Terahertz switchable VO2-Au hybrid active metasurface holographic encryption.

The combination of metasurface and holographic technology is the most cutting-edge development, but most of the proposed designs are static and do not allow active changes through external stimulation after fabrication, which takes only a limited part of the advantage provided by metasurface. Here, we propose and demonstrate a switchable hybrid active metasurface hologram in the terahertz (THz) regime composed of dynamic pixels (VO2-CSRR) and static pixels (Au-CSRR) based on an intelligent algorithm, which can display some/all information in different temperature ranges. In particular, such performance shows excellent potential in the field of optical communication security, making it a promising candidate. To prove this possibility, we propose a scheme for optical information encryption/decryption and transmission, which takes metasurfaces as carriers of encrypted information and state/polarization/positions as the secret key components. Only when the two matches correctly can we get the hidden real information. The security of our proposed scheme has reached an unprecedented level, providing a new road for communication security.

Single Pixel Imaging Key for Holographic Encryption Based on Spatial Multiplexing Metasurface.

Compared with the traditional holographic technology, metasurface holography is a promising technology due to the large field angle and high spatial resolution. Thanks to the precise control of phase, amplitude, polarization and so on, metasurface holography provides a flexible platform for light modulation, optical encryption and so on. Besides, the process of image reconstruction by single pixel imaging is similar to a form of encoding and decoding, which is realized by calculating the correlation between a series of modulation patterns and their corresponding intensity signals. In this work, an optical encryption scheme is proposed based on spatial multiplexing metasurface, which depends on the combination of holographic technology and single pixel imaging technology. In the encryption scheme, the image transmitted by single pixel imaging based on metasurface is used as the addressing key of holography. Besides, illuminating different positions of the metasurface can generate different holographic reconstructed images, and there is 50% information overlapped between adjacent sub-holograms. This work makes use of the spatial multiplexing property of metasurface, which can complete different functions, paving the way for the application in the field of optical imaging encryption and information security.

Tailoring the Excited and Cutoff States of Spoof Surface Plasmon Polaritons for Full-Space Quadruple Functionalities.

Integrating diversified functionalities within a single aperture is crucial for microwave and optics-integrated devices. To date, research on this issue suffers from restricted bifunctionality, inadequate efficiency, and the limitation of extending to manipulate full-space wave. Here, we propose a general paradigm to achieve full-space multifunctional integration via tailoring the excited and cutoff states of spoof surface plasmon polaritons (SSPPs). A plasmonic meta-atom consisting of judiciously arranged metallic strips is used to excite and cut off the SSPP mode with uniaxially anisotropic characteristics. By shaping the topological structure of the meta-atom, the transmission and reflection phases are arbitrarily controlled at each pixel. Accordingly, the cross-placed meta-atom arrays can be designed to achieve independent phase profiles for x-/y-polarized transmission/reflection waves through dispersion engineering. A metamaterial with quadruple functionalities of backward beams scattering/anomalous reflection and electromagnetic transmission focusing/vortex is designed and fabricated as a proof-of-principle to reveal flexible manipulation. Both simulation and experimental verification are carried out in microwave frequency to demonstrate the feasibility.

Nonlinear Bicolor Holography Using Plasmonic Metasurfaces.

Nonlinear metasurface holography shows the great potential of metasurfaces to control the phase, amplitude, and polarization of light while simultaneously converting the frequency of the light. The possibility of tailoring the scattering properties of a coherent beam, as well as the scattering properties of nonlinear signals originating from the meta-atoms, facilitates a huge degree of freedom in beam shaping application. Recently, several approaches showed that virtual objects or any kind of optical information can be generated at a wavelength different from the laser input beam. Here, we demonstrate a single-layer nonlinear geometric-phase metasurface made of plasmonic nanostructures for a simultaneous second- and third-harmonic generation. Different from previous works, we demonstrate a two-color hologram with dissimilar types of nanostructures that generate the color information by different nonlinear optical processes. The amplitude ratio of both harmonic signals can be adapted depending on the nanostructures' resonance as well as the power and the wavelength of the incident laser beam. The two-color holographic image is reconstructed in the Fourier space at visible wavelengths with equal amplitudes using a single near-infrared wavelength. Nonlinear holography using multiple nonlinear processes simultaneously provides an alternative path to holographic color display applications, enhanced optical encryption schemes, and multiplexed optical data storage.

