Multiplexed Anticounterfeiting Meta-image Displays with Single-Sized Nanostructures

Metasurface-enabled optical encryption and steganography with enhanced information security

Metasurfaces have attracted considerable interest in optical encryption due to their remarkable ability to manipulate light at subwavelength scales, however the aspect of encryption security remains an area requiring deeper exploration. Here, we propose and demonstrate metasurface-enabled optical encryption and steganography that provides dual-layer information protection. A secret information is embedded within multiple carrier images using a run-length encoding algorithm, dispersing the data to safeguard it against direct observation and brute-force attacks, thereby establishing the first layer of security. The second layer is achieved by encoding the multiple carrier images onto a silicon metasurface, leveraging light wavelength and polarization to generate diverse optical keys post-steganography. To validate the proposed scheme, several silicon metasurface samples are fabricated and characterized in the visible spectrum. By adjusting various combinations of optical keys, three encrypted carrier images are retrieved with high fidelity and negligible crosstalk, and the concealed secret information is successfully extracted through a corresponding decryption algorithm. The proposed approach enhances optical information security at the hardware level, making it less susceptible to leakage. It is anticipated that the demonstrated advancement will hold significant potential for applications in information security and optical anti-counterfeiting.

High-Capacity Multilayered Luminescent Encryption Technology Based on Er-Implanted Silicon Treated by Pulsed Laser Annealing

Luminescent encrypted labels can effectively solve the problem of counterfeiting. However, they suffer from complex design and fabrication, low space utilization, and limited capacity of encrypted information. Herein, we create multilayer infrared luminescent encryption labels using femtosecond-laser-activated Er-doped silicon. In comparison with other annealing techniques that treat the whole sample at once, femtosecond lasers have a high spatial precision and flexibility, which can locally anneal the Er-doped Si, generating stable and controllable multilayered photoluminescent patterns. It therefore can significantly enhance security and increase the information storage capacity. This work demonstrates a low-cost, high-capacity, and high-security encrypted label with great application value. Fs-laser-annealed Er-doped silicon is also a promising material to realize quantum light sources or gain materials for lasers at the communication band.

Enhanced Reconfigurable Visual Cryptography Strategies Utilizing Optical Metasurfaces

The secure storage and transmission of information are core measures that ensure data security. Within intricate information landscapes, reconfigurable encryption mechanisms serve as robust defenses against potential breaches of encrypted data. Here, we propose a reconfigurable visual secret sharing (VSS) encryption strategy that leverages the capabilities of optical metasurfaces and exhibits a dual-channel secondary encryption scheme. This method employs three shared keys (SKs) hidden in the near-field and spin-decoupled far-field channels of the metasurface to perform the initial encryption, enabling the decryption of two distinct encrypted information. Subsequently, leveraging clues from the initial decryption process, the SKs in two far-field channels, in conjunction with the corresponding reconfigurable codebook, can achieve dual-channel reconfigurable encryption. As a proof of concept, we conduct experiments to decrypt the first-time encrypted information and the secondary encrypted information that is reconfigurable. The enhanced reconfigurable encryption strategy not only improves the information security and parallel encryption capacity but also offers repetitive encryption and dynamic encryption without updating the metasurface hardware.

Print evaluation of inks with stealth nanobeacons

Plasmonic structures using noble metal nano-assemblies are created and printed or stamped with a seal for use as information tags that carry both authenticity and information. We created an ink that contains stealth nanobeacons and evaluated its printing characteristics. Stealth nanobeacons are composed of noble metal nano-assemblies, which are fabricated via a self-assembly process and have indefinite shapes. This plasmonic structure was made into simple ink by mixing it with pure water or existing inkjet printer inks. We discharged this adjusted ink on an inkjet printer to evaluate its surface-enhanced Raman scattering activity and other properties, and confirmed that the ink containing stealth nanobeacons can be printed successfully. The printable ink is expected to be developed into a "Nanotag" information tag and an authenticity tag.

Strategic engineering of cationic systems for spatial & temporal anti-counterfeiting applications in zero-dimensional Mn(II) halides

While spatial and time-resolved anti-counterfeiting technologies have gained increasing attention owing to their excellent tunable photoluminescence, achieving high-security-level anti-counterfeiting remains a challenge. Herein, we developed a spatial-time-dual-resolved anti-counterfeiting system using zero-dimensional (0D) organic-inorganic Mn(II) metal halides: (EMMZ)2MnBr4 (named M-1, EMMZ=1-Ethyl-3-Methylimidazolium Bromide) and (EDMMZ)2MnBr4 (named M-2, EDMMZ=1-Ethyl-2,3-Dimethylimidazolium Bromide). M-1 shows a bright green emission with a quantum yield of 78 %. It undergoes a phase transformation from the crystalline to molten state with phosphorescence quenching at 350 K. Reversible phase and luminescent conversion was observed after cooling down for 15 s. Notably, M-2 exhibits green light emission similar to M-1 but undergoes phase conversion and phosphorescence quenching at 390 K, with reversible conversion observed after cooling down for 5 s. The photoluminescence switching mode of on(green)-off-on(green) can be achieved by temperature control, demonstrating excellent performance with short response times and ultra-high cyclic reversibility. By leveraging the different quenching temperatures and reversible PL conversion times of M-1 and M-2, we propose a spatial-time-dual-resolved photoluminescence (PL) switching system that combines M-1 and M-2. This system enables multi-fold tuning of the PL switch for encryption and decryption through cationic engineering strategies by modulating temperature and cooling time. This work presents a novel and feasible design strategy for advanced-level anti-counterfeiting technology based on a spatial-time-dual-resolved system.

