Metasurface-Empowered Optical Multiplexing and Multifunction

Metasurfaces are planar photonic elements composed of subwavelength nanostructures, which can deeply interact with light and exploit new degrees of freedom (DOF) to manipulate optical fields. In the past decade, metasurfaces have drawn great interest from the scientific community due to their profound potential to arbitrarily control light. Here, recent developments of multiplexing and multifunctional metasurfaces, which enable concurrent tasks through a dramatic compact design, are reviewed. The fundamental properties, design strategies, and applications of multiplexing and multifunctional metasurfaces are then discussed. First, recent progress on angular momentum multiplexing, including its behavior under different incident conditions, is considered. Second, a detailed overview of polarization-controlled, wavelength-selective, angle-selective, and reconfigurable multiplexing/multifunctional metasurfaces is provided. Then, the integrated and on-chip design of multifunctional metasurfaces is addressed. Finally, future directions and potential applications are presented.     

From Single‐Dimensional to Multidimensional Manipulation of Optical Waves with Metasurfaces

Metasurfaces, 2D artificial arrays of subwavelength elements, have attracted great interest from the optical scientific community in recent years because they provide versatile possibilities for the manipulation of optical waves and promise an effective way for miniaturization and integration of optical devices. In the past decade, the main efforts were focused on the realization of single‐dimensional (amplitude, frequency, polarization, or phase) manipulation of optical waves. Compared to the metasurfaces with single‐dimensional manipulation, metasurfaces with multidimensional manipulation of optical waves show significant advantages in many practical application areas, such as optical holograms, sub‐diffraction imaging, and the design of integrated multifunctional optical devices. Nowadays, with the rapid development of nanofabrication techniques, the research of metasurfaces has been inevitably developed from single‐dimensional manipulation toward multidimensional manipulation of optical waves, which greatly boosts the application of metasurfaces and further paves the way for arbitrary design of optical devices. Herein, the recent advances in metasurfaces are briefly reviewed and classified from the viewpoint of different dimensional manipulations of optical waves. Single‐dimensional manipulation and 2D manipulation of optical waves with metasurfaces are discussed systematically. In conclusion, an outlook and perspectives on the challenges and future prospects in these rapidly growing research areas are provided.     

All‐Silicon Broadband Ultraviolet Metasurfaces

Featuring high photon energy and short wavelength, ultraviolet (UV) light enables numerous applications such as high‐resolution imaging, photolithography, and sensing. In order to manipulate UV light, bulky optics are usually required, and hence do not meet the fast‐growing requirements of integration in compact systems. Recently, metasurfaces have shown unprecedented control of light, enabling substantial miniaturization of photonic devices from terahertz to visible regions. However, material challenges have hampered the realization of such functionalities at shorter wavelengths. Herein, it is experimentally demonstrated that all‐silicon (Si) metasurfaces with thicknesses of only one‐tenth of the working wavelength can be designed and fabricated to manipulate broadband UV light with efficiencies comparable to plasmonic metasurface performance in infrared (IR). Also, for the first time, photolithography enabled by metasurface‐generated UV holograms is shown. Such performance enhancement is attributed to increased scattering cross sections of Si antennas in the UV range, which is adequately modeled via a circuit. The new platform introduced here will deepen the understanding of light–matter interactions and introduce even more material options to broadband metaphotonic applications, including those in integrated photonics and holographic lithography technologies.     

Twisted Acoustics: Metasurface‐Enabled Multiplexing and Demultiplexing

Metasurfaces are used to enable acoustic orbital angular momentum (a‐OAM)‐based multiplexing in real‐time, postprocess‐free, and sensor‐scanning‐free fashions to improve the bandwidth of acoustic communication, with intrinsic compatibility and expandability to cooperate with other multiplexing schemes. The metasurface‐based communication relying on encoding information onto twisted beams is numerically and experimentally demonstrated by realizing real‐time picture transfer, which differs from existing static data transfer by encoding data onto OAM states. With the advantages of real‐time transmission, passive and instantaneous data decoding, vanishingly low loss, compact size, and high transmitting accuracy, the study of a‐OAM‐based information transfer with metasurfaces offers new route to boost the capacity of acoustic communication and great potential to profoundly advance relevant fields.     

