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In this paper, a novel method is proposed to achieve two distinct information channels by simultaneously manipulating both the transmitted cross- and co-polarized components of a 1-bit coding metasurface under linearly polarized incidence. Compared to previously demonstrated incidence-switchable or position multiplexed holograms, our proposed coding meta-hologram can simultaneously project two independent holographic images without inevitable change of the incidence state and can at the same time also avoid crosstalk between different channels. Moreover, the orientation of the double-layered split ring (SR) apertures is specially designed to be 45° or 135° to achieve identical multiplexed functionality for both -polarized and -polarized incidences. The proof-of-concept experimental demonstrations present total transmittance efficiency above 30% for the dual linearly polarized incidences at 15 GHz, and good imaging performances with 53.98%/48.18% imaging efficiency, 1.55%/1.46% RMSE, and 29.9/28.72 peak signal-to-noise ratio for the cross-/co-polarized channels under -polarized incidence, and 47.27%/45.75% imaging efficienc , 1.55%/1.43% RMSE, and 18.74/25.93 peak signal-to-noise ratio under -polarized incidence, demonstrating great potential of the proposed multiplexed coding meta-hologram in practical applications such as data storage and information processing.

The interplay between two paradigms, artificial intelligence (AI) and optical metasurfaces, nowadays appears obvious and unavoidable. AI is permeating literally all facets of human activity, from science and arts to everyday life. On the other hand, optical metasurfaces offer diverse and sophisticated multifunctionalities, many of which appeared impossible only a short time ago. The use of AI for optimization is a general approach that has become ubiquitous. However, here we are witnessing a two-way process—AI is improving metasurfaces but some metasurfaces are also improving AI. AI helps design, analyze and utilize metasurfaces, while metasurfaces ensure the creation of all-optical AI chips. This ensures positive feedback where each of the two enhances the other one: this may well be a revolution in the making. A vast number of publications already cover either the first or the second direction; only a modest number includes both. This is an attempt to make a reader-friendly critical overview of this emerging synergy. It first succinctly reviews the research trends, stressing the most recent findings. Then, it considers possible future developments and challenges. The author hopes that this broad interdisciplinary overview will be useful both to dedicated experts and a general scholarly audience.

The metaverse has captured significant attention as it provides a virtual realm that cannot be experienced in the physical world. Programmable optical holograms, integral components of the metaverse, allow users to access diverse information without needing external equipment. Meta-devices composed of artificially customized nano-antennas are excellent candidates for programmable optical holograms due to their compact footprint and flexible electromagnetic manipulation. Programmable optical meta-holograms can dynamically alter reconstructed images in real-time by directly modulating the optical properties of the metasurface or by modifying the incident light. Information can be encoded across multiple channels and freely selected through switchable functionality. These advantages will broaden the range of virtual scenarios in the metaverse, facilitating further development and practical applications. This review concentrates on recent advancements in the fundamentals and applications of programmable optical meta-holograms. We aim to provide readers with general knowledge and potential inspiration for applying programmable optical meta-holograms, both intrinsic and external ways, into the metaverse for better performance. An outlook and perspective on the challenges and prospects in these rapidly growing research areas are provided.

Holography is a highly desired technology in modern photonics, yet setups for traditional generating methods suffer from complexity and bulky sizes. While metasurface-based holography exhibits advantages of compactness and easy-fabrication, most meta-holograms realized so far exhibit only single functionality, with a few multifunctional ones suffering from imbalances of efficiency and device-thickness. Here, we propose a generic approach to design metasurfaces for realization of multiple holographic images with , and experimentally verify the concept in the telecom regime. We first design a series of high-efficiency reflective meta-atoms exhibiting incident-spin-delinked reflection phases governed by geometric and resonant mechanisms, and experimentally characterize their optical properties at wavelengths around 1,064 nm. We next experimentally demonstrate a single-functional meta-hologram as a benchmark test. Finally, we employ the designed meta-atoms to construct a metasurface with the thickness ∼1/4 , and experimentally demonstrate its capability of generating two distinct holographic images under illuminations of circularly polarized light beams with different helicities, possessing generation efficiencies ∼48.08 %. Our work provides a highly-efficient and ultra-compact platform to generate multifunctional holographic images, which may inspire numerous applications in integration optics.