Experimental retrieval of metaholographic images via evanescent wave using a single-layer nanostructure for encryption

Metasurfaces have demonstrated significant potential in optical encryption and anti-counterfeiting due to their incredible capability of manipulating various light properties. However, previous metasurface-encryption methods did not sufficiently explore the spatial frequency aspect, particularly regarding evanescent waves. Here, we propose an encryption scheme by introducing evanescent waves into the encoding and decoding processes. Different parts of the target image are individually encoded at distinct spatial frequencies within the evanescent-wave region. Only when an evanescent wave with a specific transverse wave vector (TWV) serves as the key can the complete target image be retrieved. Our work exemplifies a thorough exploration of the mathematical framework underlying metasurface-based holography. It enables the feasibility of a single metasurface in concealing and retrieving information in the evanescent-wave region, thus opening up a new, to our knowledge, methodology for high-security optical information encryption.

Scalable Multistep Imprinting of Multiplexed Optical Anti-counterfeiting Patterns with Hierarchical Structures

Multiplexed optical techniques with multichannel patterns provide powerful strategies for high-capacity anti-counterfeiting. However, it is still a big challenge to meet the demands of achieving high encryption levels, excellent readability, and simple preparation simultaneously. Herein, we use a multistep imprinting technique, leveraging surface work-hardening to massively produce multiplexed encrypted patterns with hierarchical structures. These patterns with coupled nano- and microstructures can be instantaneously decoded into different pieces of information at different view angles under white light illumination. By incorporating perpendicular nano- and microgratings, we achieve four-channel encoded patterns, enhancing anti-counterfeiting capacity. This versatile method works on various metal/polymer materials, offering high-density information storage, direct visibility, broad material compatibility, and low-cost mass production. Our high-performance anti-counterfeiting patterns show significant potential in real-world applications.

Uncovering the Recent Progress of CNC-Derived Chirality Nanomaterials: Structure and Functions

Cellulose nanocrystal (CNC), as a renewable resource, with excellent mechanical performance, low thermal expansion coefficient, and unique optical performance, is becoming a novel candidate for the development of smart material. Herein, the recent progress of CNC-based chirality nanomaterials is uncovered, mainly covering structure regulations and function design. Undergoing a simple evaporation process, the cellulose nanorods can spontaneously assemble into chiral nematic films, accompanied by a vivid structural color. Various film structure-controlling strategies, including assembly means, physical modulation, additive engineering, surface modification, geometric structure regulation, and external field optimization, are summarized in this work. The intrinsic correlation between structure and performance is emphasized. Next, the applications of CNC-based nanomaterials is systematically reviewed. Layer-by-layer stacking structure and unique optical activity endow the nanomaterials with wide applications in the mineralization, bone regeneration, and synthesis of mesoporous materials. Besides, the vivid structural color broadens the functions in anti-counterfeiting engineering, synthesis of the shape-memory and self-healing materials. Finally, the challenges for the CNC-based nanomaterials are proposed.

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.

Upper limit on the polarization-assisted amplitude modulation capability of cascaded single-cell wave-plate-like metasurfaces

The Jones matrix method offers a robust framework for designing polarization multiplexed metasurfaces (PMMs). Traditional PMMs design involves initially defining functions and working channels, then mapping feature functions to adjustable parameters of metasurfaces. However, this approach makes it difficult to predict how working channels affect metasurface features. Here, we employ the generalized Malus law and Rodriguez rotation matrix on the Poincare Sphere to analyze diverse working channels' impact on PMMs' amplitude modulation capacity. For single-celled waveplate-like PMMs, up to three distinct images can be displayed. We demonstrate this in both theoretic method and numerical simulations. Our study establishes a framework for multi-channel amplitude modulation design of metasurfaces, applicable in information encryption, optical computation, diffraction neural networks, etc.

Fabrication of Large-Area Nanostructures Using Cross-Nanoimprint Strategy

Nanostructures with sufficiently large areas are necessary for the development of practical devices. Current efforts to fabricate large-area nanostructures using step-and-repeat nanoimprint lithography, however, result in either wide seams or low efficiency due to ultraviolet light leakage and the overflow of imprint resin. In this study, we propose an efficient method for large-area nanostructure fabrication using step-and-repeat nanoimprint lithography with a composite mold. The composite mold consists of a quartz support layer, a soft polydimethylsiloxane buffer layer, and multiple intermediate polymer stamps arranged in a cross pattern. The distance between the adjacent stamp pattern areas is equal to the width of the pattern area. This design combines the high imprinting precision of hard molds with the uniform large-area imprinting offered by soft molds. In this experiment, we utilized a composite mold consisting of three sub-molds combined with a cross-nanoimprint strategy to create large-area nanostructures measuring 5 mm × 30 mm on a silicon substrate, with the minimum linewidth of the structure being 100 nm. Compared with traditional step-and-flash nanoimprint lithography, the present method enhances manufacturing efficiency and generates large-area patterns with seam errors only at the micron level. This research could help advance micro-nano optics, flexible electronics, optical communication, and biomedicine studies.