Quasicrystal Photonic Metasurfaces for Radiation Controlling of Second Harmonic Generation

Photonic metasurfaces, a kind of 2D structured medium, represent a novel platform to manipulate the propagation of light at subwavelength scale. In linear optical regime, many interesting topics such as planar meta‐lenses, metasurface optical holography, and so on have been widely investigated. Recently, metasurfaces have gone into the nonlinear optical regime. While it is recognized that the local symmetry of the meta‐atoms plays a vital role in determining the polarization, phase, and intensity of the nonlinear waves, much less attention has been paid to the global symmetry of the nonlinear metasurfaces. According to the Penrose tiling and the newly proposed hexagonal quasicrystalline tiling, nonlinear optical quasicrystal metasurfaces are designed and fabricated based on the geometric‐phase‐controlled plasmonic meta‐atoms with local rotational symmetry. It is found that the far‐field radiation behavior of second harmonic generation waves are determined by both the tiling schemes of quasicrystal metasurfaces and the local symmetry of meta‐atoms they consist of. The proposed concept may open new avenues for designing nonlinear optical sources with metasurface crystals.     

Multichannel‐Independent Information Encoding with Optical Metasurfaces

Optical metasurfaces, as an emerging platform, have been shown to be capable of effectively manipulating the local properties (amplitude, phase, and polarization) of the reflected or transmitted light and have unique strengths in high‐density optical storage, holography, display, etc. The reliability and flexibility of wavefront manipulation makes optical metasurfaces suitable for information encryption by increasing the possibility of encoding combinations of independent channels and the capacity of encryption, and thus the security level. Here, recent progress in metasurface‐based information encoding is reviewed, in which the independent channels for information encoding are built with wavelength and/or polarization in one‐dimensional/two‐dimensional (1D/2D) modes. The way to increase information encoding capacity and security level is proposed, and the opportunities and challenges of information encoding with independent channels based on metasurfaces are discussed.     

Tailorable Dynamics in Nonlinear Optical Metasurfaces

Controlling light with light is essential for all-optical switching, data processing in optical communications and computing. Until now, all-optical control of light has relied almost exclusively on nonlinear optical interactions in materials. Achieving giant nonlinearities under low light intensity is essential for weak-light nonlinear optics. In the past decades, such weak-light nonlinear phenomena have been demonstrated in photorefractive and photochromic materials. However, their bulky size and slow speed have hindered practical applications. Metasurfaces, which enhance light–matter interactions at the nanoscale, provide a new framework with tailorable nonlinearities for weak-light nonlinear dynamics. Current advances in nonlinear metasurfaces are introduced, with a special emphasis on all-optical light controls. The tuning of the nonlinearity values using metasurfaces, including enhancement and sign reversal is presented. The tailoring of the transient behaviors of nonlinearities in metasurfaces to achieve femtosecond switching speed is also discussed. Furthermore, the impact of quantum effects from the metasurface on the nonlinearities is introduced. Finally, an outlook on the future development of this energetic field is offered.     

A Reconfigurable Active Huygens' Metalens

Metasurfaces enable a new paradigm to control electromagnetic waves by manipulating subwavelength artificial structures within just a fraction of wavelength. Despite the rapid growth, simultaneously achieving low‐dimensionality, high transmission efficiency, real‐time continuous reconfigurability, and a wide variety of reprogrammable functions is still very challenging, forcing researchers to realize just one or few of the aforementioned features in one design. This study reports a subwavelength reconfigurable Huygens' metasurface realized by loading it with controllable active elements. The proposed design provides a unified solution to the aforementioned challenges of real‐time local reconfigurability of efficient Huygens' metasurfaces. As one exemplary demonstration, a reconfigurable metalens at the microwave frequencies is experimentally realized, which, to the best of the knowledge, demonstrates for the first time that multiple and complex focal spots can be controlled simultaneously at distinct spatial positions and reprogrammable in any desired fashion, with fast response time and high efficiency. The presented active Huygens' metalens may offer unprecedented potentials for real‐time, fast, and sophisticated electromagnetic wave manipulation such as dynamic holography, focusing, beam shaping/steering, imaging, and active emission control.     