Securing optical information to avoid counterfeiting and manipulation by unauthorized persons and agencies requires innovation and enhancement of security beyond basic intensity encryption. In this paper, we present a new method for polarization-dependent optical encryption that relies on extremely high-resolution near-field phase encoding at metasurfaces, down to the diffraction limit. Unlike previous intensity or color printing methods, which are detectable by the human eye, analog phase decoding requires specific decryption setup to achieve a higher security level. In this work, quadriwave lateral shearing interferometry is used as a phase decryption method, decrypting binary quick response (QR) phase codes and thus forming phase-contrast images, with phase values as low as 15°. Combining near-field phase imaging and far-field holographic imaging under orthogonal polarization illumination, we enhanced the security level for potential applications in the area of biometric recognition, secure ID cards, secure optical data storage, steganography, and communications.

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 2 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.

Compound meta-optics, characterized by the unprecedented complex optical architectures containing single or multiple meta-optics elements, has emerged as a powerful paradigm for overcoming the physical limitations of single-layer metasurfaces. This review systematically examines the recent progress in this burgeoning field, primarily focusing on the development of high-performance optical systems for imaging, display, sensing, and computing. We first focus on the design of compound metalens architectures that integrate metalenses with additional elements such as iris, refractive optics, or other meta-optics elements. These configurations effectively succeed in providing multiple high-quality image quality metrics simultaneously by correcting monochromatic and chromatic aberrations, expanding the field of view, enhancing overall efficiency, and so on. Thus, the compound approach enables practical applications in next-generation cameras and sensors. Furthermore, we explore the advancement of cascaded metasurfaces in the realm of wave-optics, specifically for advanced meta-holography and optical computing. These multi-layered systems facilitate complex wavefront engineering, leading to significant increases in information capacity and functionality for security and analog optical computing applications. By providing a comprehensive overview of fundamental principles, design strategies, and emerging applications, this review aims to offer a clear perspective on the pivotal role of compound meta-optics in devising and optimizing compact, multifunctional optical systems to optics engineers with a variety of professional knowledge backgrounds and techniques.

Optical metasurfaces enable compact, lightweight and planar optical devices. Their performances, however, are still limited by design approximations imposed by their macroscopic dimensions. To address this problem, we propose a neural network-based multi-stage gradient optimization method to efficiently modulate nonlocal interactions between meta-atoms, which is one of the major effects neglected by current design methods. Our strategy allows for the use of these interactions as an additional design dimension to enhance the performance of metasurfaces and can be used to optimize large-scale metasurfaces with multiple parameters. As an example of application, we design a meta-hologram with a zero-order energy suppressed to 26% (theoretically) and 57% (experimentally) of its original value. Our results suggest that neural networks can be used as a powerful design tool for the next generation of high-performance metasurfaces with complex functionalities.

Chirality, as one of the ubiquitous properties in nature, has aroused striking attention in the fields of physics and materials sciences. For tailoring of light with more degree of freedom, circular dichroism has been considered as the auxiliary dimension to improving such an issue. Inspired by the Born-Kuhn plasmonic oscillation model, we demonstrate and discuss a chiral resonator to reveal reverse circular dichroism within two separate frequency bands in the microwave regime. The underlying physical mechanisms of bonding and anti-bounding modes are analyzed via transmission/reflection spectra and surface current distributions for different chiral enantiomers. To leverage the proposed paradigm to the application levels, especially for meta-holography, we conduct the numerical simulation and experimental demonstration of two proofs of principles. Based on the Pancharatnam-Berry phase/amplitude modulation and complex-amplitude manipulation, respectively, the meta-holograms with independent targets of reconstructing images in full-space and reflected regions are achieved. Significantly, our paradigm may promise further applications in spin-controlled meta-devices for image processing, information encryption, anti-counterfeiting, remote sensing, and radar systems.