arbitrary order, which is quantified by the intrinsic topological charge. Notably distinct from the conventional spin-orbit coupling-based theories, the proposed geometric phase is the direct result of the cyclic evolution of orbital-angular-momentum transformation in mode space, thereby exhibiting universality across classical wave systems. As a proof of concept, the existence of this geometric phase is experimentally demonstrated using scalar acoustic waves, showcasing the remarkable ability in the precise manipulation of acoustic waves at subwavelength scales. These findings engender a fresh understanding of wave-matter interaction in compact structures and establish a promising platform for exploring geometric phase, offering significant opportunities for diverse applications in wave systems.

Dual-mode vortex beam transmission metasurface antenna based on linear-to-circular polarization converter

The generation of multi-mode vortex beams at the same aperture is currently emerging as a research hotspot. In this paper, a method based on a linearly polarized-circularly polarized translational transmission metasurface (TM) is proposed to enable a dual-circularly polarized dual-mode vortex beam generation. Through the judicious implementation of an additional rotational phase and the combination of the initial transmission phase, the phases of the left-hand circularly polarized (LHCP) and right-hand circularly polarized (RHCP) waves can be manipulated arbitrarily and independently. Meanwhile, the design of the array phase is utilized for the dual-mode dual-circularly polarized beam generation. Simulation and sample measurements provide validation data for the feasibility of this method, in which the measurement results are in excellent consistency with the simulation ones. This proposed method paves the way toward the enhancement of the channel capacity of mobile communication.

Optical reflective metasurfaces enable spin-decoupled OAM and focusing

Due to the physically unrestricted set of orthogonally helical modes of orbital angular momentum (OAM), it has contributed significantly to wireless communication and information capacity. Meanwhile, focusing has important applications in fields such as super-resolution microscopic imaging and optical integration. Plasmonic metasurfaces have a powerful ability to modulate electromagnetic (EM) waves, and diversified functionalities in them are strongly desired. As of today, few plasmonic metasurfaces are reported which have multi-function in a single flat device. Herein, by fine-tuning the geometric dimensions and orientation angle of the meta-atom, the geometric phase is combined with the propagation phase to produce an independent phase response when left-handed circular polarization (LCP) and right-handed circular polarization (RCP) waves illuminate the metasurface. This paper presents three plasmonic metasurfaces, and each of them implements multiple functions on a single plasmonic metasurface. Firstly, normal reflection of OAM and a focused beam is achieved. Secondly, we realize anomalous reflection of OAM by convolving a gradient sequence and implement computational focusing at any point. Finally, addition theorem is adopted to implement the above two functions, and this design contains normal and inclined output beams. Our work provides novel approaches for the integration of multifunctional EM modulation.

Design of a Metasurface with Long Depth of Focus Using Superoscillation

Longitudinal optical field modulation is very important for applications such as optical imaging, spectroscopy, and optical manipulation. It can achieve high-resolution imaging or manipulation of the target object, but it is also limited by its depth of focus. The depth of focus determines whether the target object can be clearly imaged or manipulated at different distances, so extending the depth of focus can improve the adaptability and flexibility of the system. However, how to extend the depth of focus is still a significant challenge. In this paper, we use a super-oscillation phase modulation optimization method to design a polarization-independent metalens with extended focal depth, taking the axial focal depth length as the optimization objective. The optimized metalens has a focal depth of 13.07 μm (about 22.3 λ), and in the whole focal depth range, the transverse full width at half maximum values are close to the Rayleigh diffraction limit, and the focusing efficiency is above 10%. The results of this paper provide a new idea for the design of a metalens with a long focal depth and may have application value in imaging, lithography, and detection.

Miniaturized solar-blind ultraviolet imaging system enabled by a diffractive/refractive hybrid

In this paper, we demonstrated a miniaturized diffractive/refractive hybrid system based on a diffractive optical element and three refractive lenses to achieve solar-blind ultraviolet imaging within a range of 240-280 nm. We experimentally demonstrate the optical system has both outstanding resolution and excellent imaging capability. The experiments demonstrate that the system could distinguish the smallest line pair with a width of 16.7 µm. The modulation transfer function (MTF) at the target maximum frequency (77 lines pair/mm) is great than 0.76. The strategy provides significant guidance for the mass production of solar-blind ultraviolet imaging systems towards miniaturization and lightweight.

