Polarization Encoded Color Image Embedded in a Dielectric Metasurface

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.

Multilevel Intelligent Anti-Counterfeiting Label with Spatially Selective Dynamic Aurora Response and 3D Mesoscopic Physical Unclonable Function Fingerprint

With the increasing demand for anti-counterfeiting measures, the efficient integration of multi-level optical anti-counterfeiting information has become a critical challenge. In this study, a novel bottom-up self-assembly technique is introduced for fabricating composite integrated films. This method overcomes the size limitations of phosphors that achieve circularly polarized light (CPL) through co-assembly with cellulose nanocrystals. Specifically, rare earth metal-organic frameworks with a length of 140 µm can generate CPL with an asymmetry factor of 0.65. Moreover, the introduction of random defects in the film imparts unpredictable CPL properties, enabling dynamic auroral anti-counterfeiting within the decryption optical path. Additionally, an innovative two-stage serial decryption process is proposed by leveraging the non-correlation between orthogonal decryption patterns. Notably, the label surface features biomimetic fingerprint textures that exhibit 3D physical unclonable functions (PUFs) at the mesoscopic scale. These textures possess high entropy close to the ideal value of 1, and an encoding capacity in a 175 × 175 µm2 area reaches 262500. In summary, the composite label achieves a high degree of integration by combining three levels optical anti-counterfeiting information: full-chromatographic tunable photoluminescence, spatially selective random dynamic aurora responses, and 3D bionic mesoscopic PUFs fingerprints.

2D/3D Geometric Multiplexing via Orthogonal Control of Circularly Polarized Transmission and Long Afterglow Imaging

Multiplexed optical imaging is highly desirable for enhancing information security. However, shaping optically active materials with circularly polarized long afterglow (CPLA) into 3D geometric structures for multiplexing stereoscopic display and multidirectional encryption remains a significant challenge. Herein, a novel strategy is proposed for designing multiplexed encryption patterns using twisted-stacking hierarchical structures that exhibit remarkable optical activity and CPLA properties. The hybrid films display dynamically orthogonal control of circularly polarized transmission patterns in daylight and switchable CPLA images in darkness, both of which can be directly viewed by the naked eye using left- or right-handed circularly polarized filters, and independently modulated without mutual interference during dynamic regulation process. Furthermore, it is demonstrated that this highly integrated platform can be utilized as 3D geometric multimodal image multiplexing toward advanced anti-counterfeiting and information encryption applications.

Programmable electron-induced color router array

The development of color routers (CRs) realizes the splitting of dichromatic components, contributing to the modulation of photon momentum that acts as the information carrier for optical information technology on the frequency and spatial domains. However, CRs with optical stimulation lack active control of photon momentum at deep subwavelength scale because of the optical diffraction limit. Here, we experimentally demonstrate an active manipulation of dichromatic photon momentum at a deep subwavelength scale via electron-induced CRs, where the CRs radiation patterns are manipulated by steering the electron impact position within 60 nm in a single nanoantenna unit. Moreover, an encrypted display device based on programmable modulation of the CR array is designed and implemented. This approach with enhanced security, large information capacity, and high-level integration at a deep subwavelength scale may find applications in photonic devices and emerging areas in quantum information technologies.

Structural Colors Go Active

Structural colors find wide applications for color printing, intelligent display, filtering imaging, etc., owing to their benefits, including high resolution, stable properties, and dynamic tunability. This review first illustrates the mechanisms of structural color generation, such as surface plasmon resonances, localized surface plasmon resonances, Fabry-Perot resonances, Mie resonances, etc. It then proposes the recent technological strategies employed to realize dynamic structural colors. The integration of structural colors with functional materials like phase-change, along with the development of color dynamic control mechanisms such as microfluidic chips, micro-electro-mechanical system drivers, and microheaters, represents key approaches for spectrum regulation. Furthermore, the review assesses the performance, advantages, and limitations of various technologies for dynamic structural colors. Finally, this review concluded with a section on the future challenges and prospects in large-area fabrication, practical applications, and performance improvement. It explains the current typical applications, including smart windows, adaptive camouflage, sensors, etc., and explores the processing methods that can achieve large-area, high-fidelity preparation of structural colors, such as nanoimprint, deep ultraviolet lithography, immersion lithography, laser printing, etc. This field promises advancements in high-density data storage, information encryption, and broader market applications.

3D-architected gratings for polarization-sensitive, nature-inspired structural color

Structural coloration, a color-generation mechanism often found in nature, arises from light-matter interactions such as diffraction, interference, and scattering, with micro- and nanostructured elements. Herein, we systematically study anisotropic, 3D-architected grating structures with polarization-tunable optical properties, inspired by the vivid blue of Morpho butterfly wings. Using two-photon lithography, we fabricate multilayered gratings, varying parameters such as height (through scanning speed and laser power), periodicity, and number of layers. In transmission, significant color transitions from blue to brown were identified when varying structural parameters and incident light polarization conditions (azimuthal angle and ellipticity). Based on thin film diffraction efficiency theory in the Raman-Nath regime, optical characterization results are analytically explained, evaluating the impact of each parameter variation. Overall, these findings contribute to technological implementations of polarization-sensitive, 3D-architected gratings for structural color applications.

Single-cell bilayer design of a terahertz six-channel metasurface for simultaneous holographic and grayscale images

Metasurfaces have exhibited excellent capabilities in controlling main characteristics of electromagnetic fields. Thus, a lot of significant achievements have been attained in many areas especially in the fields of hologram and near-field imaging. However, some of these designs are implemented in a manner of interleaved subarrays that complicates the design and makes them difficult to achieve integration. Here, an innovative stacking technique of metasurface is combined with vanadium dioxide (VO2) to achieve independent imaging of six channels in terahertz band. Our research combines intensity modulation controlled by the Malus's law and phase modulation of geometry and propagation to merge amplitude, phase, and polarization manipulation of electromagnetic wave. A "six-in-one" meta-device is constructed by combining phase change properties of VO2 to realize simultaneous near-field grayscale imaging and far-field holography. This design has advantages of wide bandwidth and low crosstalk. Based on the advantage of low crosstalk, single-cell bilayer design allows the number of independent channels to be doubled within an acceptable error range. The proposed metasurface introduces a fresh viewpoint for the design of multi-purpose meta-devices, and has broad application prospects in information encryption and multi-channel image display.