Photorewriting, Time-Resolved Encryption, and Unclonable Anticounterfeiting with Artificial Intelligence Authentication via a Reversible Photoswitchable System

In the fields of photolithographic patterning, optical anticounterfeiting, and information encryption, reversible photochromic materials with solid-state fluorescence are emerging as a potential class of systems. A design strategy for reversible photochromic materials has been proposed and synthesized through the introduction of photoactive thiophene groups into the molecular backbone of aryl vinyls, compounds with unique aggregation-induced emission properties, and solid-state reversible photocontrollable fluorescence and color-changing properties. This work develops novel photochromic inks, films, and cellulose hydrogels for enhancing the security of information encryption and anticounterfeiting technologies. They achieve rapid and reversible color change under ultraviolet light irradiation. Dependent upon the rate of color change, higher levels of time-resolved security can be achieved. This feature is important for enhancing the confidentiality of encrypted information and the reliability of security labels. Color-changing cellulose hydrogels, inks, and films consisting of three photochromic fluorescent molecules have quick photoactivity, great photoreversibility and photostability, and good processability, making them ideal for time-delayed anticounterfeiting and smart encryption. Furthermore, specialized algorithms are used to construct convolutional neural networks, and image analysis is performed on these systems, thus solving the current problem of the time-consuming information decryption process. This artificial intelligence method offers new opportunities for enhanced data encryption.

Structural-color meta-nanoprinting embedding multi-domain spatial light field information

Recently, multifunctional metasurface has showcased its powerful functionality to integrate nanoprinting and holography, and display ultracompact meta-images in near- and far-field simultaneously. Herein, we propose a tri-channel metasurface which can further extend the meta-imaging ranges, with three independent images located at the interface, Fresnel and Fourier domains, respectively. Specifically, a structural-color nanoprinting image is decoded right at the interface of the metasurface, enabled by varying the dimensions of nanostructures; a Fresnel holographic image and another Fourier holographic image are present at the Fresnel and Fourier (far-field) domains, respectively, enabled by geometric phase. The spectral and phase manipulation capabilities of nanostructures have been maximized, and the spatial multiplexing capabilities for diffraction in metasurfaces have also been fully exploited. By leveraging the design freedom enabled through the tuning of the geometric size and orientation of nanostructures, as well as optimizing the diffraction spatial light wave transformation, the encoding of multiple images on the single-celled metasurface is achieved. More interestingly, due to the spatial separation of images across different channels, crosstalk is virtually eliminated, effectively enhancing imaging quality. The proposed metasurface offers several advantages, including a compact design, easiness of fabrication, minimal crosstalk, and high storage density. Consequently, it holds promising applications in image display, data storage, information encryption, anti-counterfeiting, and various other fields.

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.

Tristate Photonic Crystal Film with Structural, Fluorescent, and Up-Conversion Luminescent Color for Multilevel Anticounterfeiting

Counterfeit items are growing worldwide, affecting the global economy and human health. Anticounterfeiting tags based on a physical microstructure or chemical materials have enjoyed long-term commercial success due to their visualization and inexpensive production. However, conventional anticounterfeiting tags can be readily imitated. Herein, we have overcome this limitation by assembling colloidal nanospheres and two luminescent micromaterials into a composited photonic crystal (PhC) and achieved massive scale-up fabrication of multilevel anticounterfeiting PhC films in just several minutes of thermal rolling. The fabricated PhC film exhibits three optical states, including angle-dependent structural color (reflectivity = 66%) under white light, emits green light under 980 nm light, and emits red light under ultraviolet light. Multilevel anticounterfeiting colorful images were obtained by further use of masking templates, which integrate colors from both physically colored microstructures and chemical luminescent materials. Besides, the thermal-rolling process also shows excellent feasibility for assembling microunits with different sizes into high-quality functional PhC films.

Preparation of Self-Healing Anthocyanidin-Containing Thermochromic Alginate Ink for Authentication Purposes

Thermochromic inks have proven to be a promising security encoding approach for making commercially available products less susceptible to forgery. However, thermochromic inks have been plagued with poor durability. Thus, self-healable hydrogels can be used as self-repair inks with better durability. Herein, we combined hybrid cellulose nanofibers (CNFs) and sodium alginate (SA) with anthocyanidin(Cy)-based Brassica oleracea L. var. capitata extract in the existence of mordant (ferrous sulfate) to create a self-healing ink for authentication. CNFs were used as a reinforcement agent to enhance the mechanical strength of the sodium alginate hydrogel. Both durability and thermal stability were ensured using self-healing inks. Red cabbage was used to extract Cy-based chromophore as an environmentally friendly spectroscopic probe for immobilization into SA. Using varying concentrations of anthocyanidin, self-healable composite hydrogels (Cy@SA) with thermochromic properties were provided. Using the CIE Lab color coordinate system, homogeneous purple (569 nm) films were printed onto a sheet surface. Upon heating from 25 to 70 °C, the purple color changed to red (433 nm). Transmission electron microscopy was applied to study anthocyanidin/mordant (Cy/M) nanoparticles (NPs). The properties of the applied prints were analyzed using several methods. Both the hydrogel and stamped sheets were tested for their mechanical and rheological characteristics, respectively. Research on the nanocomposite ink (Cy@SA) antibacterial properties and cytotoxicity was also conducted.