Meta-optics empowered vector visual cryptography for high security and rapid decryption

Optical encryption is a promising approach to protecting secret information owing to the advantages of low-power consumption, parallel, high-speed, and multi-dimensional processing capabilities. Nevertheless, conventional strategies generally suffer from bulky system volume, relatively low security level, redundant measurement, and/or requirement of digital decryption algorithms. Here, we propose a general optical security strategy dubbed meta-optics-empowered vector visual cryptography, which fully exploits the abundant degrees of freedom of light as well as the spatial dislocation as key parameters, significantly upgrading the security level. We also demonstrate a decryption meta-camera that can implement the reversal coding procedure for real-time imaging display of hidden information, avoiding redundant measurement and digital post-processing. Our strategy features the merits of a compact footprint, high security, and rapid decryption, which may open an avenue for optical information security and anti-counterfeiting.

High Performance Graphene-C60 -Bismuth Telluride-C60 -Graphene Nanometer Thin Film Phototransistor with Adjustable Positive and Negative Responses

Graphene is a promising candidate for the next-generation infrared array image sensors at room temperature due to its high mobility, tunable energy band, wide band absorption, and compatibility with complementary metal oxide semiconductor process. However, it is difficult to simultaneously obtain ultrafast response time and ultrahigh responsivity, which limits the further improvement of graphene photoconductive devices. Here, a novel graphene/C₆₀ /bismuth telluride/C₆₀ /graphene vertical heterojunction phototransistor is proposed. The response spectral range covers 400-1800 nm; the responsivity peak is 10⁶ A W^-1 ; and the peak detection rate and peak response speed reach 10¹⁴ Jones and 250 µs, respectively. In addition, the regulation of positive and negative photocurrents at a gate voltage is characterized and the ionization process in impurities of the designed phototransistor at a low temperature is analyzed. Tunable bidirectional response provides a new degree of freedom for phototransistors' signal resolution. The analysis of the dynamic change process of impurity energy level is conducted to improve the device's performance. From the perspective of manufacturing process, the ultrathin phototransistor (20-30 nm) is compatible with functional metasurface to realize wavelength or polarization selection, making it possible to achieve large-scale production of integrated spectrometer or polarization imaging sensor by nanoimprinting process.

Generation of Perfect Electron Vortex Beam with a Customized Beam Size Independent of Orbital Angular Momentum

The electron vortex beam (EVB)-carrying quantized orbital angular momentum (OAM) plays an essential role in a series of fundamental research. However, the radius of the transverse intensity profile of a doughnut-shaped EVB strongly depends on the topological charge of the OAM, impeding its wide applications in electron microscopy. Inspired by the perfect vortex in optics, herein, we demonstrate a perfect electron vortex beam (PEVB), which completely unlocks the constraint between the beam size and the beam's OAM. We design nanoscale holograms to generate PEVBs carrying different quanta of OAM but exhibiting almost the same beam size. Furthermore, we show that the beam size of the PEVB can be readily controlled by only modifying the design parameters of the hologram. The generation of PEVB with a customized beam size independent of the OAM can promote various in situ applications of free electrons carrying OAM in electron microscopy.

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

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

Full-space wavefront manipulation enabled by asymmetric photonic spin-orbit interactions

Optical metasurfaces empower complete wavefront manipulation of electromagnetic waves and have been found in extensive applications, whereas most of them work in either transmission or reflection space. Here, we demonstrate that two independent and arbitrary phase profiles in transmission and reflection spaces could be produced by a monolayer all-dielectric metasurface based on the asymmetric photonic spin-orbit interactions, realizing full-space wavefront independent manipulation. Furthermore, the supercell-based non-local approach is employed to suppress crosstalk between adjacent nanopillars in one supercell for broadband and high-efficiency wavefront manipulation in full space. Compared with the conventional unit cell-based local approach, such a method could improve efficiency about 10%. As a proof of concept, two metadevices are designed, in which the maximum diffraction efficiencies are ∼95.53%/∼74.07% within the wavelength range of 1500-1600 nm in reflection/transmission space under circularly polarized light incidence. This configuration may offer an efficient way for 2π-space holographic imaging, augmented reality, virtual reality technologies, three-dimensional imaging, and so forth.