Stretchable plasmonic metasurfaces for deformation monitoring

Metasurfaces have recently gained significant attention due to the strong capacity in light field manipulation. However, most traditional metasurfaces are fabricated on rigid substrates, which fix their functionality after fabrication and limit their applications in dynamic measurement fields. In this work, we designed and fabricated a silver metasurface embedded in a stretchable substrate for sensing applications. This metasurface can generate different point cloud patterns under varying stretch ratios when illuminated by a laser beam. By collecting and analyzing the patterns, we can precisely reconstruct the deformation of the metasurface. Furthermore, the sample exhibits excellent performance under incident light of various wavelengths. These results pave the way for developing microdevices with novel capabilities based on flexible metamaterials.

Directional Phase and Polarization Manipulation Using Janus Metasurfaces

Janus metasurfaces, exemplifying two-faced 2D metamaterials, have shown unprecedented capabilities in asymmetrically manipulating the wavefront of electromagnetic waves in both forward and backward propagating directions, enabling novel applications in asymmetric information processing, security, and signal multiplexing. However, current Janus metasurfaces only allow for directional phase manipulation, hindering their broader application potential. Here, the study proposes a versatile Janus metasurface platform that can directionally control the phase and polarization of terahertz waves by integrating functionalities of half-wave plates, quarter-wave plates, and metallic gratings within a cascaded metasurface structure. As a proof-of-principle, the study experimentally demonstrates Janus metasurfaces capable of independent and simultaneous control over phase and polarization, showcasing propagation direction-encoded focusing and polarization conversion. Moreover, the directionally focused points are utilized with distinct polarization states for advanced applications in direction- and polarization-sensitive detection and imaging. This unique strategy for simultaneous phase and polarization control with direction-dependent versatility opens new avenues for designing ultra-compact devices with significant implications in imaging, encryption, and data storage.

Dual-band complex-amplitude metasurface empowered high security cryptography with ultra-massive encodable patterns

The significance of a cryptograph method lies in its ability to provide high fidelity, high security, and large capacity. The emergence of metasurface-empowered cryptography offers a promising alternative due to its unparalleled wavefront modulation capabilities and easy integration with traditional schemes. However, the majority of reported strategies suffer from limited capacity as a result of restricted independent information channels. In this study, we present a novel method of cryptography that utilizes a dual-band complex-amplitude meta-hologram. The method allows for the encoding of 225 different patterns by combining a modified visual secret-sharing scheme (VSS) and a one-time-pad private key. The use of complex-amplitude modulation and the modified VSS enhances the quality and fidelity of the decrypted results. Moreover, the transmission of the private key through a separate mechanism can greatly heighten the security, and different patterns can be generated simply by altering the private key. To demonstrate the feasibility of our approach, we design, fabricate, and characterize a meta-hologram prototype. The measured results are in good agreement with the numerical ones and the design objectives. Our proposed strategy offers high security, ultra-capacity, and high fidelity, making it highly promising for applications in information encryption and anti-counterfeiting.

Unlocking ultra-high holographic information capacity through nonorthogonal polarization multiplexing

Contemporary studies in polarization multiplexing are hindered by the intrinsic orthogonality constraints of polarization states, which restrict the scope of multiplexing channels and their practical applications. This research transcends these barriers by introducing an innovative nonorthogonal polarization-basis multiplexing approach. Utilizing spatially varied eigen-polarization states within metaatoms, we successfully reconstruct globally nonorthogonal channels that exhibit minimal crosstalk. This method not only facilitates the generation of free-vector holograms, achieving complete degrees-of-freedom in three nonorthogonal channels with ultra-low energy leakage, but it also significantly enhances the dimensions of the Jones matrix, expanding it to a groundbreaking 10 × 10 scale. The fusion of a controllable eigen-polarization engineering mechanism with a vectorial diffraction neural network culminates in the experimental creation of 55 intricate holographic patterns across these expanded channels. This advancement represents a profound shift in the field of polarization multiplexing, unlocking opportunities in advanced holography and quantum encryption, among other applications.

Programmable Optical Encryption Based on Electrical-Field-Controlled Exciton-Trion Transitions in Monolayer WS2

Optical encryption is receiving much attention with the rapid growth of information technology. Conventional optical encryption usually relies on specific configurations, such as metasurface-based holograms and structure colors, not meeting the requirements of increasing dynamic and programmable encryption. Here, we report a programmable optical encryption approach using WS2/SiO2/Au metal-oxide-semiconductor (MOS) devices, which is based on the electrical-field-controlled exciton-trion transitions in monolayer WS2. The modulation depth of the MOS device reflection amplitude up to 25% related to the excitons ensures the fidelity of information, and the decryption based on the near excitonic resonance assures security. With such devices, we successfully demonstrate their applications in real-time encryption of ASCII codes and visual images. For the latter, it can be implemented at the pixel level. The strategy shows significant potential for low-cost, low-energy-consumption, easily integrated, and high-security programmable optical encryptions.

Arbitrarily rotating polarization direction and manipulating phases in linear and nonlinear ways using programmable metasurface

Independent controls of various properties of electromagnetic (EM) waves are crucially required in a wide range of applications. Programmable metasurface is a promising candidate to provide an advanced platform for manipulating EM waves. Here, we propose an approach that can arbitrarily control the polarization direction and phases of reflected waves in linear and nonlinear ways using a stacked programmable metasurface. Further, we extend the space-time-coding theory to incorporate the dimension of polarization, which provides an extra degree of freedom for manipulating EM waves. As proof-of-principle application examples, we consider polarization rotation, phase manipulation, and beam steering at linear and nonlinear frequencies. For validation, we design, fabricate, and measure a metasurface sample. The experimental results show good agreement with theoretical predictions and simulations. The proposed approach has a wide range of applications in various areas, such as imaging, data storage, and wireless communication.

Angle-Multiplexed 3D Photonic Superstructures with Multi-Directional Switchable Structural Color for Information Transformation, Storage, and Encryption

Creating photonic crystals that can integrate and switch between multiple structural color images will greatly advance their utility in dynamic information transformation, high-capacity storage, and advanced encryption, but has proven to be highly challenging. Here, it is reported that by programmably integrating newly developed 1D quasi-periodic folding structures into a 3D photonic crystal, the generated photonic superstructure exhibits distinctive optical effects that combine independently manipulatable specular and anisotropic diffuse reflections within a versatile protein-based platform, thus creating different optical channels for structural color imaging. The polymorphic transition of the protein format allows for the facile modulation of both folding patterns and photonic lattices and, therefore, the superstructure's spectral response within each channel. The capacity to manipulate the structural assembly of the superstructure enables the programmable encoding of multiple independent patterns into a single system, which can be decoded by the simple adjustment of lighting directions. The multifunctional utility of the photonic platform is demonstrated in information processing, showcasing its ability to achieve multimode transformation of information codes, multi-code high-capacity storage, and high-level numerical information encryption. The present strategy opens new pathways for achieving multichannel transformable imaging, thereby facilitating the development of emerging information conversion, storage, and encryption media using photonic crystals.