Three-fold information encryption based on polarization- and wavelength-multiplexed metasurfaces

Metasurface has garnered significant attention in the field of optical encryption as it allows the integration and occultation of multiple grayscale nanoprinting images on a single platform. However, in most cases, polarization serves as the only key for encryption/decryption, and the risk of being cracked is relatively high. In this study, we propose a three-fold information encryption strategy based on a dielectric metasurface, in which a colorful nanoprinting image and two grayscale images are integrated on such a single platform. Unlike previous works based on the orientation-angle degenerated light intensity, the proposed image encryptions are realized by customizing nanobricks with polarization-mediated similar/different transmission characteristics in either broadband or at discrete wavelengths. Different combinations of polarization and monochromatic wavelengths can form three keys with different levels of decryption complexity as compared to the previous counterpart based merely on polarization. Once illuminated by non-designed wavelengths or polarized light, messy images with false information will be witnessed. Most importantly, all images are safely secured by the designated incidence polarization and cannot be decrypted via an additional analyzer as commonly happens in conventional metasurface-based nanoprinting. The proposed metasurface provides an easy-to-design and easy-to-disguise scheme for multi-channel display and optical information encryption.

Polarization-encoded optical secret sharing based on a dielectric metasurface incorporating near-field nanoprinting and far-field holography

Metasurface encryption with high concealment and resolution is promising for information security. To improve the encryption security, a polarization-encoded secret sharing scheme based on dielectric metasurface by combining the secret sharing method with nanoprinting and holography is proposed. In this encryption scheme, the secret image is split into camouflaged holograms of different polarization channels and shares a total of 24-1 encryption channels. Benefiting from the secret sharing mechanism, the secret image cannot be obtained by decoding the hologram with a single shared key. Specifically, the secret hologram of a specific channel in the far field can be obtained by specifying the optical key, acquiring the near-field nanoprinting image to determine the combination order for the shared key, and decoding using multiple shared keys. The secret sharing encryption scheme can not only enhance the security level of metasurface encryption, but also increase the number of information channels by predefining camouflage information. We believe that it has important potential applications in large-capacity optical encryption and information storage.

High-Temperature, Reversible, and Robust Thermochromic Fluorescence Based on Rb2 MnBr4 (H2 O)2 for Anti-Counterfeiting

Thermochromic fluorescent materials (TFMs) characterized by noticeable emission color variation with temperature have attracted pervasive attention for their frontier application in stimulus-response and optical encryption technologies. However, existing TFMs typically suffer from weak PL reversibility as well as limited mild operating temperature and severe temperature PL quenching. PL switching under extreme conditions such as high temperature will undoubtedly improve encryption security, while it is still challenging for present TFMs. In this work, high-temperature thermochromic fluorescence up to 473 K and robust structural and optical reversibility of 80 cycles are observed in Rb2 MnBr4 (H2 O)2 and related crystals, which is seldom reported for PL changes at such a high temperature. Temperature-driven nonluminous, red and green light emission states can be achieved at specific temperatures and the modulation mechanism is verified by in situ optical and structural measurements and single particle transition. By virtue of this unique feature, a multicolor anti-counterfeiting label based on a broad temperature gradient and multidimensional information encryption applications are demonstrated. This work opens a window for the design of inorganic materials with multi-PL change and the development of advanced encryption strategies with extreme stimuli source.

Dielectric diatomic metasurface-assisted versatile bifunctional polarization conversions and incidence-polarization-secured meta-image

Dielectric metasurface empowering efficient light polarization control at the nanoscale, has recently garnered tremendous research interests in the field of high-resolution image encryption and display, particularly at low-loss wavelengths in the visible band. Nevertheless, due to the single fixed polarization conversion function, the image (either positive or negative image) can always be decrypted in a host-uncontrollable manner as long as the user applies an analyzer to select the polarization component of the output light. Here, we resort to half-waveplate- and quarter-waveplate-like silicon nanopillars to form a metamolecule of a dielectric diatomic metasurface, which can yield versatile linearly polarized (LP) and circularly polarized (CP) light upon orthogonally linear-polarized incidences, providing new degrees of freedom for image display and encryption. We show both theoretically and numerically that versatile different paired LP and CP combinations could be achieved by simply adjusting the orientation angles of the two nanopillars. The bifunctional polarization conversion functions make possible that a meta-image can only be seen when incident light is linearly polarized at a specific polarization angle, whereas no image can be discerned for the orthogonal polarization incidence case, indicating the realization of incidence-polarization secured meta-image. This salient feature holds for all individual metamolecules, reaching a remarkable image resolution of 52,916 dots per inch. By fully exploiting all polarization conversions of four designed metamolecules, three-level incidence polarization-secured meta-image can also be expected.

Two-level optical encryption platform via an electrically driven liquid-crystal-integrated tri-channel metasurface

Metasurface-based optical encryption techniques have garnered significant attention due to their ultracompact nature and ability to support multichannel optical responses. Here, we present a liquid-crystal (LC)-integrated metasurface that enables polarized-encrypted amplitude and phase multiplexing. This approach allows for simultaneously realizing trifold displays of both meta-holography and meta-nanoprinting. By combining propagation and geometric phase modulation, we meticulously screen the unit cells of the metasurface, establishing a comprehensive structural dictionary. As a proof-of-concept, we developed an electrically driven advanced optical encryption platform that boasts multifunctional channels and two-level encryption capabilities. This study paves the way for advanced optical encryption and identification techniques.