Actively tunable linear and circular dichroic metamirrors based on single-layer graphene

Aiming at the problems of low efficiency, single function and complex structure of the existing dichroic metamirrors, the actively tunable linear and circular dichroic metamirrors based on single-layer graphene are proposed in this study. The designed metamirrors are mainly composed of the ion-gel, patterned graphene, polyimide, polysilicon and gold substrates. The anisotropy of the achiral structures can be used to realize circular dichroism (0.8) and linear dichroism (0.9) in two directions at the same time without functional switching. Additionally, the incidence angle of electromagnetic waves, rather than the structural chirality, is used to create the exceptionally strong dichroism. The proposed metamirrors not only increase the integration, but also reduce the angular dispersion and complexity of the structure. What's more, by changing the Fermi level of graphene, the CD function of the metamirrors can be tuned in the range of 0 - 0.8, and the LD function can be tuned in the range of 0.22 - 0.9. The designed metamirrors can achieve dual functions under a wide range of incident angles, and can be widely used in various fields such as terahertz imaging, biological detection, optical sensing, and spectrometry.

Fiber-integrated catenary ring-array metasurfaces for beam shaping

Catenary is referred to as "the real mathematical and mechanical form" in the architectural field. Because of the unique phase control characteristic of the catenary, it has excellent ability in optical manipulation. Here, we propose an optical waveform conversion device based on optical fiber-integrated catenary ring-array metasurfaces. The device consists of a cascade structure of a single-mode fiber (SMF) and a graded-index fiber (GIF). At the GIF end, two kinds of catenary ring-array metasurfaces are introduced to realize beam shaping from Gaussian beam (GB) to Bessel beam. The device can selectively generate a focused or non-diffracting Bessel beam by changing the circular polarization state of the incident light. It is worth noting that under some parameters of the device, the output Bessel beam can break through the diffraction limit, which has potential applications in the fields of optical imaging, optical communication, and optical trapping.

Sub-wavelength patterned pulse laser lithography for efficient fabrication of large-area metasurfaces

Rigorously designed sub-micrometer structure arrays are widely used in metasurfaces for light modulation. One of the glaring restrictions is the unavailability of easily accessible fabrication methods to efficiently produce large-area and freely designed structure arrays with nanoscale resolution. We develop a patterned pulse laser lithography (PPLL) approach to create structure arrays with sub-wavelength feature resolution and periods from less than 1 μm to over 15 μm on large-area thin films with substrates under ambient conditions. Separated ultrafast laser pulses with patterned wavefront by quasi-binary phase masks rapidly create periodic ablated/modified structures by high-speed scanning. The gradient intensity boundary and circular polarization of the wavefront weaken diffraction and polarization-dependent asymmetricity effects during light propagation for high uniformity. Structural units of metasurfaces are obtained on metal and inorganic photoresist films, such as antennas, catenaries, and nanogratings. We demonstrate a large-area metasurface (10 × 10 mm²) revealing excellent infrared absorption (3–7 μm), which comprises 250,000 concentric rings and takes only 5 minutes to produce.

Fabrication of metasurfaces with nanoscale structures is inefficient for large areas. Here, the authors introduce patterned pulse laser lithography for creating structured arrays with sub-wavelength feature on large-area thin films under ambient conditions.

Metasurfaces for Amplitude-Tunable Superposition of Plasmonic Orbital Angular Momentum States

The superposition of orbital angular momentum (OAM) in a surface plasmon polariton (SPP) field has attracted much attention in recent years for its potential applications in classical physics problems and quantum communications. The flexible adjustment of the amplitudes of two OAM states can provide more freedom for the manipulation of superposed states. Here, we propose a type of plasmonic metasurface consisting of segmented spiral-shaped nanoslits that not only can generate the superposition of two OAM states with arbitrary topological charges (TCs), but also can independently modulate their relative amplitudes in a flexible manner. The TCs of two OAM states can be simultaneously modulated by incident light, the rotation rate of the nanoslits, and the geometric parameters of the segmented spiral. The relative amplitudes of the two OAM states are freely controllable by meticulously tuning the width of the nanoslits. Under a circularly polarized beam illumination, two OAM states of opposite TCs can be superposed with various weightings. Furthermore, hybrid superposition with different TCs is also demonstrated. The presented design scheme offers an opportunity to develop practical plasmonic devices and on-chip applications.