Terahertz metalens for generating multi-polarized focal points and images with uniform intensity distributions

Metasurfaces have provided a flexible platform for designing ultracompact metalenses with unusual functionalities. However, traditional multi-foci metalenses are limited to generating circularly polarized (CP) or linearly polarized (LP) focal points, and the intensity distributions are always inhomogeneous/chaotical between the multiple focal points. Here, an inverse design approach is proposed to optimize the in-plane orientation of each meta-atom in a terahertz (THz) multi-foci metalens that can generate multi-polarized focal points with nearly uniform intensity distributions. As a proof-of-principle example, we numerically and experimentally demonstrate an inversely designed metalens for simultaneously generating multiple CP- and LP-based focal points with homogeneous intensity distributions, leading to a multi-polarized image (rather than the holography). Furthermore, the multi-channel and multi-polarized images consisting of multiple focal points with homogeneous intensity distributions are also numerically demonstrated. The unique approach for inversely designing multi-foci metalens that can generate multi-polarized focal points and images with uniform intensity distributions will enable potential applications in imaging and sensing.

Poincaré sphere trajectory encoding metasurfaces based on generalized Malus' law

As a fundamental property of light, polarization serves as an excellent information encoding carrier, playing significant roles in many optical applications, including liquid crystal displays, polarization imaging, optical computation and encryption. However, conventional polarization information encoding schemes based on Malus' law usually consider 1D polarization projections on a linear basis, implying that their encoding flexibility is largely limited. Here, we propose a Poincaré sphere (PS) trajectory encoding approach with metasurfaces that leverages a generalized form of Malus' law governing universal 2D projections between arbitrary elliptical polarization pairs spanning the entire PS. Arbitrary polarization encodings are realized by engineering PS trajectories governed by either arbitrary analytic functions or aligned modulation grids of interest, leading to versatile polarization image transformation functionalities, including histogram stretching, thresholding and image encryption within non-orthogonal PS loci. Our work significantly expands the encoding dimensionality of polarization information, unveiling new opportunities for metasurfaces in polarization optics for both quantum and classical regimes.

Diffractive optical computing in free space

Structured optical materials create new computing paradigms using photons, with transformative impact on various fields, including machine learning, computer vision, imaging, telecommunications, and sensing. This Perspective sheds light on the potential of free-space optical systems based on engineered surfaces for advancing optical computing. Manipulating light in unprecedented ways, emerging structured surfaces enable all-optical implementation of various mathematical functions and machine learning tasks. Diffractive networks, in particular, bring deep-learning principles into the design and operation of free-space optical systems to create new functionalities. Metasurfaces consisting of deeply subwavelength units are achieving exotic optical responses that provide independent control over different properties of light and can bring major advances in computational throughput and data-transfer bandwidth of free-space optical processors. Unlike integrated photonics-based optoelectronic systems that demand preprocessed inputs, free-space optical processors have direct access to all the optical degrees of freedom that carry information about an input scene/object without needing digital recovery or preprocessing of information. To realize the full potential of free-space optical computing architectures, diffractive surfaces and metasurfaces need to advance symbiotically and co-evolve in their designs, 3D fabrication/integration, cascadability, and computing accuracy to serve the needs of next-generation machine vision, computational imaging, mathematical computing, and telecommunication technologies.

Optical computing metasurfaces: applications and advances

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

Terahertz dynamic multichannel holograms generated by spin-multiplexing reflective metasurface

In recent years, metasurfaces have attracted considerable interest for their unprecedented capabilities to manipulate intensity, phase, and polarization of an electromagnetic wave. Although metasurface-based wavefront modulation has achieved numerous successful results, implementation of multifunctional devices in a single metasurface still meet significant challenges. Here, a novel multilayer structure is designed using properties of vanadium dioxide (VO2). Propagation phase and geometric phase are introduced in this structure to achieve multichannel holographic imaging in terahertz band. When the temperature is above 68°C, VO2 becomes a metal and it plays a role in wavefront modulation for terahertz wave. The left-handed channel realizes a hologram letter L and the right-handed channel realizes a hologram letter R. When the temperature is below 68°C, VO2 changes to an insulator, and electromagnetic wave is controlled by gold structures embedded inside a VO2 film. In this case, hologram number 2 is realized in the left-handed channel and hologram number 6 appears in the right-handed channel. Our structure has advantages of low crosstalk, multiple channels, and large bandwidth. This novel design paves a new road for multichannel imaging and information encryption.

Universal Polarization Transformations: Spatial Programming of Polarization Scattering Matrices Using a Deep Learning-Designed Diffractive Polarization Transformer

Controlled synthesis of optical fields having nonuniform polarization distributions presents a challenging task. Here, a universal polarization transformer is demonstrated that can synthesize a large set of arbitrarily-selected, complex-valued polarization scattering matrices between the polarization states at different positions within its input and output field-of-views (FOVs). This framework comprises 2D arrays of linear polarizers positioned between isotropic diffractive layers, each containing tens of thousands of diffractive features with optimizable transmission coefficients. After its deep learning-based training, this diffractive polarization transformer can successfully implement Ni No = 10 000 different spatially-encoded polarization scattering matrices with negligible error, where Ni and No represent the number of pixels in the input and output FOVs, respectively. This universal polarization transformation framework is experimentally validated in the terahertz spectrum by fabricating wire-grid polarizers and integrating them with 3D-printed diffractive layers to form a physical polarization transformer. Through this set-up, an all-optical polarization permutation operation of spatially-varying polarization fields is demonstrated, and distinct spatially-encoded polarization scattering matrices are simultaneously implemented between the input and output FOVs of a compact diffractive processor. This framework opens up new avenues for developing novel devices for universal polarization control and may find applications in, e.g., remote sensing, medical imaging, security, material inspection, and machine vision.