Triple-Mode upconversion emission for dynamic multicolor luminescent anti-Counterfeiting

Lanthanide (Ln³⁺) luminescent materials play a crucial role in information security and data storage owing to their excellent and unique optical properties. The advances in dynamic colorful luminescent anti-counterfeiting nanomaterials enable the generation of a high-level information encryption. In this work, a superior thermal, optical wavelength and excitation power triple-mode stimuli-responsive emission color modulation is demonstrated in a lanthanide-doped nanostructured luminescent material. The plentiful emission colors are manipulated by modulating the composition of a fluoride core-shell nanostructure with different Ln³⁺ at different doping concentrations. The nanomaterials display remarkable excitation wavelength/power-dependent color change, along with temperature-dependent color variation in the range from 298 K to 437 K, with a good relative sensitivity S_r of 1.1387% K^-1 at 398 K. The universal optical modulation, combined with the excellent optical and structural stability of the luminescent nanoparticles, renders many advantages for the anti-counterfeiting application. This work explores a universal strategy for the manipulation of triple-mode stimuli-responsive dynamic luminescence and demonstrates its good potential for anti-counterfeiting application.

Admirable stability achieved by ns2 ions Co-doping for all-inorganic metal halides towards optical anti-counterfeiting

Optical materials play a momentous role in anti-counterfeiting field, such as authentication, currency and security. The development of tunable optical properties and optical responses to a range of external stimuli is quite imperative for the growing demand of optical anti-counterfeiting technology. Metal halide perovskites have attracted much attention of researchers due to their excellent optical properties. In addition, co-doping methods have been gradually applied to the research of metal halide perovskites, by which more abundant luminescence phenomena can be introduced into the host perovskite. Herein, the ns² ions of bismuth (Bi³⁺) and antimony (Sb³⁺) ions co-doped zero-dimensional Cs₂SnCl₆ metal halide with an excitation-wavelength-dependent emission phenomenon is synthesized as an efficient multimodal luminescent material, the luminescence of which is tunable and covers a wide region of color. What's more, a dynamic dual-emission phenomenon is captured when the excitation wavelength changes from 320 nm to 420 nm for Cs₂SnCl₆:Bi_0.08Sb_0.12 crystals. Moreover, the Bi³⁺ and Sb³⁺ doped metal halide material shows great enhancement in solvent resistance and thermal stability compared to the pristine Cs₂SnCl₆. The admirable stability and distinguishable photoluminescence (PL) phenomenon of this all-inorganic metal halide has great potential to be applied in optical anti-counterfeiting technology. Furthermore, the co-doping method can accelerate the discovery of new luminescence phenomena in original metal halide perovskites.

Integrating biopolymer design with physical unclonable functions for anticounterfeiting and product traceability in agriculture

Smallholder farmers and manufacturers in the Agri-Food sector face substantial challenges because of increasing circulation of counterfeit products (e.g., seeds), for which current countermeasures are implemented mainly at the secondary packaging level, and are generally vulnerable because of limited security guarantees. Here, by integrating biopolymer design with physical unclonable functions (PUFs), we propose a cryptographic protocol for seed authentication using biodegradable and miniaturized PUF tags made of silk microparticles. By simply drop casting a mixture of variant silk microparticles on a seed surface, tamper-evident PUF tags can be seamlessly fabricated on a variety of seeds, where the unclonability comes from the stochastic assembly of spectrally and visually distinct silk microparticles in the tag. Unique, reproducible, and unpredictable PUF codes are generated from both Raman mapping and microscopy imaging of the silk tags. Together, the proposed technology offers a highly secure solution for anticounterfeiting and product traceability in agriculture.

Development of Thermochromic Ink Using the Anthocyanidin-Based Red-Cabbage Extract for Anticounterfeiting Applications

Temperature-driven colorful switching inks have been an interesting security encoding method to improve the anticounterfeiting properties of commercially available merchandise. Recently, thermochromic inks have faced many disadvantages, such as low efficiency, high cost, and low durability. In the current study, we developed self-healable ink from poly(2-acrylamido-2-methyl-1-propanesulfonic acid) (PAMPSA) integrated with the anthocyanidin(ACY)-based red-cabbage extract in the presence of ferrous sulfate for authentication purposes. Self-healable inks have been able to guarantee durability and thermal stability. Environmentally friendly, ACY-based chromophore was extracted from Brassica oleracea L. var. Capitata (red-cabbage) to serve as a spectroscopic probe immobilized into PAMPSA. The prepared self-healable nanocomposite ink (PAMPSA-ACY) displayed temperature-induced chromism with high reversibility and thermal stability. Different self-healable nanocomposite inks of thermochromic features were prepared employing different ratios of the ACY-based red-cabbage extract. As described by Commission Internationale de L'éclairage Lab coordinates, homogeneous films were stamped on the paper surface to show a purple color (631 nm) able to switch color into red (458 nm) with the increase in temperature from 25 to 65 °C, respectively. Transmission electron microscopy, infrared spectra (FT-IR), energy-dispersive X-ray, and scanning electron microscopy were utilized to inspect the morphological behavior and chemical compositions of thermochromic prints. Both mechanical and rheological properties of ink-printed paper substrates and ink solution were also investigated. Both of antimicrobial activity and cytotoxicity study of the nanocomposite ink (PAMPSA-ACY) were also evaluated. Various industries can take the advantage of the current ink as a competent approach for anticounterfeiting purposes.