Automatic Alignment of an Orbital Angular Momentum Sorter in a Transmission Electron Microscope Using a Convolutional Neural Network

We report on the automatic alignment of a transmission electron microscope equipped with an orbital angular momentum sorter using a convolutional neural network. The neural network is able to control all relevant parameters of both the electron-optical setup of the microscope and the external voltage source of the sorter without input from the user. It can compensate for mechanical and optical misalignments of the sorter, in order to optimize its spectral resolution. The alignment is completed over a few frames and can be kept stable by making use of the fast fitting time of the neural network.

Generation of scalar/vectorial vortex beams by using the plasmonic metasurfaces

Scalar and vector vortex beams are characterized of a helical wavefront but different polarized states, which result in different applications. In this paper, we design and fabricate a plasmonic metasurface based on the geometric phase principle. The designed metasurfaces are capable of generating a scalar vortex beam with a topological charge of ±2 and a vectorial vortex beam with a topological charge of ±1 in the near-infrared band. The experimental results are in good agreement with the simulation results, and our work provides a new idea for the development of a multivortex beam converter.

Enabling Active Nanotechnologies by Phase Transition: From Electronics, Photonics to Thermotics

Phase transitions can occur in certain materials such as transition metal oxides (TMOs) and chalcogenides when there is a change in external conditions such as temperature and pressure. Along with phase transitions in these phase change materials (PCMs) come dramatic contrasts in various physical properties, which can be engineered to manipulate electrons, photons, polaritons, and phonons at the nanoscale, offering new opportunities for reconfigurable, active nanodevices. In this review, we particularly discuss phase-transition-enabled active nanotechnologies in nonvolatile electrical memory, tunable metamaterials, and metasurfaces for manipulation of both free-space photons and in-plane polaritons, and multifunctional emissivity control in the infrared (IR) spectrum. The fundamentals of PCMs are first introduced to explain the origins and principles of phase transitions. Thereafter, we discuss multiphysical nanodevices for electronic, photonic, and thermal management, attesting to the broad applications and exciting promises of PCMs. Emerging trends and valuable applications in all-optical neuromorphic devices, thermal data storage, and encryption are outlined in the end.

Catenary optics: a perspective of applications and challenges

Catenary optics is an emerging direction of subwavelength optics, which is indispensable in describing the electric fields and dispersion property of coupled metallic subwavelength structures, and designing broadband high-efficiency geometric-phase metasurfaces. It involves catenary optical fields and catenary structures, in which both ordinary and equal-length catenary functions play important roles. In recent years, catenary optics has realized a variety of exotic phenomena and optical applications, including broadband photonic spin-Hall effect, super-resolution lithography, broadband absorbers, and extreme-angle imaging. Here, we discuss developments of catenary optics, including a brief history, physical concept and properties, applications, and future perspectives.

Broadband transparent and high-Q resonant polarization meta-grating enabled by a non-local geometric-phase metasurface

Spatial wavefront control and high-Q spectral filtering are both of great importance for various optical applications, such as eye-tracking for eyewear, planar optical modulators, and optical sensing. However, it is a great challenge to simultaneously satisfy these two functionalities in a metasurface due to the inevitable conflicts of local and non-local modes, where local modes of a single meta-atom manipulate the wavefront in a broadband range, while non-local collective modes of extended meta-atoms only support high-Q resonances at certain characteristic wavelengths. Here, we demonstrate a low-contrast dielectric non-local meta-grating that provides both spatial and spectral control of light in a broadband range of 700-1600 nm, offering elaborate wavefront shaping only for narrow-band resonances. Such counterintuitive functionality is supported by spatially tailored dark modes (quasi-bound states in the continuum) encoding with spatially varying geometric phases, while low-contrast dielectric provides broadband non-resonant transmission. Moreover, a broadband transparent polarization meta-grating with two resonance wavelengths is presented. Non-local geometric-phase metasurfaces open an exciting avenue for wavefront shaping and spectral manipulation, and may have potential applications in sensing, lasing, and spectral filtering.