Continuous-Spectrum-Polarization Recombinant Optical Encryption with a Dielectric Metasurface

The Jones matrix, with eight degrees of freedom (DoFs), provides a general mathematical framework for the multifunctional design of metasurfaces. Theoretically, the maximum eight DoFs can be further extended in the spectrum dimension to endow unique encryption capabilities. However, the topology and intrinsic spectral responses of meta-atoms constrains the continuous engineering of polarization evolution over wavelength dimension. In this work, a forward evolution strategy to quickly establish the mapping relationships between the solutions of the dispersion Jones matrix and the spectral responses of meta-atoms is reported. Based on the eigenvector transformation method, arbitrary conjugate polarization channels over the continuous-spectrum dimension are successfully reconstructed. As a proof-of-concept, a silicon metadevice is demonstrated for optical information encryption transmission. Remarkably, the arbitrary combination forms of polarization and wavelength dimension increase the information capacity (210 ), and the measured polarization contrasts of the conjugate polarization conversion are >94% in the entire wavelength range (3-4 µm). It is believed that the proposed approach will benefit secure optical and quantum information technologies.

Twofold optical display and encryption of binary and grayscale images with a wavelength-multiplexed metasurface

The remarkable capability in regulating light polarization or amplitude at the nanoscale makes metasurface a leading candidate in high-resolution image display and optical encryption. Diverse binary or grayscale meta-images were previously shown concealed in a single metasurface, yet they are mostly stored at same encryption level and share an identical decryption key, running the risk of exposing all images once the key is disclosed. Here, we propose a twofold optical display and encryption scheme demonstrating that binary and grayscale meta-images can be concurrently embedded in a nonspatially multiplexed silicon metasurface, and their decryptions demand for drastically different keys. Unlike previous metasurfaces relying on isolated transmission or phase manipulations upon orthogonal linear polarization incidences, this is made possible by exploiting silicon meta-atoms featuring joint transmission amplitude and polarization control at two wavelengths. In detail, the selected two meta-atoms exhibit large polarization-independent transmission difference (∼85 %) at a wavelength of 800 nm, while functioning as the nano-quarter-wave plate at wavelength of 1200 nm. Through elaborate design in simulation, a binary image can be witnessed when the metasurface is merely illuminated by an unpolarized light of wavelength 800 nm or under white light illumination. However, a distinct binary or grayscale image will come into view by inspecting the metasurface with an analyzer and when the incident light is circularly polarized at the wavelength of 1200 nm. Two metasurface samples are fabricated and successfully verified the claims experimentally. The proposed approach is expected to bring new insights to the field of optical display and encryption.

Structural color generation: from layered thin films to optical metasurfaces

Recent years have witnessed a rapid development in the field of structural coloration, colors generated from the interaction of nanostructures with light. Compared to conventional color generation based on pigments and dyes, structural color generation exhibits unique advantages in terms of spatial resolution, operational stability, environmental friendliness, and multiple functionality. Here, we discuss recent development in structural coloration based on layered thin films and optical metasurfaces. This review first presents fundamentals of color science and introduces a few popular color spaces used for color evaluation. Then, it elaborates on representative physical mechanisms for structural color generation, including Fabry-Pérot resonance, photonic crystal resonance, guided mode resonance, plasmon resonance, and Mie resonance. Optimization methods for efficient structure parameter searching, fabrication techniques for large-scale and low-cost manufacturing, as well as device designs for dynamic displaying are discussed subsequently. In the end, the review surveys diverse applications of structural colors in various areas such as printing, sensing, and advanced photovoltaics.

Soft Actor-Critic-Driven Adaptive Focusing under Obstacles

Electromagnetic (EM) waves that bypass obstacles to achieve focus at arbitrary positions are of immense significance to communication and radar technologies. Small-sized and low-cost metasurfaces enable the accomplishment of this function. However, the magnitude-phase characteristics are challenging to analyze when there are obstacles between the metasurface and the EM wave. In this study, we creatively combined the deep reinforcement learning algorithm soft actor-critic (SAC) with a reconfigurable metasurface to construct an SAC-driven metasurface architecture that realizes focusing at any position under obstacles using real-time simulation data. The agent learns the optimal policy to achieve focus while interacting with a complex environment, and the framework proves to be effective even in complex scenes with multiple objects. Driven by real-time reinforcement learning, the knowledge learned from one environment can be flexibly transferred to another environment to maximize information utilization and save considerable iteration time. In the context of future 6G communications development, the proposed method may significantly reduce the path loss of users in an occluded state, thereby solving the open challenge of poor signal penetration. Our study may also inspire the implementation of other intelligent devices.

Three-Channel Metasurfaces for Multi-Wavelength Holography and Nanoprinting

Metasurfaces, employed to simultaneously generate nanoprinting and holographic images, have been extensively explored recently. Among them, multi-wavelength multiplexing in a single metasurface is often accompanied by dispersion and crosstalk, which hinder the display of multicolor patterns. Here, we propose an efficient phase method to decouple the wavelength and realize a three-channel display operating at different wavelengths. Holographic images appear in the far field with the illumination of two different circularly polarized lights while a nanoprinting image is reconstructed by inserting an orthogonal optical path with the illumination of linear polarization light. The proposed metasurface is only composed of four types of unit cells, which significantly decreases the complexity of fabrication and improves the information capacity. Benefiting from its different decoding strategies and capability of multi-wavelength control, this approach may develop broad applications in information encryption, security, and color display.

Phase-assisted angular-multiplexing nanoprinting based on the Jacobi-Anger expansion

Featuring with ultracompactness and subwavelength resolution, metasurface-assisted nanoprinting has been widely researched as an optical device for image display. It also provides a platform for information multiplexing, and a series of multiplexed works based on incident polarizations, operating wavelengths and observation angles have emerged. However, the angular-multiplexing nanoprinting is realized at the cost of image resolution reduction or the increase of fabrication difficulty, hindering its practical applications. Here, inspired by the Jacobi-Anger expansion, a phase-assisted design paradigm, called Bessel metasurface, was proposed for angular multiplexing nanoprinting. By elaborately designing the phase distribution of the Bessel metasurface, the target images can be encoded into the desired observation angles, reaching angular multiplexing. With the merits of ultracompactness and easy fabrication, we believe that our design strategy would be attractive in the real-world applications, including optical information storage, encryption/concealment, multifunctional switchable optical devices, and 3D stereoscopic displays, etc.