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.

Security labeling and optical information encryption enabled by laser-printed silicon Mie resonators

Fighting against the falsification of valuable items remains a crucial social-threatening challenge stimulating a never-ending search for novel anti-counterfeiting strategies. The demanding security labels must simultaneously address multiple requirements (high density of the recorded information, high protection degree, etc.) and be realized via scalable and inexpensive technologies. Here, the direct reproducible femtosecond-laser patterning of thin glass-supported amorphous (α-)Si films is proposed for optical information encryption and the scalable and highly reproducible fabrication of security labels composed of Raman-active hemispherical Si nanoparticles (NPs). Laser printing conditions allow the precise control of the diameter of the formed NPs ensuring translation of their dipolar Mie resonance position within the entire visible spectral range. Two-temperature molecular dynamics simulations clarify the origin of α-Si NP formation by rupture of the molten Si layer driven by a negative GPa-range pressure near the liquid-solid interface. Arrangement of the laser-printed Mie-resonant NP allows the creation of hidden security labels offering several easy-to-realize information encryption strategies (for example, local laser-induced post-crystallization or mixing Mie-resonant and non-resonant NPs), additional protection modalities, facile Raman mapping readout and dense information recording (up to 60 000 dots per inch) close to the optical diffraction limit. The developed fabrication strategy is simple, inexpensive, and scalable and can be realized based on cheap Earth-abundant materials and commercially-available equipment justifying its practical applicability and attractiveness for anti-counterfeit and security applications.

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.

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.

Spray coated micropatterning of metal halide perovskite for anticounterfeiting fluorescent tags

Metal halide perovskites possess exciting optoelectronic properties and are being used for various applications, including fluorescent anticounterfeiting security tags. The existing anticounterfeitings based on perovskites have a reversible transition that does not allow to know whether the information is tampered or compromised. In this work, we developed fluorescent anticounterfeiting security tags using micropatterned metal halide perovskite nanocrystals. The micro features were created by spray coating of stabilized methylammonium lead tribromide (MAPbBr₃) nanocrystals (NCs) in polystyrene (PS) solution, which has a proper wettability to various rigid and flexible substrates. The PS provides additional optical and structural stability to the MAPbBr₃NCs against polar solvents. By combining stable and unstable MAPbBr₃nanocrystals, we created a double-layer fluorescent anticounterfeiting security tag, and the information is hidden under both ambient light and UV illumination. An irreversible decryption is possible after treating the security tags with particular solvents, thus tampering of the security tag is easily detectable.

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.

Single-sized metasurface for simultaneous pseudo-color nanoprinting and holographic image display

Metasurfaces provide a novel platform for multifunctional devices due to their incomparable competence for the manipulation of different optical properties. Recently, many works have emerged to merge distinct functions into a single metasurface, which effectively increase the information density and capacity of meta-devices. In this work, combining the dual-wavelength polarizer and the orientation degeneracy of the Malus law, we further exploit the design degree of freedom of the metasurface, and realize color control and phase manipulation simultaneously with single-sized nanostructures. We experimentally demonstrate our concept by integrating the function of pseudo-color nanoprinting and holographic image display together. Our research can effectively improve the functionalities of metasurface and promote advanced research of multimode displays, information encryption, optical multiplexing, and many other related fields.

Three-channel metasurface based on simultaneous and independent control of near and far field under a single line light source

Metasurface based on independent and simultaneous control of near field and far field has significant potential for use in multichannel optics platform devices. However, the previous studies cannot satisfy independent and simultaneous control of near field and far field under a single line source, which made a significant challenge to multichannel optical platforms working in a compact environment. To manipulate effectively and freely the amplitude and phase of transmission under line source, Marius' law and Propagation phase was introduced on all-dielectric encoding metasurfaces meta-atoms. The Marius' law and Propagation phase can control the size and rotation angle of meta-atoms to encode grayscale amplitude images and holographic phase images. Finite-difference time-domain simulation results reveal that dual channel metasurface under a single line source achieves the same display effect as the dual channel metasurface under multiple light sources, which proves the feasibility of our studies. Moreover, under different angles of the line source, we encode the near-field binary image by using the degeneracy rotation angle of meta-atoms. Finally, a three-channel metasurface was obtained without affecting the display of the previous two-channel metasurface. As a result, the independent control amplitude, phase, and polarization of the incident light wave were achieved. The proposed metasurface could be applied in creating a multi-channel metasurface optical platform in a compact environment, which has application potential in image displays, optical storage, optical anti-counterfeiting, and information encryption technology.