Broadband and high-efficiency photonic spin-Hall effect with all-metallic metasurfaces

In this paper, all-metallic reflective metasurfaces comprising S-shape streamline structures are proposed to achieve the photonic spin-Hall effect with average cross-polarization conversion efficiency exceeding ∼84% in the range of 8-14 µm. By comparing with all-metallic nanobricks, it is demonstrated that the electric field coupling could be enhanced by constructing a similar split ring resonator between adjacent unit elements to further improve its efficiency and bandwidth. As a proof of concept, the photonic spin Hall effect and spin-to-orbit angular momentum conversion could be observed by two metadevices with the maximum diffraction efficiency of ∼95.7%. Such an all-metallic configuration may provide a platform for various high-efficiency electromagnetic components, catenary optics, and practical applications.

Generation of A Space-Variant Vector Beam with Catenary-Shaped Polarization States

We demonstrate the generation of a space-variant vector beam with catenary-shaped polarization states based on the polarization interferometry. With a spatial light modulator and a common path interferometric configuration, two orthogonally circularly polarized beams with different phase modulation overlap each other, yielding the vector beams. In addition, the polarization states of this vector beam are scalable to the arbitrary spatial distribution because of its great flexibility and universal applicability. It is expected that this vector beam may have many potential and intriguing applications in the micro/nano material processing, liquid crystal elements fabrication and optical micro-manipulation, and so on.

Controllable perfect optical vortex generated by complex amplitude encoding

We propose a new paradigm for generating the perfect optical vortex (POV) with a controlled structure and orbital angular momentum (OAM) distribution in the focal region of a tightly focused system. The superiority of the proposed technique is demonstrated with an experiment involving the dynamic manipulation of small particles. This technique for creating the POV could open new routes to optical manipulation based on OAM.

Dual-band chiral metasurface for independent controls of spin-selective reflections

The development of chiral metasurfaces with spin-selective reflection or transmission provides a new way to control the circularly polarized (CP) waves. However, it is still a great challenge to independently manipulate the polarization, frequency, and phase of the spin-selective reflected waves in different operating bands, which may have potential applications in improving the data capacity of microwave and optical communication systems. Here, a dual-band chiral metasurface is proposed to generate gigantic intrinsic chirality with strong circular dichroism (CD) in two different frequency bands by piecing two typical mono-chiral units together. The polarization, frequency and phase of the spin-selective reflected waves can also be independently designed in the two operating bands by adjusting the configuration of the chiral unit structures. Based on the proposed chiral structures, a dual-band chiral metasurface with spin-selective anomalous reflections is designed and demonstrated by both simulations and experiments. The results show that the polarization of spin-selective reflected waves can be customized by selecting appreciate chiral structures, while the wavefront of the spin-selective reflected waves can be further controlled by designing their arrangement.

Emerging Long-Range Order from a Freeform Disordered Metasurface

Recently, disordered metasurfaces have attracted considerable interest due to their potential applications in imaging, holography, and wavefront shaping. However, how to emerge long-range ordered phase distribution in disordered metasurfaces remains an outstanding problem. Here, a general framework is proposed to generate a spatially homogeneous in-plane phase distribution from a disordered metasurface, by engineering disorder parameters together with topology optimization. As a proof-of-concept demonstration, an all-dielectric disordered supercell metasurface with relatively homogeneous in-plane phase fluctuation is designed by disorder parameter engineering, manifesting as polarization conversion-dependent random scattering or unidirectional transmission. Then, a topology optimization approach is utilized to overcome the lattice coupling effect and to further improve the homogeneity of complex electric field fluctuation. In comparison with the initial supercell metasurface, both the phase fluctuation range and the relative efficiency of the topology-optimized freeform metasurface are significantly improved, leading to a long-range ordered electric field distribution. Moreover, three experimental realizations are performed, all of which agree well with the theoretical results. This methodology may inspire more exotic optical phenomena and find more promising applications in disordered metasurfaces and disordered optics.