A Polarization-Modulated Information Metasurface for Encryption Wireless Communications

Programmable and information metasurfaces have shown great potentials in wireless communications, but there are few reports on encrypted communications. In this paper, a programmable polarization-modulated (PoM) information metasurface is proposed, which can not only customize arbitrarily linearly polarized reflected waves, but also modulate their amplitudes in real time. Based on this feature, a physical-level wireless communication encryption scheme is presented and experimentally demonstrated by introducing a meta-key, which can be encrypted and sent by the programmable PoM information metasurface. To be specific, the key is encoded and concealed into different linear polarization channels, and then modulated and transmitted by the information metasurface at the transmitting end. At the receiving end, the modulated signal can be received and decoded by using a pair of polarization discrimination antennas. A wireless transceiver system is established to verify the feasibility of the scheme. It is shown that, once the meta-key is obtained, the corresponding encrypted target information that has been sent to the user in advance can be recovered.

Grayscale Image Display Based on Nano-Polarizer Arrays

Optical metasurfaces have shown unprecedented capabilities to control the two-dimensional distributions of phase, polarization, and intensity profiles of optical waves. Here, a TiO2 nanostructure functioning as a nano-polarizer was optimized considering that an anisotropic nanostructure is sensitive to the polarization states of incident light. We demonstrate two metasurfaces consisting of nano-polarizer arrays featured with different orientations, which can continuously manipulate the intensity distribution of the output light cell by cell according to Malus law and clearly display the detailed information of the target image. These metasurfaces have potential application in ultracompact displays, high-density optical information storage, and many other related polarization optics fields.

Non-orthogonal polarization encoding/decoding assisted by structured optical pattern recognition

The complex vector beams yield up an abundance of polarization information that has not yet been well utilized in information encoding. In this paper, we propose a polarization encoding scheme with the non-orthogonal polarization states using a stationary vector beam. Recognizing those non-orthogonal polarization states is assisted by the structured patterns of the single vector beams under different polarization projections. We show that one can achieve different capacities of encoding bits by changing the step of the polarization angle with the single vector beam. We also demonstrate the non-orthogonal polarization encoding scheme can be well decoded with the machine learning classification algorithm. A 64×64 gray image is successfully transmitted by using 4 bits/symbol encoding-decoding scheme with 99.94 % transmission accuracy. Besides, by extending the encoding-decoding scheme to 8 bits/symbol based on the same single vector beam, we achieve a higher transmission rate with 65.58% transmission accuracy. Our work holds promise for small-angle non-orthogonal polarization encoding for free-space optical communications.

Stokes meta-hologram toward optical cryptography

Optical cryptography manifests itself a powerful platform for information security, which involves encrypting secret images into visual patterns. Recently, encryption schemes demonstrated on metasurface platform have revolutionized optical cryptography, as the versatile design concept allows for unrestrained creativity. Despite rapid progresses, most efforts focus on the functionalities of cryptography rather than addressing performance issues, such as deep security, information capacity, and reconstruction quality. Here, we develop an optical encryption scheme by integrating visual cryptography with metasurface-assisted pattern masking, referred to as Stokes meta-hologram. Based on spatially structured polarization pattern masking, Stokes meta-hologram allows multichannel vectorial encryption to mask multiple secret images into unrecognizable visual patterns, and retrieve them following Stokes vector analysis. Further, an asymmetric encryption scheme based on Stokes vector rotation transformation is proposed to settle the inherent problem of the need to share the key in symmetric encryption. Our results show that Stokes meta-hologram can achieve optical cryptography with effectively improved security, and thereby paves a promising pathway toward optical and quantum security, optical communications, and anticounterfeiting.

Achieving optical cryptography scheme with both high capacity and security is highly desirable. Here, authors report a Stokes meta-hologram with a hierarchical encryption strategy that allows vector encryptions to produce depth-masked ciphertexts.

Color-selective three-dimensional polarization structures

Polarization as an important degree of freedom for light plays a key role in optics. Structured beams with controlled polarization profiles have diverse applications, such as information encoding, display, medical and biological imaging, and manipulation of microparticles. However, conventional polarization optics can only realize two-dimensional polarization structures in a transverse plane. The emergent ultrathin optical devices consisting of planar nanostructures, so-called metasurfaces, have shown much promise for polarization manipulation. Here we propose and experimentally demonstrate color-selective three-dimensional (3D) polarization structures with a single metasurface. The geometric metasurfaces are designed based on color and phase multiplexing and polarization rotation, creating various 3D polarization knots. Remarkably, different 3D polarization knots in the same observation region can be achieved by controlling the incident wavelengths, providing unprecedented polarization control with color information in 3D space. Our research findings may be of interest to many practical applications such as vector beam generation, virtual reality, volumetric displays, security, and anti-counterfeiting.

Tri-channel metasurface for watermarked structural-color nanoprinting and holographic imaging

Structural-color nanoprinting, which can generate vivid colors with spatial resolution at subwavelength level, possesses potential market in optical anticounterfeiting and information encryption. Herein, we propose an ultracompact metasurface with a single-cell design strategy to establish three independent information channels for simultaneous watermarked structural-color nanoprinting and holographic imaging. Dual-channel spectrum manipulation and single-channel phase manipulation are combined together by elaborately introducing the orientation degeneracy into the design of variable dielectric nanobricks. Hence, a structural-color nanoprinting image covered with polarization-dependent watermarks and a holographic image can be respectively generated under different decoded environments. The proposed metasurface shows a flexible method for tri-channel image display with high information capacity, and exhibits dual-mode anticounterfeiting with double safeguards, i.e., polarization-controlled watermarks and a far-field holographic image. This study provides a feasible route to develop multifunctional metasurfaces for applications including optical anticounterfeiting, information encryption and security, information multiplexing, etc.

Dual-channel anticounterfeiting color-nanoprinting with a single-size nanostructured metasurface

Metasurface-based structural-colors are usually implemented by changing the dimensions of nanostructures to produce different spectral responses. Therefore, a single-size nanostructured metasurface usually cannot display structural-colors since it has only one design degree of freedom (DOF), i.e., the orientation angles of nanostructures. Here, we show structural-color nanoprinting images can be generated with a single-size nanostructured metasurface, enabled by designing the anisotropic nanostructure with different spectral responses along its long- and short-axis directions, respectively. More interestingly, the concept of orientation degeneracy of nanostructures can be applied in the metasurface design, which shows two spectral modulations can be implemented under different polarization directions of output light, thus extending the color-nanoprinting from single-channel to dual-channel. The proposed dual-channel metasurface used for anticounterfeiting color-nanoprinting has presented the advantages of ultra-compactness, high information capacity, and vivid colors, which can develop broad applications in fields such as high-end anticounterfeiting, high-density information storage, optical encryption, etc.