Mitigating Chromatic Dispersion with Hybrid Optical Metasurfaces

Metasurfaces control various properties of light via scattering across a large number of subwavelength‐spaced nanostructures. Although metasurfaces appear to be ideal photonic platforms for realizing and designing miniaturized devices, their chromatic aberrations have hindered the large‐scale deployment of this technology in numerous applications. Wavelength‐dependent diffraction and resonant scattering effects usually limit their working operation wavelengths. In refractive optics, chromatic dispersion is a significant problem and is generally treated by cascading multiple lenses into achromatic doublets, triplets, and so on. Recently, broadband achromatic metalenses in the visible have been proposed to circumvent chromatic aberration but their throughput efficiency is still limited. Here, the dispersion of refractive components is corrected by leveraging the inherent dispersion of metasurfaces. Hybrid refractive‐metasurface devices, with nondispersive refraction in the visible, are experimentally demonstrated. The dispersion of this hybrid component, characterized by using a Fourier plane imaging microscopy setup, is essentially achromatic over about 150 nm in the visible. Broadband focusing with composite plano‐convex metasurface lenses is also proposed. These devices could find applications in numerous consumer optics, augmented reality components, and all applications including imaging for which monochromatic performance is not sufficient.     

Compounding Meta-Atoms into Metamolecules with Hybrid Artificial Intelligence Techniques

Molecules composed of atoms exhibit properties not inherent to their constituent atoms. Similarly, metamolecules consisting of multiple meta-atoms possess emerging features that the meta-atoms themselves do not possess. Metasurfaces composed of metamolecules with spatially variant building blocks, such as gradient metasurfaces, are drawing substantial attention due to their unconventional controllability of the amplitude, phase, and frequency of light. However, the intricate mechanisms and the large degrees of freedom of the multielement systems impede an effective strategy for the design and optimization of metamolecules. Here, a hybrid artificial-intelligence-based framework consolidating compositional pattern-producing networks and cooperative coevolution to resolve the inverse design of metamolecules in metasurfaces is proposed. The framework breaks the design of the metamolecules into separate designs of meta-atoms, and independently solves the smaller design tasks of the meta-atoms through deep learning and evolutionary algorithms. The proposed framework is leveraged to design metallic metamolecules for arbitrary manipulation of the polarization and wavefront of light. Moreover, the efficacy and reliability of the design strategy are confirmed through experimental validations. This framework reveals a promising candidate approach to expedite the design of large-scale metasurfaces in a labor-saving, systematic manner.     

High‐Speed 3D Printing of Millimeter‐Size Customized Aspheric Imaging Lenses with Sub 7 nm Surface Roughness

Advancements in three‐dimensional (3D) printing technology have the potential to transform the manufacture of customized optical elements, which today relies heavily on time‐consuming and costly polishing and grinding processes. However the inherent speed‐accuracy trade‐off seriously constrains the practical applications of 3D‐printing technology in the optical realm. In addressing this issue, here, a new method featuring a significantly faster fabrication speed, at 24.54 mm³ h^?1, without compromising the fabrication accuracy required to 3D‐print customized optical components is reported. A high‐speed 3D‐printing process with subvoxel‐scale precision (sub 5 µm) and deep subwavelength (sub 7 nm) surface roughness by employing the projection micro‐stereolithography process and the synergistic effects from grayscale photopolymerization and the meniscus equilibrium post‐curing methods is demonstrated. Fabricating a customized aspheric lens 5 mm in height and 3 mm in diameter is accomplished in four hours. The 3D‐printed singlet aspheric lens demonstrates a maximal imaging resolution of 373.2 lp mm^?1 with low field distortion less than 0.13% across a 2 mm field of view. This lens is attached onto a cell phone camera and the colorful fine details of a sunset moth's wing and the spot on a weevil's elytra are captured. This work demonstrates the potential of this method to rapidly prototype optical components or systems based on 3D printing.     

Breaking Reciprocity with Space‐Time‐Coding Digital Metasurfaces

Metasurfaces are artificially engineered ultrathin structures that can finely tailor and control electromagnetic wavefronts. There is currently a strong interest in exploring their capability to lift some fundamental limitations dictated by Lorentz reciprocity, which have strong implications in communication, heat management, and energy harvesting. Time‐varying approaches have emerged as attractive alternatives to conventional schemes relying on magnetic or nonlinear materials, but experimental evidence is currently limited to devices such as circulators and antennas. Here, the recently proposed concept of space‐time‐coding digital metasurfaces is leveraged to break reciprocity. Moreover, it is shown that such nonreciprocal effects can be controlled dynamically. This approach relies on inducing suitable spatiotemporal phase gradients in a programmable way via digital modulation of the metasurface‐elements' phase repsonse, which enable anomalous reflections accompanied by frequency conversions. A prototype operating at microwave frequencies is designed and fabricated for proof‐of‐concept validation. Measured results are in good agreement with theory, hence providing the first experimental evidence of nonreciprocal reflection effects enabled by space‐time‐modulated digital metasurfaces. The proposed concept and platform set the stage for “on‐demand” realization of nonreciprocal effects, in programmable or reconfigurable fashions, which may find several promising applications, including frequency conversion, Doppler frequency illusion, optical isolation, and unidirectional transmission.     