Optical secret sharing with cascaded metasurface holography.

Secret sharing is a well-established cryptographic primitive for storing highly sensitive information like encryption keys for encoded data. It describes the problem of splitting a secret into different shares, without revealing any information to its shareholders. Here, we demonstrate an all-optical solution for secret sharing based on metasurface holography. In our concept, metasurface holograms are used as spatially separable shares that carry encrypted messages in the form of holographic images. Two of these shares can be recombined by bringing them close together. Light passing through this stack of metasurfaces accumulates the phase shift of both holograms and optically reconstructs the secret with high fidelity. In addition, the hologram generated by each single metasurface can uniquely identify its shareholder. Furthermore, we demonstrate that the inherent translational alignment sensitivity between two stacked metasurface holograms can be used for spatial multiplexing, which can be further extended to realize optical rulers.

Creating a three-dimensional surface with antireflective properties by using femtosecond-laser Bessel-beam-assisted thermal oxidation.

Metal surfaces with low reflectance have received considerable attention for their great optical, electrical, and thermal properties. However, the difficulty in achieving low reflectance on curved metal surfaces has hindered their practical applications. We propose a rapid and flexible method for processing a three-dimensional surface with antireflective properties. A Bessel beam created using an axicon is employed to generate ripple structures on the curved surface, thereby assisting subsequent thermal oxidation. Ripple structures coated with oxide semiconductor nanowires are then processed on a Cu substrate, thus further reducing reflectance. Antireflective properties with a minimum reflectance of less than 0.015 at a wavelength of 500-1200 nm could be achieved by using this method. This presented approach reduces dimensionality in laser processing, subsequently improving processing efficiency, and provides a foundation for the practical application of metal antireflective surfaces.

Polarization-Encrypted Orbital Angular Momentum Multiplexed Metasurface Holography.

Metasurface holography has the advantage of realizing complex wavefront modulation by thin layers together with the progressive technique of computer-generated holographic imaging. Despite the well-known light parameters, such as amplitude, phase, polarization, and frequency, the orbital angular momentum (OAM) of a beam can be regarded as another degree of freedom. Here, we propose and demonstrate orbital angular momentum multiplexing at different polarization channels using a birefringent metasurface for holographic encryption. The OAM selective holographic information can only be reconstructed with the exact topological charge and a specific polarization state. By using an incident beam with different topological charges as erasers, we mimic a super-resolution case for the reconstructed image, in analogy to the well-known STED technique in microscopy. The combination of multiple polarization channels together with the orbital angular momentum selectivity provides a higher security level for holographic encryption. Such a technique can be applied for beam shaping, optical camouflage, data storage, and dynamic displays.

Simultaneous Spectral and Spatial Modulation for Color Printing and Holography Using All-Dielectric Metasurfaces.

Metasurfaces possess the outstanding ability to tailor phase, amplitude, and even spectral responses of light with an unprecedented ultrahigh resolution and thus have attracted significant interest. Here, we propose and experimentally demonstrate a novel meta-device that integrates color printing and computer-generated holograms within a single-layer dielectric metasurface by modulating spectral and spatial responses at subwavelength scale, simultaneously. In our design, such metasurface appears as a microscopic color image under white light illumination, while encrypting two different holographic images that can be projected at the far-field when illuminated with red and green laser beams. We choose amorphous silicon dimers and nanofins as building components and use a modified parallel Gerchberg-Saxton algorithm to obtain multiple subholograms with arbitrary spatial shapes for image-indexed arrangements while avoiding the loss of phase information. Such a method can further extend the design freedom of metasurfaces. By exploiting spectral and spatial control at the level of individual pixels, multiple sets of independent information can be introduced into a single-layer device; the additional complexity and enlarged information capacity are promising for novel applications such as information security and anticounterfeiting.