Four-channel display and encryption by near-field reflection on nanoprinting metasurface

Multichannel metasurfaces become one of the most significant development trends, as they exhibit versatile manipulation abilities on electromagnetic fields and provide a promising approach to constitute compact devices with various complex functions, especially in optical encryption due to its capabilities of multichannel, high complexity, and high concealment. However, the existent multichannel metasurfaces based optical encryption technology can only realize two channels in the near-field, or perform three channels in near- and far-field. In this paper, a four-channel display metasurface used to encrypt information by three optical parameters as security keys is firstly proposed and experimentally demonstrated, which is different from the previous three-channel metasurface combined nanoprinting and hologram in near- and far-field. The novel design strategy of the four-channel metasurface can effectively enhance the information capacity and increase the difficulty of leaks without causing manufacturing challenges and additional costs. In addition, the simulation and experimental results demonstrate that the designed metasurface with four independent channels can separately display distinguishable nanoprinting images under decoding keys of special optical parameters. The proposed four-channel display metasurface with advantages of high capacity and ultracompactness will pave a way for multichannel applications in nano display, information storage, optical anticounterfeiting, and other relevant fields.

Terahertz Airy beam generated by Pancharatnam-Berry phases in guided wave-driven metasurfaces

Metasurface antennas scatter traveling guided waves into spatial waves, which act as extendable subsources to overcome the size limitation on emission sources. With the use of a Pancharatnam-Berry phase metasurface stimulated by a circularly polarized wave in a waveguide, the local phase distributions of scattered spatial waves can be made consistent with those of an Airy beam, thereby allowing the generation of high-quality Airy beams. In a slab waveguide, circularly polarized waves are synthesized through superposition of in-plane transverse electric modes. Simulations demonstrate that a 20 mm × 20 mm footprint all-dielectric guided wave-driven metasurface generates a 2D Airy beam at a frequency of 0.6 THz. Furthermore, we employ a metasurface deposited on a strip waveguide to generate a 1D Airy beam under direct stimulation by the fundamental transverse electric mode. Our work not only provides a large-scale emitter, but it also suggests promising potential applications in on-chip imaging and holography.

Near-infrared plasmonic sensing and digital metasurface via double Fano resonances

Plasmonic sensing that enables the detection of minute events, when the incident light field interacts with the nanostructure interface, has been widely applied to optical and biological detection. Implementation of the controllable plasmonic double Fano resonances (DFRs) offers a flexible and efficient way for plasmonic sensing. However, plasmonic sensing and digital metasurface induced by tailorable plasmonic DFRs require further study. In this work, we numerically and theoretically investigate the near-infrared plasmonic DFRs for plasmonic sensing and digital metasurface in a hybrid metasurface with concentric ϕ-shaped-hole and circular-ring-aperture unit cells. We show that a plasmonic Fano resonance, resulting from the interaction between a narrow and a wide effective dipolar modes, can be realized in the ϕ-shaped hybrid metasurface. In particular, we demonstrate that the tailoring plasmonic DFRs with distinct mechanisms of actions can be accomplished in three different ϕ-shaped hybrid metasurfaces. Moreover, the resonance mode-broadening and mode-shifting plasmonic sensing can be fulfilled by modulating the polarization orientation and the related geometric parameters of the unit cells in the near-infrared waveband, respectively. In addition, the plasmonic switch with a high ON/OFF ratio can not only be achieved but also be exploited to establish a single-bit digital metasurface, even empower to implement two- and three-bit digital metasurface characterized by the plasmonic DFRs in the telecom L-band. Our results offer a new perspective toward realizing polarization-sensitive optical sensing, passive optical switches, and programmable metasurface devices, which also broaden the landscape of subwavelength nanostructures for biosensors and optical communications.

Mass-Manufactured Beam-Steering Metasurfaces for High-Speed Full-Duplex Optical Wireless-Broadcasting Communications

Beam-steering devices, which are at the heart of optical wireless-broadcasting communication links, play an important role in data allocation and exchange. An ideal beam-steering device features large steering angles, arbitrary channel numbers, reconfigurability, and ultracompactness. However, these criteria have been achieved only partially with conventional beam-steering devices based on waveguides, micro-electricalmechanical systems, spatial light modulators, and gratings, which will substantially limit the application of optical wireless-broadcasting communication techniques. In this study, an ultracompact full-duplex metabroadcasting communication system is designed and experimentally demonstrated, which exhibits beam steering angles up to ±40°, 14 broadcasting channels with capacity for downstream and upstream links up to 100 and 10 Gbps for each user channel, three operating modes for flexible signal switching, and metadevice dimensions as small as 2 mm × 2 mm. In particular, the beam-steering metadevices are mass-manufactured by a complementary metal-oxide-semiconductor (CMOS) processing platform, which shows their potential for large-scale commercial applications. The demonstrated metabroadcasting communication system merges optical wireless-broadcasting communications and metasurfaces, which reduces the complexity of beam-steering devices while significantly increasing their performance, opening up a new avenue for high-quality optical wireless-broadcasting communications.

Multiplexed multi-focal and multi-dimensional SHE (spin Hall effect) metalens

Metalenses are two-dimensional ultrathin metalenses composed of subwavelength artificial microstructures. In this paper, various multi-focal spin Hall effect (SHE)-based metalenses are designed to provide spin-dependent splitting in transverse and longitudinal directions, which possess spin-dependent two focal points under left-circularly polarized (LCP) or right-circularly polarized (RCP) incidence, and all four focal points can be observed under the linearly polarized (LP) incidence. A spin-independent bifocal metalens was investigated, which possesses the same bifocal focusing phenomena for LCP and RCP incidences. Our method is significant for designing high-efficiency multifunctional optics devices based on optical SHE.

Nanophotonic Color Routing

Recent advances in low-dimensional materials and nanofabrication technologies have stimulated many breakthroughs in the field of nanophotonics such as metamaterials and plasmonics that provide efficient ways of light manipulation at a subwavelength scale. The representative structure-induced spectral engineering techniques have demonstrated superior design of freedom compared with natural materials such as pigment/dye. In particular, the emerging spectral routing scheme enables extraordinary light manipulation in both frequency-domain and spatial-domain with high-efficiency utilization of the full spectrum, which is critically important for various applications and may open up entirely new operating paradigms. In this review, a comparative introduction on the operating mechanisms of spectral routing and spectral filtering schemes is given and recent progress on various color nanorouters based on metasurfaces, plasmonics, dielectric antennas is reviewed with a focus on the potential application in high-resolution imaging. With a thorough analysis and discussion on the advanced properties and drawbacks of various techniques, this report is expected to provide an overview and vision for the future development and application of nanophotonic color (spectral) routing techniques.