Single-sized multifunctional metasurfaces for simultaneous nanoprinting and holography inspired by tri-redundancy

Multifunctional metasurfaces, where multiple functions can be integrated into a piece of metasurface, are preferably desired for compact systems with higher integration and subwavelength footprint. Particularly, metasurfaces for simultaneous nanoprinting and holography are one of the promising directions of development image display and information hiding in meta-devices. Here, inspired by tri-redundancy, a new, to the best of our knowledge, approach is proposed for generating a nanoprinting image in the near field and holographic image in the far field simultaneously, which can solve the extremum-mapping problem existing in single-sized scheme without increasing the complexity of the nanostructures. The tri-redundancy of image recognition, hologram designing and intensity modulation introduce an extra degree of freedom, which helps to find a balance between the two types of meta-images generated by utilizing the simulated annealing algorithm. A multifunctional metasurface composed of single-sized silver nanobricks with in-plane orientation has been fabricated to demonstrate the feasibility of encoding a binary image in the near field while reconstructing a 16-steps holographic image without twin-image in the far field. This multifunctional metasurface has flexible working modes, broadband working window and large robustness for fabrication errors, and it provides a simple design scheme for multifunctional integration. We expect it can empower advanced research and applications in high-end optical anticounterfeiting, image hiding and so on.

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.

Predicting Laser-Induced Colors of Random Plasmonic Metasurfaces and Optimizing Image Multiplexing Using Deep Learning

Structural colors of plasmonic metasurfaces have been promised to a strong technological impact thanks to their high brightness, durability, and dichroic properties. However, fabricating metasurfaces whose spatial distribution must be customized at each implementation and over large areas is still a challenge. Since the demonstration of printed image multiplexing on quasi-random plasmonic metasurfaces, laser processing appears as a promising technology to reach the right level of accuracy and versatility. The main limit comes from the absence of physical models to predict the optical properties that can emerge from the laser processing of metasurfaces in which random metallic nanostructures are characterized by their statistical properties. Here, we demonstrate that deep neural networks trained from experimental data can predict the spectra and colors of laser-induced plasmonic metasurfaces in various observation modes. With thousands of experimental data, produced in a rapid and efficient way, the training accuracy is better than the perceptual just noticeable change. This accuracy enables the use of the predicted continuous color charts to find solutions for printing multiplexed images. Our deep learning approach is validated by an experimental demonstration of laser-induced two-image multiplexing. This approach greatly improves the performance of the laser-processing technology for both printing color images and finding optimized parameters for multiplexing. The article also provides a simple mining algorithm for implementing multiplexing with multiple observation modes and colors from any printing technology. This study can improve the optimization of laser processes for high-end applications in security, entertainment, or data storage.

Vivid Structural Color Macropatterns Created by Flexible Nanopainting of Ultrafast Lasers

Structural colors based on the macro- or nanostructure formation are ubiquitous in nature, having great prospects in many fields as a result of their environmentally friendly and long-term stable characteristics compared to pigments or dyes. However, the current fabrication techniques still face challenges for the generation of high-quality structural color patterns, especially at the macroscale, in an efficient way. Here, we demonstrate a method that exploits a flexible scanning process of generating macropatterns to convert the contour profiles into well-defined sub-micrometer grating structures with unprecedented vivid structural colors, at high speed and low cost on the graphene oxide film. The nature of dynamic beam shaping of the laser line spot allows us to flexibly construct the complex patterns at high speed, in sharp contrast to the traditional point-by-point laser processing. Moreover, the multicolor display of the patterns can be carried out by simply modulating the laser polarization to change the orientation of the sub-micrometer structures, and this nanopainting strategy is further explored to flexibly design the composite image for potential anti-counterfeiting applications.

Multifold Integration of Printed and Holographic Meta-Image Displays Enabled by Dual-Degeneracy

By virtue of the unprecedented ability of manipulating the optical parameters, metasurfaces open up a new avenue for realizing ultra-compact image displays, e.g., nanoprinting on the surface and holographic displaying in the far-field. The multifold integration of these two functions into a single metasurface can undoubtedly expand the functionality and increase the information capacity. In this study, a minimalist tri-channel metasurface is proposed and experimentally demonstrated with multifold integration of printed and holographic displaying, which can generate two N-bit grayscale images and a four-step holographic image simultaneously. Benefiting from exploiting the degeneracy of energy allocation and the degeneracy of nanostructure orientations, the functionalities of nanoprinting and holography are combined without the need of a large amount of nanostructures with varied dimensions, which would facilitate both the metasurface design and fabrication. The proposed scheme provides a new idea in enhancing the functionality and capacity of metasurfaces without complicating their design, which has promising prospects for applications in ultra-compact image displays, high-density optical storage, optical anti-counterfeiting and many other related fields.

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.

Anti-Counterfeiting Tags Using Flexible Substrate with Gradient Micropatterning of Silver Nanowires

Anti-counterfeiting technologies for small products are being developed. We present an anti-counterfeiting tag, a grayscale pattern of silver nanowires (AgNWs) on a flexible substrate. The anti-counterfeiting tag that is observable with a thermal imaging camera was fabricated using the characteristics of silver nanowires with high visible light transmittance and high infrared emissivity. AgNWs were patterned at microscale via a maskless lithography method using UV dicing tape with UV patterns. By attaching and detaching an AgNW coated glass slide and UV dicing tape irradiated with multiple levels of UV, we obtained AgNW patterns with four or more grayscales. Peel tests confirmed that the adhesive strength of the UV dicing tape varied according to the amount of UV irradiation, and electrical resistance and IR image intensity measurements confirmed that the pattern obtained using this tape has multi-level AgNW concentrations. When applied for anti-counterfeiting, the gradient-concentration AgNW micropattern could contain more information than a single-concentration micropattern. In addition, the gradient AgNW micropattern could be transferred to a flexible polymer substrate using a simple method and then attached to various surfaces for use as an anti-counterfeiting tag.