Single-layered phase-change metasurfaces achieving efficient wavefront manipulation and reversible chiral transmission

Efficient control of the phase and polarization of light is of significant importance in modern optics and photonics. However, traditional methods are often accompanied with cascaded and bulky designs that cannot fulfill the ongoing demand for further integrations. Here, a single-layered metasurface composed of nonvolatile phase-change material Ge₂Sb₂Se₄Te₁ (GSST) is proposed with tunable spin-orbit interactions in subwavelength scale. According to the spin-dependent destructive or constructive interference, asymmetric transmission for circularly polarized incidence (extinction ratio > 8:1) can be achieved when GSST is in an amorphous state. Moreover, when GSST changes to crystalline state, reversed chiral transmission (extinction ratio > 12:1) can be observed due to the existence of intrinsic chirality. In addition, as the average cross-polarized transmitted amplitude is larger than 85%, arbitrary wavefront manipulations can be achieved in both states simultaneously based on the theory of Pancharatnam-Berry phase. As a proof of concept, several functional metasurface devices are designed and characterized to further demonstrate the validation of our design methodology. It is believed that these multifunctional devices with ultrahigh compactness are promising for various applications including chiroptical spectroscopy, EM communication, chiral imaging, and information encryption.

Ultra-Broadband Polarization Conversion Metasurface with High Transmission for Efficient Multi-Functional Wavefront Manipulation in the Terahertz Range

Recently, terahertz (THz) wireless communication has been widely investigated as the future prospect of wireless network architecture. However, most of the natural existing materials are inapplicable for THz devices, which hinder their further development. To promote the integration and channel capacity of the THz wireless communication systems, an ultrabroadband polarization conversion metasurface for efficient multi-functional wavefront manipulation is proposed. The designed metasurface is composed of an arrow-type structure sandwiched by a pair of orthogonal gratings, which can induce the Fabry-Pérot-like cavity for improving the transmission. Simulated results indicate that the transmission coefficient of the cross-polarization metasurface is higher than 90% from 0.73 THz to 2.24 THz, and the corresponding polarization conversion ratio is greater than 99.5%. Moreover, the phase coverage of 0–2π at operation frequency can be easily obtained by altering the geometric parameter of the metasurface. To demonstrate the concept of wavefront manipulation, anomalous refraction, focusing metalens, and vortex beam generation are investigated in detail. All of these applications exhibit a remarkable performance of the proposed metasurface that has great potential in prompting the efficient, broadband and compact systems for THz wireless communication.

Phase gradient protection of stored spatially multimode perfect optical vortex beams in a diffused rubidium vapor

We experimentally investigate the optical storage of perfect optical vortex (POV) and spatially multimode perfect optical vortex (MPOV) beams via electromagnetically induced transparency (EIT) in a hot vapor cell. In particular, we study the role that phase gradients and phase singularities play in reducing the blurring of the retrieved images due to atomic diffusion. Three kinds of manifestations are enumerated to demonstrate such effect. Firstly, the suppression of the ring width broadening is more prominent for POVs with larger orbital angular momentum (OAM). Secondly, the retrieved double-ring MPOV beams' profiles present regular dark singularity distributions that are related to their vortex charge difference. Thirdly, the storage fidelities of the triple-ring MPOVs are substantially improved by designing line phase singularities between multi-ring MPOVs with the same OAM number but π offset phases between adjacent rings. Our experimental demonstration of MPOV storage opens new opportunities for increasing data capacity in quantum memories by spatial multiplexing, as well as the generation and manipulation of complex optical vortex arrays.

Effect of microstructures on temperature and stress distributions of an irradiated alumina surface

Three types of alumina surface irradiated by laser are simulated in this study to investigate stray light ablation. Results indicate that temperature fields of triangular and rectangular microstructures exhibit the "head effect," while overall still exhibit Gaussian distributions. For the stress, there is a notable difference between the microstructure surface and the ideal surface. The most stress concentration occurs at the corners on the microstructure surface termed as the "bottom effect." The maximum tensile stress of a triangular microstructure appears below the midline of the slope. The location of the maximum tensile stress on the triangle first shifts down and then up. The inflection point is 0.9 µm in height of the triangle.