3D Hierarchical Nanorod@Nanobowl Array Photoanode with a Tunable Light‐Trapping Cutoff and Bottom‐Selective Field Enhancement for Efficient Solar Water Splitting

Constructing 3D nanophotonic structures is regarded as an effective means to realize both efficient light absorption and efficient charge separation. However, most of the 3D structures reported so far enhance light trapping beyond the absorption onset wavelength, and thus greatly attentuate or even completely block the long‐wavelength light, which could otherwise be efficiently absorbed by narrow‐bandgap materials in a Z‐scheme or tandem device. In addition, constructing a 3D conductive substrate often involves complex processes causing increased cost and upscaling problems. To overcome these shortcomings, a novel 3D hematite nanorod@nanobowl array nanophotonic structure is designed and fabricated by a low‐cost method. This unique structure can enhance light absorption with tunable cutoffs and rationally concentrate photons right above the bowl bottom, enabling efficient charge separation. By loading NiFeO_x as a cocatalyst, a high photocurrent density of 3.41 ± 0.2 mA cm^?2 at 1.23 V versus reversible hydrogen electrode (RHE) can be obtained, which is 2.35 times that with a planar structure in otherwise the same system.     

0.2 λ0 Thick Adaptive Retroreflector Made of Spin‐Locked Metasurface

The metasurface concept is employed to planarize retroflectors by stacking two metasurfaces with separation that is two orders larger than the wavelength. Here, a retroreflective metasurface using subwavelength‐thick reconfigurable C‐shaped resonators (RCRs) is reported, which reduces the overall thickness from the previous record of 590 λ₀ down to only 0.2 λ₀. The geometry of RCRs could be in situ controlled to realize equal amplitude and phase modulation onto transverse magnetic (TM)‐polarized and transverse electric (TE)‐polarized incidences. With the phase gradient being engineered, an in‐plane momentum could be imparted to the incident wave, guaranteeing the spin state of the retro‐reflected wave identical to that of the incident light. Such spin‐locked metasurface is natively adaptive toward different incident angles to realize retroreflection by mechanically altering the geometry of RCRs. As a proof of concept, an ultrathin retroreflective metasurface is validated at 15 GHz, under various illumination angles at 10°, 12°, 15°, and 20°. Such adaptive spin‐locked metasurface could find promising applications in spin‐based optical devices, communication systems, remote sensing, RCS enhancement, and so on.     

超表面的电磁特性研究进展

超表面由于具有自然界不存在的独特电磁特性而引起了人们极大的兴趣,它是占有更少物理空间,提供更小损耗,更容易被制造的一类超材料。简要介绍了超表面的概念和背景,并着重阐述了超表面结构在微波段、太赫兹波段以及光频段的模拟计算及实验研究进展。这些超表面在微波、光波、光电子器件中具有潜在的应用价值。     

Chromatic Dispersion Manipulation Based on Metalenses

Metasurfaces are 2D metamaterials composed of subwavelength nanoantennas according to specific design. They have been utilized to precisely manipulate various parameters of light fields, such as phase, polarization, amplitude, etc., showing promising functionalities. Among all meta-devices, the metalens can be considered as the most basic and important application, given its significant advantage in integration and miniaturization compared with traditional lenses. However, the resonant dispersion of each nanoantenna in a metalens and the intrinsic chromatic dispersion of planar devices and optical materials result in a large chromatic aberration in metalenses that severely reduces the quality of their focusing and imaging. Consequently, how to effectively suppress or manipulate the chromatic aberration of metalenses has attracted worldwide attention in the last few years, leading to variety of excellent achievements promoting the development of this field. Herein, recent progress in chromatic dispersion control based on metalenses is reviewed.     