Metasurface-Driven Optically Variable Devices

Optically variable devices (OVDs) are in tremendous demand as optical indicators against the increasing threat of counterfeiting. Conventional OVDs are exposed to the danger of fraudulent replication with advances in printing technology and widespread copying methods of security features. Metasurfaces, two-dimensional arrays of subwavelength structures known as meta-atoms, have been nominated as a candidate for a new generation of OVDs as they exhibit exceptional behaviors that can provide a more robust solution for optical anti-counterfeiting. Unlike conventional OVDs, metasurface-driven OVDs (mOVDs) can contain multiple optical responses in a single device, making them difficult to reverse engineered. Well-known examples of mOVDs include ultrahigh-resolution structural color printing, various types of holography, and polarization encoding. In this review, we discuss the new generation of mOVDs. The fundamentals of plasmonic and dielectric metasurfaces are presented to explain how the optical responses of metasurfaces can be manipulated. Then, examples of monofunctional, tunable, and multifunctional mOVDs are discussed. We follow up with a discussion of the fabrication methods needed to realize these mOVDs, classified into prototyping and manufacturing techniques. Finally, we provide an outlook and classification of mOVDs with respect to their capacity and security level. We believe this newly proposed concept of OVDs may bring about a new era of optical anticounterfeit technology leveraging the novel concepts of nano-optics and nanotechnology.

Mechanical modulation of multifunctional responses in three-dimensional terahertz metamaterials

Reconfigurable metamaterials have attracted a surge of attention for their formidable capability to dynamically manipulate the electromagnetic wave. Among the multifarious modulation methods, mechanical deformation is widely adopted to tune the electromagnetic response of the stereotype metamaterial owing to its straightforward and continuous controllability on the metamaterial structure. However, previous morphologic reconfigurations of metamaterials are typically confined in planar deformation that renders limited tunable functionalities. Here we have proposed a novel concept of out-of-plane deformation to broaden the functionalities of mechanically reconfigurable metamaterials via introducing a cross-shaped metamaterial. Our results show that the out-of-plane mechanical modulation dramatically enhances the magnetic response of the pristine metamaterial. Furthermore, by uncrossing the bars of cross-shaped meta-atoms, a L-shaped metamaterial is proposed to verify the effectiveness of such a mechanical method on the handedness switching via changing mechanical loading-paths. More importantly, the differential transmission for circularly polarized incidences can be continuously modulated from -0.45 to 0.45, and the polarization states of the transmission wave can be dynamically manipulated under the linearly polarized illumination. Our proposed mechanical modulation principle might open a novel avenue toward the three-dimensional reconfigurable metamaterials and shows their ample applications in the areas of chiroptical control, tunable polarization rotator and converter.

Vectorial Compound Metapixels for Arbitrary Nonorthogonal Polarization Steganography

Malus' law regulating the intensity of light when passed through a polarizer, forms the solid basis for image steganography based on orthogonal polarizations of light to convey hidden information without adverse perceptions, which underpins important practices in information encryptions, anti-counterfeitings, and security labels. Unfortunately, the restriction to orthogonal states being taken for granted in the common perceptions fails to advance cryptoinformation to upgraded levels of security. By introducing a vectorial compound metapixel design, arbitrary nonorthogonal polarization multiplexing of independent grayscale images with high fidelity and strong concealment is demonstrated. The Jones matrix treatment of compound metapixels consisting of double atoms with tailored in-plane orientation sum and difference allows point-by-point configuring of both the amplitude and polarization rotations of the output beam in an analytical and linear form. With this, both multiplexing two continuous grayscale images in arbitrary nonorthogonal polarization angles and concealing grayscale image on another in an arbitrary disclosure angle window are experimentally demonstrated in the visible TiO₂ metasurface platform. The methods shed new light on multifarious metaoptics by harnessing the new degree of freedom and unlock the full potential of metasurface polarization optics.

Facile full-color printing with a single transparent ink

Structural colors are promising candidates for their antifading and eco-friendly characteristics. However, high cost and complicated processing inevitably hinder their development. Here, we propose a facile full-color structural-color inkjet printing strategy with a single transparent ink from the common polymer materials. This structural color arisen from total internal reflections is prepared by digitally printing the dome-shaped microstructure (microdome) with well-controlled morphology. By controlling the ink volume and substrate wettability, the microdome color can be continuously regulated across whole visible regions. The gamut, saturation, and lightness of the printed structural-color image are precisely adjusted via the programmable arrangement of different microdomes. With the advantages of simple manufacturing and widely available inks, this color printing approach presents great potential in imaging, decoration, sensing, and biocompatible photonics.

Preselectable Optical Fingerprints of Heterogeneous Upconversion Nanoparticles

The control in optical uniformity of single nanoparticles and tuning their diversity in multiple dimensions, dot to dot, holds the key to unlocking nanoscale applications. Here we report that the entire lifetime profile of the single upconversion nanoparticle (τ² profile) can be resolved by confocal, wide-field, and super-resolution microscopy techniques. The advances in both spatial and temporal resolutions push the limit of optical multiplexing from microscale to nanoscale. We further demonstrate that the time-domain optical fingerprints can be created by utilizing nanophotonic upconversion schemes, including interfacial energy migration, concentration dependency, energy transfer, and isolation of surface quenchers. We exemplify that three multiple dimensions, including the excitation wavelength, emission color, and τ² profile, can be built into the nanoscale derivative τ²-dots. Creating a vast library of individually preselectable nanotags opens up a new horizon for diverse applications, spanning from sub-diffraction-limit data storage to high-throughput single-molecule digital assays and super-resolution imaging.

Electrically Switchable, Polarization-Sensitive Encryption Based on Aluminum Nanoaperture Arrays Integrated with Polymer-Dispersed Liquid Crystals

Metasurface-based structural coloration is a promising enabling technology for advanced optical encryption with a high-security level. Herein, we propose a paradigm of electrically switchable, polarization-sensitive optical encryption based on designed metasurfaces integrated with polymer-dispersed liquid crystals. The metasurfaces consist of anisotropic and isotropic aluminum nanoaperture arrays. Optical images can be encrypted by elaborately arranging anisotropic and isotropic nanoapertures based on their polarization-dependent plasmonic resonance characteristics. We demonstrate high-quality encrypted images and QR codes with electrically switchable, polarization-sensitive properties based on PDLC-integrated aluminum nanoaperture arrays. The proposed technique can be applied to many fields including high-security optical encryption, security tags, anticounterfeiting, multichannel imaging, and dynamic displays.