Ultra-dense perfect optical orbital angular momentum multiplexed holography

Optical orbital angular momentum (OAM) has been recently implemented in holography technologies as an independent degree of freedom for boosting information capacity. However, the holography capacity and fidelity suffer from the limited space-bandwidth product (SBP) and the channel crosstalk, albeit the OAM mode set exploited as multiplexing channels is theoretically unbounded. Here, we propose the ultra-dense perfect OAM holography, in which the OAM modes are discriminated both radially and angularly. As such, the perfect OAM mode set constructs the two-dimensional spatial division multiplexed holography (conventional OAM holography is 1D). The extending degree of freedom enhances the holography capacity and fidelity. We have demonstrated an ultra-fine fractional OAM holography with the topological charge resolution of 0.01. A 20-digit OAM-encoded holography encryption has also been exhibited. It harnesses only five angular OAM topological charges ranging from -16 to +16. The SBP efficiency is about 20 times larger than the conventional phase-only OAM holography. This work paves the way to compact, high-security and high-capacity holography.

Photonic orbital angular momentum with controllable orientation

Vortices are whirling disturbances, commonly found in nature, ranging from tremendously small scales in Bose-Einstein condensations to cosmologically colossal scales in spiral galaxies. An optical vortex, generally associated with a spiral phase, can carry orbital angular momentum (OAM). The optical OAM can either be in the longitudinal direction if the spiral phase twists in the spatial domain or in the transverse direction if the phase rotates in the spatiotemporal domain. In this article, we demonstrate the intersection of spatiotemporal vortices and spatial vortices in a wave packet. As a result of this intersection, the wave packet hosts a tilted OAM that provides an additional degree of freedom to the applications that harness the OAM of photons.

Catenary-based phase change metasurfaces for mid-infrared switchable wavefront control

Active wave manipulation by ultracompact meta-devices is highly embraced in recent years, but a major concern still exists due to the lack of functional reconfigurability. Moreover, the phase or amplitude discontinuities introduced by collective response of discrete meta-atoms make current meta-devices far from practical applications. Here, we demonstrate actively tunable wavefront control with high-efficiency by combining catenary-based meta-atoms for intrinsic continuous phase regulation with the chalcogenide phase change material (PCM) of Ge₂Sb₂Te₅. First, switchable beam deflection is demonstrated in a wide mid-IR range between 8 μm and 9.5 μm with 'on' and 'off' states for beam steering between anomalous and normal specular reflections. Second, a switchable meta-axicon for zero order Bessel beam generation is demonstrated with full width at half maximum (FWHM) as small as ∼0.41 λ (λ = 12 µm). As a result, our scheme for active and continuous phase control potentially paves an avenue to construct active photonic devices especially for applications where large contrast ratio is highly desirable, such as optoelectronic integration, wavefront engineering and so on.

Polarization-dependent spatial channel multiplexing dynamic hologram in the visible band

In this work, we propose dynamic holography based on metasurfaces combining spatial channel multiplexing and polarization multiplexing. In this design, spatial channels can provide up to 3^N holographic frames, which not only increase the possibility of dynamic control but also increase the privacy of the holographic display. This design is also sensitive to polarization, so it further expands the spatial channel capacity. For the left and right circular polarization incident light, there are different dynamic pixel schemes. Therefore, this approach holds promise in the holographic display, optical storage, optics communication, optical encryption, and information processing.

Generalized Pancharatnam-Berry Phase in Rotationally Symmetric Meta-Atoms

Pancharatnam-Berry geometric phase has attracted enormous interest in subwavelength optics and electromagnetics during the past several decades. Traditional theory predicts that the geometric phase is equal to twice the rotation angle of anisotropic elements. Here, we show that high-order geometric phases equal to multiple times the rotation angle could be achieved by meta-atoms with highfold rotational symmetries. As a proof of concept, the broadband angular spin Hall effect of light and optical vortices is experimentally demonstrated by using plasmonic metasurfaces consisting of space-variant nanoapertures with C2, C3, and C5 rotational symmetries. The results provide a fundamentally new understanding of the geometric phase as well as light-matter interaction in nanophotonics.