Flat Lenses Based on 2D Perovskite Nanosheets

Ultrathin flat lenses based on metasurfaces or metamaterials have shown great promise in recent years as essential components in nano-optical system, with capability of abrupt changes of light wavefronts. However, such structural designs require complex nanopatterns and a time-consuming nanofabrication process. In this regard, flat lenses are developed based on 2D perovskite nanosheets, using a cost-effective mask-free femtosecond direct laser writing system. The optical properties of the 2D perovskite are rationally adjusted through facile composition engineering as well as thickness-dependent quantum-size confinement. A diffraction theory model is derived to understand the focusing mechanism of the 2D perovskite nanosheets flat lenses. The as-fabricated lenses exploit the tunable material property variations to effectively manipulate not only the amplitude but also the phase of the incident light to focus into a 3D focal spot with a sub-wavelength resolution in the range of 0.5–0.9λ. The results pave the way toward low-cost and large-scale high-resolution imaging applications in the future.     

Metasurfaces and Colloidal Suspensions Composed of 3D Chiral Si Nanoresonators

High‐refractive‐index silicon nanoresonators are promising low‐loss alternatives to plasmonic particles in CMOS‐compatible nanophotonics applications. However, complex 3D particle morphologies are challenging to realize in practice, thus limiting the range of achievable optical functionalities. Using 3D film structuring and a novel gradient mask transfer technique, the first intrinsically chiral dielectric metasurface is fabricated in the form of a monolayer of twisted silicon nanocrescents that can be easily detached and dissolved into colloidal suspension. The metasurfaces exhibit selective handedness and a circular dichroism as large as 160° µm^?1 due to pronounced differences in induced current loops for left‐handed and right‐handed polarization. The detailed morphology of the detached particles is analyzed using high‐resolution transmission electron microscopy. Furthermore, it is shown that the particles can be manipulated in solution using optical tweezers. The fabrication and detachment method can be extended to different nanoparticle geometries and paves the way for a wide range of novel nanophotonic experiments and applications of high‐index dielectrics.     

2D Perovskites with Giant Excitonic Optical Nonlinearities for High‐Performance Sub‐Bandgap Photodetection

Two‐dimensional (2D) perovskites have proved to be promising semiconductors for photovoltaics, photonics, and optoelectronics. Here, a strategy is presented toward the realization of highly efficient, sub‐bandgap photodetection by employing excitonic effects in 2D Ruddlesden–Popper‐type halide perovskites (RPPs). On near resonance with 2D excitons, layered RPPs exhibit degenerate two‐photon absorption (D‐2PA) coefficients as giant as 0.2–0.64 cm MW^?1. 2D RPP‐based sub‐bandgap photodetectors show excellent detection performance in the near‐infrared (NIR): a two‐photon‐generated current responsivity up to 1.2 × 10⁴ cm² W^?2 s^?1, two orders of magnitude greater than InAsSbP‐pin photodiodes; and a dark current as low as 2 pA at room temperature. More intriguingly, layered‐RPP detectors are highly sensitive to the light polarization of incoming photons, showing a considerable anisotropy in their D‐2PA coefficients (β_[001]/β_[011] = 2.4, 70% larger than the ratios reported for zinc‐blende semiconductors). By controlling the thickness of the inorganic quantum well, it is found that layered RPPs of (C₄H₉NH₃)₂(CH₃NH₃)Pb₂I₇ can be utilized for three‐photon photodetection in the NIR region.     

Coherent Perfect Diffraction in Metagratings

Metasurfaces are 2D engineered structures with subwavelength granularity, offering a wide range of opportunities to tailor the impinging wavefront. However, fundamental limitations on their efficiency in wave transformation, associated with their deeply subwavelength thickness, challenge their implementation in practical application scenarios. Here, it is shown how the coherent control of metagratings through multiple wave excitations can provide new opportunities to achieve highly reconfigurable broadband metasurfaces with large diffraction efficiency, beyond the limitations of conventional approaches. Remarkably, energy distribution between the 0th and higher diffraction orders can be continuously tuned by changing the relative phase difference between two excitation waves, enabling coherent control, with added benefits of enhanced efficiency and bandwidth. This concept is demonstrated for a thin electric metagrating operating at terahertz frequencies, showing that coherent control can overcome several of the limitations of single-layer ultrathin metastructures, and extend their feasibility in various practical scenarios.