Multifunctional terahertz metasurfaces for polarization transformation and wavefront manipulation

Conventionally, the realization of polarization transformation and wavefront manipulation in metasurfaces relies on the Pancharatnam-Berry (PB) phase together with the dynamic phase. However, the reported polarization transformation and wavefront manipulation were limited to spin-dependent wavefront manipulation for circular polarization (CP). To obtain more abundant functions, we propose a novel technology that relies on the dynamic phase with a spatial interleaving unit arrangement. With the functions of a quarter wave plate, the metasurfaces we designed can achieve multiple wavefront manipulations which are not only for the spin polarization transformation but also for the linear polarization transformation. Specifically, we design a bifocal metasurface, which can focus on one circularly polarized component as a point and spin-opposite component as a vortex under the linearly polarized (LP) incidence. With the further adjustment of the unit arrangement, the left-hand circularly polarized (LCP) and right-hand circularly polarized (RCP) components under the LP incidence can be refocused on the same point and then composited, resulting in a new LP exit wave. Furthermore, we prove theoretically that the desired x-LP component and y-LP component under the arbitrary CP incidence can also be manipulated independently. We believe that the versatility of this method will provide a novel platform for the development of terahertz integrated photonics.

Dynamic Bifunctional Metasurfaces for Holography and Color Display

Metasurfaces have shown their unprecedented ability in wavefront shaping and triggered various applications with state-of-the-art performances, e.g., color nanoprinting and metaholograms. Recently, these two functions have been combined into a single metasurface to further expand its capabilities. Despite the progress, the current dual-mode metasurfaces are mostly static and strongly hinder their practical applications. Herein, the realization of dynamic bifunctional metasurfaces is reported. Five metaholograms at two different wavelengths are multiplexed with structural colors by controlling the spectral and phase response of metasurface. Owing to the destructive interference and the resonance on external environment, the light diffraction at particular wavelengths can be switched between "ON" and "OFF" states, or remain unchanged with the change of surrounding refractive index. Consequently, the encoded metaholograms are selectively turned on and off, making the overall holographic image dynamically switchable. This concept paves a solid step toward practical applications of all-dielectric metasurfaces.

Multiplexing meta-hologram with separate control of amplitude and phase

Metasurfaces have shown their unique capabilities to manipulate the phase and/or amplitude properties of incident light at the subwavelength scale, which provides an effective approach for constructing amplitude-only, phase-only or even complexed amplitude meta-devices with high resolution. Most of meta-devices control the amplitude and/or phase of the incident light with the same polarization state; however, separately controlling of amplitude and phase of the incident light with different polarization states can provide a new degree of freedom for improving the information capacity of metasurfaces and designing multifunctional meta-devices. Herein, we combine the amplitude manipulation and geometric phase manipulation by only reconfiguring the orientation angle of the nanostructure and present a single-sized design strategy for a multiplexing meta-hologram which plays the dual roles: a continuous amplitude-only meta-device and a two-step phase-only meta-device. Two different modulation types can be readily switched merely by polarization controls. Our approach opens up the possibilities for separately and independently controlling of amplitude and phase of light to construct a multiplexing meta-hologram with a single-sized metasurface, which can contribute to the advanced research and applications in multi-folded optical anti-counterfeiting, optical information hiding and optical information encoding.

Dual-color meta-image display with a silver nanopolarizer based metasurface

Plasmonic metallic nanostructures with anisotropic design have unusual polarization-selective characteristic which can be utilized to build nanopolarizers at the nanoscale. Herein, we propose a dual-color image display platform by reconfiguring two types of silver nanoblocks in a single-celled metasurface. Governed by Malus's law, the two types of silver nanoblocks both acting as nanopolarizers with different orientations can continuously modulate the intensity of incident linearly polarized red and green light pixel-by-pixel, respectively. As a result, an ultra-compact, high-resolution, and continuous-greyscale dual-color image can be recorded right at the surface of the meta-device. We demonstrate the dual-color Malus metasurface by successfully encoding and decoding a red-green continuously-grayscale image into a metasurface sample. The experimentally captured meta-image with high-fidelity and resolution as high as 63500 dots per inch (dpi) has verified our proposal. With the advantages such as continuous grayscale modulation, ultrathin, high stability and high density, the proposed dual-color encoded metasurfaces can be readily used in ultra-compact image displays, high-end anti-counterfeiting, high-density optical information storage and information encryption, etc.

Broadband and switchable terahertz polarization converter based on graphene metasurfaces

In this work, we propose broadband and switchable terahertz (THz) polarization converters based on either graphene patch metasurface (GPMS) or its complementary structure (graphene hole metasurface, GHMS). The patch and hole are simply cross-shaped, composed of two orthogonal arms, along which plasmonic resonances mediated by Fabry-Perot cavity play a key role in polarization conversion (PC). An incidence of linear polarization will be converted to its cross-polarization (LTL) or circular polarization (LTC), as the reflected wave in the direction of two arms owning the same amplitude and π phase difference (LTL), or ±π/2 phase difference (LTC). Such requirements can be met by optimizing the width and length of two arms, thickness of dielectric layer, and Fermi level E_F of graphene. By using GPMS, LTL PC of polarization conversion ratio (PCR) over 90% is achieved in the frequency range of 2.92 THz to 6.26 THz, and by using GHMS, LTC PC of ellipticity χ ≤ -0.9 at the frequencies from 4.45 THz to 6.47 THz. By varying the Fermi level, the operating frequency can be actively tuned, and the functionality can be switched without structural modulation; for instance, GPMS supports LTL PC as E_F = 0.6 eV and LTC PC of χ ≥ 0.9 as E_F = 1.0 eV, in the frequency range of 2.69 THz to 4.19 THz. Moreover, GHMS can be optimized to sustain LTL PC and LTC PC of |χ| ≥ 0.9, in the frequency range of 4.96 THz to 6.52 THz, which indicates that the handedness of circular polarization can be further specified. The proposed polarization converters of broad bandwidth, active tunability, and switchable functionality will essentially make a significant progress in THz technology and device applications, and can be widely utilized in THz communications, sensing and spectroscopy.