Reconfigurable Continuous Meta-Grating for Broadband Polarization Conversion and Perfect Absorption

As promising building blocks for functional materials and devices, metasurfaces have gained widespread attention in recent years due to their unique electromagnetic (EM) properties, as well as subwavelength footprints. However, current designs based on discrete unit cells often suffer from low working efficiencies, narrow operation bandwidths, and fixed EM functionalities. Here, by employing the superior performance of a continuous metasurface, combined with the reconfigurable properties of a phase change material (PCM), a dual-functional meta-grating is proposed in the infrared region, which can achieve a broadband polarization conversion of over 90% when the PCM is in an amorphous state, and a perfect EM absorption larger than 91% when the PCM changes to a crystalline state. Moreover, by arranging the meta-grating to form a quasi-continuous metasurface, subsequent simulations indicated that the designed device exhibited an ultralow specular reflectivity below 10% and a tunable thermal emissivity from 14.5% to 91%. It is believed that the proposed devices with reconfigurable EM responses have great potential in the field of emissivity control and infrared camouflage.

Reflective Quasi-Continuous Metasurface with Continuous Phase Control for Light Focusing

Benefitting from the arbitrary and flexible light modulation, metasurface has attracted extensive attention and been demonstrated in different applications. However, most reported metasurface-based devices were normally composed of discrete micro or nano structures, therefore the discretization precision limited the performance of the device, including the focusing efficiency, stray light suppression, and broadband performance. In this work, an all-metallic reflective metasurface consisting of numerous quasi-continuous nanostructures is proposed to realize high-efficiency and broadband focusing. The constructed quasi-continuous metasurface (QCMS) is then verified numerically through electromagnetic simulation, and the numerical results show a higher focusing efficiency and a better stray light suppression effect, compared to a binary-phase-based metalens. Through the same design strategy, a QCMS with the ability to overcome the diffraction limit can also be constructed, and a focal spot with the size of 0.8 times the diffraction limit can be achieved. We expect that this quasi-continuous structure could be utilized to construct other high-performance devices that require continuous phase controls, such as the beam deflector, orbital angle momentum generator, and self-accelerating beam generator.

From Lingering to Rift: Metasurface Decoupling for Near- and Far-Field Functionalization

Metasurfaces, simultaneously operating in near- and far-fields, can be employed as a promising candidate to implement different functions, thus significantly improving the information density, security, and system integration. Recent works have showcased some approaches for decoupling-at-large between near- and far-field functionalities, but unfortunately, their coupling effects are just reduced and mitigated to some extent rather than eradicated, which in turn leads to the performance limitation of metadevices. Herein, we propose a general platform for the complete rift between near- and far-field functionalities, enabled by strictly decoupled manipulation of optical amplitude and phase, leading to their distinct functions in the near- and far-fields, respectively. This concept is experimentally demonstrated by integrating the functions of a phase-only metalens and an amplitude-only grayscale-imaging nanoprint into a single-cell metasurface. Because of their completely decoupled functions, both meta-elements show high-performance characteristics, i.e., imaging quality close to the diffraction limit and high-definition grayscale-imaging with resolution as high as 63 500 dots per inch (dpi). The validated recipe may empower advanced explorations and applications in highly integrated nano-optoelectronics requiring high performance and less crosstalk.

Topology-optimized catenary-like metasurface for wide-angle and high-efficiency deflection: from a discrete to continuous geometric phase

We investigate the topology optimization of geometric phase metasurfaces for wide-angle and high-efficiency deflection, where adjoint-based multi-object optimization approach is adopted to improve the absolute efficiency while maintaining the polarization conversion characteristic of geometric phase metasurfaces. We show that, for the initially discrete geometric phase metasurfaces with different materials and working wavelengths, the topology shapes gradually evolve from discrete structures to quasi-continuous arrangements with the increment of optimization iteration operations. More importantly, the finally optimized metasurfaces manifest as catenary-like structure, providing significant improvements of absolute efficiency. Furthermore, for the initial structure with catenary distribution, the corresponding optimized metasurface also has a catenary-like topology shape. Our results on the topology-optimized geometric phase metasurfaces reveal that, from the perspective of numerical optimization, the continuous catenary metasurfaces is superior to the discrete geometric phase metasurfaces.