Physically unclonable functions taggant for universal steganographic prints

Counterfeiting of financial cards and marketable securities is a major social problem globally. Electronic identification and image recognition are common anti-counterfeiting techniques, yet they can be overcome by understanding the corresponding algorithms and analysis methods. The present work describes a physically unclonable functions taggant, in an aqueous-soluble ink, based on surface-enhanced Raman scattering of discrete self-assemblies of Au nanoparticles. Using this stealth nanobeacon, we detected a fingerprint-type Raman spectroscopy signal that we clearly identified even on a business card with a pigment mask such as copper-phthalocyanine printed on it. Accordingly, we have overcome the reverse engineering problem that is otherwise inherent to analogous anti-counterfeiting techniques. One can readily tailor the ink to various information needs and application requirements. Our stealth nanobeacon printing will be particularly useful for steganography and provide a sensitive fingerprint for anti-counterfeiting.

Bidirectional nanoprinting based on bilayer metasurfaces

Bidirectional nanoprinting, has received significant attention in image display and on-chip integration, due to its superior advantages. By manipulating the amplitude in a narrow- or broad-band wavelength range of forward and backward incident light, different spatially varied intensities or color distributions can be generated on the structure plane. However, the current scheme cannot fully decouple the bidirectional light intensity due to the limitation of design degree of freedom, and it would hinder the development of asymmetric photonic devices. In this paper, we propose and demonstrate bidirectional nanoprinting based on an all-dielectric bilayer metasurface, which can independently control the intensity of forward and backward incident light, resulting in two different continuous grayscale meta-image displaying in the visible region. This asymmetric but still bidirectional optical response is introduced by stacking two layers of nanostructures with different functionality in space, in which the first- and second-layer nanostructures act as a half-wave plate and a polarizer, respectively. Interestingly, these bidirectional nanoprinting metasurfaces have flexible working modes and may bring great convenience for practical applications. Specifically, two different meta-images generated by a bidirectional nanoprinting metasurface can be displayed not only on two sides of the metasurface (working mode in transmission or reflection), but on the same side due to the forward transmitted light and backward reflected light also having asymmetric optical properties. Similar phenomena also exist for forward reflected light and backward transmitted light. Our work extremely expands the design freedom for metasurface devices and may play a significant role in the field of optical display, information multiplexing, etc.

Metasurface-enabled three-in-one nanoprints by multifunctional manipulations of light

In metasurface-based ultra-compact image display, color-nanoprints, gray-imaging elements, and binary-pattern-imaging elements are three different types of nanoprints, implemented with different mechanisms of light manipulation. Here, we show the three functional elements can be integrated together to form a "three-in-one" nanoprint with negligible crosstalk, merely with a single-cell nanostructured design approach. Specifically, by decoupling spectrum and polarization-assisted intensity manipulations of incident light, the proposed metasurface appears as a dual-color nanoprint under a broadband unpolarized light source illumination, while simultaneously displaying an independent continuous gray image and another binary-pattern in an orthogonal-polarization optical setup with different polarization controls. Our approach can increase the system integration and security of metasurfaces, which can be of interest to many advanced applications such as data storage, optical information encoding, high-end optical anti-counterfeiting, and optical information hiding.

Synergistic strategy of rare-earth doped nanoparticles for NIR-II biomedical imaging

Featuring simultaneous multicolor imaging for multiple targets, a synergistic strategy has become promising for fluorescence imaging applications. Visible and first near infrared (NIR-I, 700-900 nm) fluorophores have been explored for multicolor imaging to achieve good multi-target capacity, but they are largely hampered by the narrow imaging bands available (400-900 nm, bandwidth 500 nm), the broad emission spectra of many fluorophores, shallow tissue penetration and scattering loss. With attractive characteristic emission peaks in the second NIR window (NIR-II, 1000-1700 nm), a narrow emission spectrum, and deeper tissue penetration capability, rare-earth doped nanoparticles (RENPs) have been considered by us to be outstanding candidates for multicolor bioimaging. Herein, two RENPs, NaYF4:Yb20Er2@NaYF4 and NaYF4:Nd5@NaYF4, were prepared and modified with polyethylene glycol (PEG) to explore simultaneous imaging in the NIR-IIb (1530 nm, under 980 nm laser excitation) and the NIR-II (1060 nm, under 808 nm laser excitation) windows. The PEGylated-RENPs (RENPs@PEG) were able to simultaneously visualize the circulatory system, trace the lymphatic system, and evaluate the skeletal system. Our study demonstrates that RENPs have high synergistic imaging capability in multifunctional biomedical applications using their NIR-II fluorescence. Importantly, the two RENPs@PEG are complementary to each other for higher temporal resolution in NaYF4:Nd5@NaYF4@PEG and higher spatial resolution in NaYF4:Yb20Er2@NaYF4@PEG, which may provide more comprehensive and accurate imaging diagnosis. In conclusion, RENPs are highly promising nanomaterials for multicolor imaging in the NIR-II window.

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.

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.