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

Metasurfaces have shown unparalleled controllability of electromagnetic (EM) waves. However, most of the metasurfaces need external spatial feeding sources, which renders practical implementation quite challenging. Here, a low‐profile programmable metasurface with 0.05λ0 thickness driven by guided waves is proposed to achieve dynamic control of both amplitude and phase simultaneously. The metasurface is fed by a guided wave traveling in a substrate‐integrated waveguide, avoiding external spatial sources and complex power divider networks. By manipulating the state of the p‐i‐n diodes embedded in each meta‐atom, the proposed metasurface enables 1‐bit amplitude switching between radiating and nonradiating states, as well as a 1‐bit phase switching between 0° and 180°. As a proof of concept, two advanced functionalities, namely, low sidelobe‐level beam scanning and Airy beam generation, are experimentally demonstrated with a single platform operating in the far‐ and near‐field respectively. Such complex‐amplitude, programmable, and low‐profile metasurfaces can overcome integration limitations of traditional metasurfaces, and open up new avenues for more accurate and advanced EM wave control within an unprecedented degree of freedom. A low‐profile programmable metasurface with 0.05λ0 thickness driven by guided waves is proposed to achieve dynamic control of both amplitude and phase simultaneously. As a proof of concept, two advanced functionalities, namely, low sidelobe‐level beam scanning and Airy beam generation, are experimentally demonstrated with a single platform operating in the far‐ and near‐field respectively.

Catenary functions are exciting and fundamental building blocks in constructing various kinds of waves in subwavelength structures. Here, a simple yet powerful approach inspired by catenary optics is proposed to realize efficient manipulation of electromagnetic waves in terms of both amplitude and phase. By properly engineering the catenary electromagnetic fields and frequency dispersion, lightweight metafilm‐based broadband absorbers with polarization‐independent bandwidth covering 0.65–6.2 GHz are experimentally achieved, and the bandwidth is further broadened to 0.9–40 GHz. With the same approach, a large‐scale flat antenna based on generalized reflection is demonstrated in the satellite communication system. To enable the batch manufacturing, a flexible substrate–based microfabrication process is developed with a minimum feature size of down to sub‐micrometer and total size up to almost 1 m. These results may provide important guidance for the design of metasurface‐based devices. A systematic, general way to achieve efficient design of flat electromagnetic devices based on the catenary model is demonstrated. By employing the proposed method, broadband absorbers and flat antennas are experimentally achieved with lightweight properties and higher efficiencies compared with their traditional counterparts. Besides, a film‐based microfabrication process is developed to enable batch manufacturing for these devices.

Catenary functions are exciting and fundamental building blocks in constructing various kinds of waves in subwavelength structures. In article number 1801691, Xiangang Luo and co‐workers propose a simple yet powerful approach to realize efficient design of metasurfaces inspired by catenary electromagnetics. By employing this methodology, flexible‐substrate‐based large‐scale broadband absorbers and flat antennas are experimentally achieved with lightweight properties and outstanding operation performances.

Plasmonic metasurfaces, which can be considered as the two-dimensional analog of metal-based metamaterials, have attracted progressively increasing attention in recent years because of the ease of fabrication and unprecedented control over the reflected or transmitted light while featuring relatively low losses even at optical wavelengths. Among all the different design approaches, gap-surface plasmon metasurfaces – a specific branch of plasmonic metasurfaces – which consist of a subwavelength thin dielectric spacer sandwiched between an optically thick metal film and arrays of metal subwavelength elements arranged in a strictly or quasi-periodic fashion, have gained awareness from researchers working at practically any frequency regime as its realization only requires a single lithographic step, yet with the possibility to fully control the amplitude, phase, and polarization of the reflected light. In this paper, we review the fundamentals, recent developments, and opportunities of gap-surface plasmon metasurfaces. Starting with introducing the concept of gap-surface plasmon metasurfaces, we present three typical gap-surface plasmon resonators, introduce generalized Snell’s law, and explain the concept of Pancharatnam-Berry phase. We then overview the main applications of gap-surface plasmon metasurfaces, including beam-steerers, flat lenses, holograms, absorbers, color printing, polarization control, surface wave couplers, and dynamically reconfigurable metasurfaces. The review is ended with a short summary and outlook on possible future developments.

Optical metasurfaces allow the ability to precisely manipulate the wavefront of light, creating many interesting and exotic optical phenomena. However, they generally lack dynamic control over their optical properties and are limited to passive optical elements. In this work, we report the nontrivial infiltration of nanostructured metalenses with three respective nematic liquid crystals of different refractive index and birefringence. The optical properties of the metalens are evaluated after liquid-crystal infiltration to quantify its effect on the intended optical design. We observe a significant modification of the metalens focus after infiltration for each liquid crystal. These optical changes result from modification of local refractive index surrounding the metalens structure after infiltration. We report qualitative agreement of the optical experiments with finite-difference time-domain solver (FDTD) simulation results. By harnessing the tunability inherent in the orientation dependent refractive index of the infiltrated liquid crystal, the metalens system considered here has the potential to enable dynamic reconfigurability in metasurfaces.