Reconfigurable metasurface hologram by utilizing addressable dynamic pixels.

The flatness, compactness and high-capacity data storage capability make metasurfaces well-suited for holographic information recording and generation. However, most of the metasurface holograms are static, not allowing a dynamic modification of the phase profile after fabrication. Here, we propose and demonstrate a dynamic metasurface hologram by utilizing hierarchical reaction kinetics of magnesium upon a hydrogenation/dehydrogenation process. The metasurface is composed of composite gold/magnesium V-shaped nanoantennas as building blocks, leading to a reconfigurable phase profile in a hydrogen/oxygen environment. We have developed an iterative hologram algorithm based on the Fidoc method to build up a quantified phase relation, which allows the reconfigurable phase profile to reshape the reconstructed image. Such a strategy introduces actively controllable dynamic pixels through a hydrogen-regulated chemical process, showing unprecedented potentials for optical encryption, information processing and dynamic holographic image alteration.

Dynamic control of mode modulation and spatial multiplexing using hybrid metasurfaces.

Designing reconfigurable metasurfaces that can dynamically control scattered electromagnetic waves and work in the near-infrared (NIR) and optical regimes remains a challenging task, which is hindered by the static material property and fixed structures. Phase change materials (PCMs) can provide high contrast optical refractive indexes at high frequencies between amorphous and crystal states, therefore are promising as feasible materials for reconfigurable metasurfaces. Here, we propose a hybrid metasurface that can arbitrarily modulate the complex amplitude of incident light with uniform amplitude and full 2π phase coverage by utilizing composite concentric rings (CCRs) with different ratios of gold and PCMs. Our designed metasurface possesses a bi-functionality that is capable of splitting beams or generating vortex beams by thermal switching between metal and semiconductor states of vanadium oxide (VO2), respectively. It can be easily integrated into low loss photonic circuits with an ultra-small footprint. Our metadevice serves as a novel paradigm for active control of beams, which may open new opportunities for signal processing, memory storage, holography, and anti-counterfeiting.

Nonreciprocal Asymmetric Polarization Encryption by Layered Plasmonic Metasurfaces.

As flexible optical devices that can manipulate the phase and amplitude of light, metasurfaces would clearly benefit from directional optical properties. However, single layer metasurface systems consisting of two-dimensional nanoparticle arrays exhibit only a weak spatial asymmetry perpendicular to the surface and therefore have mostly symmetric transmission features. Here, we present a metasurface design principle for nonreciprocal polarization encryption of holographic images. Our approach is based on a two-layer plasmonic metasurface design that introduces a local asymmetry and generates a bidirectional functionality with full phase and amplitude control of the transmitted light. The encoded hologram is designed to appear in a particular linear cross-polarization channel, while it is disappearing in the reverse propagation direction. Hence, layered metasurface systems can feature asymmetric transmission with full phase and amplitude control and therefore expand the design freedom in nanoscale optical devices toward asymmetric information processing and security features for anticounterfeiting applications.

Multichannel vectorial holographic display and encryption.

Since its invention, holography has emerged as a powerful tool to fully reconstruct the wavefronts of light including all the fundamental properties (amplitude, phase, polarization, wave vector, and frequency). For exploring the full capability for information storage/display and enhancing the encryption security of metasurface holograms, smart multiplexing techniques together with suitable metasurface designs are highly demanded. Here, we integrate multiple polarization manipulation channels for various spatial phase profiles into a single birefringent vectorial hologram by completely avoiding unwanted cross-talk. Multiple independent target phase profiles with quantified phase relations that can process significantly different information in different polarization states are realized within a single metasurface. For our metasurface holograms, we demonstrate high fidelity, large efficiency, broadband operation, and a total of twelve polarization channels. Such multichannel polarization multiplexing can be used for dynamic vectorial holographic display and can provide triple protection for optical security. The concept is appealing for applications of arbitrary spin to angular momentum conversion and various phase modulation/beam shaping elements.