While elastic metasurfaces offer a remarkable and very effective approach to the subwavelength control of stress waves, their use in practical applications is severely hindered by intrinsically narrow band performance. In applications to electromagnetic and photonic metamaterials, some success in extending the operating dynamic range was obtained by using nonlocality. However, while electronic properties in natural materials can show significant nonlocal effects, even at the macroscales, in mechanics, nonlocality is a higher-order effect that becomes appreciable only at the microscales. This study introduces the concept of intentional nonlocality as a fundamental mechanism to design passive elastic metasurfaces capable of an exceptionally broadband operating range. The nonlocal behavior is achieved by exploiting nonlocal forces, conceptually akin to long-range interactions in nonlocal material microstructures, between subsets of resonant unit cells forming the metasurface. These long-range forces are obtained via carefully crafted flexible elements, whose specific geometry and local dynamics are designed to create remarkably complex transfer functions between multiple units. The resulting nonlocal coupling forces enable achieving phase-gradient profiles that are functions of the wavenumber of the incident wave. The identification of relevant design parameters and the assessment of their impact on performance are explored via a combination of semianalytical and numerical models. The nonlocal metasurface concept is tested, both numerically and experimentally, by embedding a total-internal-reflection design in a thin-plate waveguide. Results confirm the feasibility of the intentionally nonlocal design concept and its ability to achieve a fully passive and broadband wave control

Emerging photonic functionalities are mostly governed by the fundamental principle of Lorentz reciprocity. Lifting the constraints imposed by this principle could circumvent deleterious effects that limit the performance of photonic systems. Most efforts to date have been limited to waveguide platforms. Here, we propose and experimentally demonstrate a spatio-temporally modulated metasurface capable of complete violation of Lorentz reciprocity by reflecting an incident beam into far-field radiation in forward scattering, but into near-field surface waves in reverse scattering. These observations are shown both in nonreciprocal beam steering and nonreciprocal focusing. We also demonstrate nonreciprocal behavior of propagative-only waves in the frequency- and momentum-domains, and simultaneously in both. We develop a generalized Bloch-Floquet theory which offers physical insights into Lorentz nonreciprocity for arbitrary spatial phase gradients, and its predictions are in excellent agreement with experiments. Our work opens exciting opportunities in applications where free-space nonreciprocal wave propagation is desired. Overcoming reciprocity is important for novel functionalities. Here, the authors demonstrate a spatio-temporally modulated metasurface capable of complete violation of Lorentz reciprocity by reflecting an incident beam into far-field radiation in forward scattering, but into near-field surface waves in reverse scattering.

Metasurface is an artificial material composed of a series of subwavelength structure units and has unique electromagnetic characteristics. Based on the ability of manipulating the phase, amplitude, and polarization of electromagnetic wave, various kinds of metasurfaces are designed to realize wavefront manipulations, such as beam focusing, beam steering, vector beams generating, and holographic imaging. This paper reviews the design methods of metasurfaces for wavefront modulation and evolution of the metasurfaces designed for wavefront manipulation in the terahertz (THz) region. The metasurfaces can be divided into two categories: passive and active metasurfaces. For the passive metasurfaces, the single-functional metasurfaces, multifunctional metasurfaces, and high diffraction efficient metasurfaces designed for various THz wavefront shaping, such as focusing, imaging, and special beams generating, are reviewed. For the active metasurfaces, the metasurfaces with fixed structure and all-optical metasurfaces without fixed structure for THz wavefront modulation are summarized. Furthermore, a comparison on the performance of different kinds of metasurfaces for THz wavefront modulation is presented and the development direction and challenges of the THz wavefront modulation metasurfaces in the future are discussed.

This paper presents a metasurface antenna with both radiation and scattering characteristics. First, a symmetric slotted patch antenna unit is designed and its operating frequency is tuned to the 8.4–9.4 GHz (11.2%) range. Subsequently, incorporating the principles of propagation phase, this study designs eight phase gradient metasurface units by varying the length of the I-shaped metallic arms on the unit surface, achieving a complete 2 π phase coverage. By arranging such metasurface antenna units in different configurations, functions such as radar cross-section (RCS) reduction, vortex wave generation, and beam deflection can be achieved in the operating frequency band. To achieve the integration of the radiation and scattering functions, a power divider is designed to feed the metasurface antenna. This approach achieves a unified system capable of transmitting and manipulating electromagnetic waves in multiple ways. According to the simulation and measurement results, the meta-plane antenna, after forming an array, can operate in the frequency band up to 8.5 GHz–10.6 GHz (22%), and the radiation gain in the frequency band can reach 20.1 dBi–21.4 dBi, which can effectively reduce the RCS in and out of the frequency band, as well as realize the functions of vortex wave generation and beam deflection, and, in short, the meta-plane antenna achieves the integration of radiation and scattering functions.