Embedded Metal Oxide Plasmonics Using Local Plasma Oxidation of AZO for Planar Metasurfaces

New methods for achieving high-quality conducting oxide metasurfaces are of great importance for a range of emerging applications from infrared thermal control coatings to epsilon-near-zero nonlinear optics. This work demonstrates the viability of plasma patterning as a technique to selectively and locally modulate the carrier density in planar Al-doped ZnO (AZO) metasurfaces without any associated topographical surface profile. This technique stands in strong contrast to conventional physical patterning which results in nonplanar textured surfaces. The approach can open up a new route to form novel photonic devices with planar metasurfaces, for example, antireflective coatings and multi-layer devices. To demonstrate the performance of the carrier-modulated AZO metasurfaces, two types of devices are realized using the demonstrated plasma patterning. A metasurface optical solar reflector is shown to produce infrared emissivity equivalent to a conventional etched design. Second, a multiband metasurface is achieved by integrating a Au visible-range metasurface on top of the planar AZO infrared metasurface. Independent control of spectral bands without significant cross-talk between infrared and visible functionalities is achieved. Local carrier tuning of conducting oxide films offers a conceptually new approach for oxide-based photonics and nanoelectronics and opens up new routes for integrated planar metasurfaces in optical technology.     

可调谐手征超表面电磁特性研究进展

手征超表面是由具有特定电磁响应的平面手征单元结构构成的超薄超材料,由于其具有自由控制电磁波的奇异能力而引起了极大的关注.通过在超表面设计中加入可调谐材料,可以实现其功能受外部激发控制的可调谐或可重构的超器件,为动态调谐电磁波开辟了新的道路.本文介绍了可调/可重构手征超表面电磁特性的一些理论基础,当线偏振光进入可调谐手征...     

All‐Dielectric Active Terahertz Photonics Driven by Bound States in the Continuum

The remarkable emergence of all‐dielectric meta‐photonics governed by the physics of high‐index dielectric materials offers a low‐loss platform for efficient manipulation and subwavelength control of electromagnetic waves from microwaves to visible frequencies. Dielectric metasurfaces can focus electromagnetic waves, generate structured beams and vortices, enhance local fields for advanced sensing, and provide novel functionalities for classical and quantum technologies. Recent advances in meta‐photonics are associated with the exploration of exotic electromagnetic modes called the bound states in the continuum (BICs), which offer a simple interference mechanism to achieve large quality factors (Q) through excitation of supercavity modes in dielectric nanostructures and resonant metasurfaces. Here, a BIC‐driven terahertz metasurface with dynamic control of high‐Q silicon supercavities that are reconfigurable at a nanosecond timescale is experimentally demonstrated. It is revealed that such supercavities enable low‐power, optically induced terahertz switching and modulation of sharp resonances for potential applications in lasing, mode multiplexing, and biosensing.     

Ultrathin Planar Cavity Metasurfaces

An ultrathin planar cavity metasurface is proposed based on ultrathin film interference and its practicability for light manipulation in visible region is experimentally demonstrated. Phase of reflected light is modulated by finely adjusting the thickness of amorphous silicon (a‐Si) by a few nanometers on an aluminum (Al) substrate via nontrivial phase shifts at the interfaces and interference of multireflections generated from the planar cavity. A phase shift of π, the basic requirement for two‐level phase metasurface systems, can be accomplished with an 8 nm thick difference. For proof of concept, gradient metasurfaces for beam deflection, Fresnel zone plate metalens for light focusing, and metaholograms for image reconstruction are presented, demonstrating polarization‐independent and broadband characteristics. This novel mechanism for phase modulation with ultrathin planar cavity provides diverse routes to construct advanced flat optical devices with versatile applications.     

Multiplexing Optical Images for Steganography by Single Metasurfaces

Image steganography based on intelligent devices is one of the effective routes for safely and quickly transferring secret information. However, optical image steganography has attracted far less attention than digital one due to the state-of-the-art technology limitations of high-resolution optical imaging in integrated devices. Optical metasurfaces, composed of ultrathin subwavelength meta-atoms, are extensively considered for flat optical-imaging nano-components with high-resolutions as competitive candidates for next-generation miniaturized devices. Here, multiplex imaging metasurfaces composed of single nanorods are proposed under a detailed strategy to realize optical image steganography. The simulation and experimental results demonstrate that an optical steganographic metasurface can simultaneously transfer independent secret image information to two receivers with special keys, without raising suspicions for the general public under the cloak of a cover image. The proposed optical steganographic strategy by metasurfaces can arbitrarily distribute a continuous grayscale image together with a black-and-white image in separate channels, implying the distinguishing feature of high-density information capacity for integration and miniaturization in optical meta-devices.     

声学超表面及其声波调控特性研究

声学超材料作为近年来研究的一个新兴领域,极大地拓宽了声学材料在各个领域的应用,声学超表面作为声学超材料的一个分支,近几年也受到了国内外研究者的广泛关注.声学超表面是一种由超材料结构单元阵列构成的基于亚波长尺寸的新型声学超材料,能够利用广义Snell定理实现对反射声波或透射声波的定向调控,具有利用亚波长厚度调控声波的独特...     

卷曲空间型超表面对声波的调控

声学超表面具有天然材料所不具备的独特属性,为声学器件的设计提供了多样性。以广义斯涅尔定律为理论基础,设计了具有多种声波调控能力的折射型相位梯度超表面。该超表面由8个具有不同结构参数的卷曲空间单元结构组合而成,在中心频率3 500 Hz附近,8个单元结构的相位覆盖π范围且声波透射率较高。通过合理地设计超表面水平方向上的相位梯度变化,能够实现对声波的任意调控,在理论和有限元仿真上依次实现了异常折射、无衍射贝塞尔声束和声聚焦。这种厚度薄、透射率高的声超表面,在声学器件设计方面具有潜在的应用价值。     

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.     

Polarization Encoded Color Image Embedded in a Dielectric Metasurface

Optical metasurfaces have shown unprecedented capabilities in the local manipulation of the light's phase, intensity, and polarization profiles, and represent a new viable technology for applications such as high‐density optical storage, holography and display. Here, a novel metasurface platform is demonstrated for simultaneously encoding color and intensity information into the wavelength‐dependent polarization profile of a light beam. Unlike typical metasurface devices in which images are encoded by phase or amplitude modulation, the color image here is multiplexed into several sets of polarization profiles, each corresponding to a distinct color, which further allows polarization modulation‐induced additive color mixing. This unique approach features the combination of wavelength selectivity and arbitrary polarization control down to a single subwavelength pixel level. The encoding approach for polarization and color may open a new avenue for novel, effective color display elements with fine control over both brightness and contrast, and may have significant impact for high‐density data storage, information security, and anticounterfeiting.     

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.     

Tri-functional metasurface enhanced with a physically unclonable function

In optical anti-counterfeiting, several distinct optically variable devices (OVDs) are often concurrently employed to compensate for the insufficient security level of constituent OVDs. Alternatively, metasurfaces that exhibit multiple optical responses effectively combine multiple OVDs into one, thus significantly enhancing their security and hindering fraudulent replication. This work demonstrates the simultaneous control of three separate optical responses, i.e., phase, amplitude, and luminescence, using anisotropic gap-plasmon metasurfaces. Due to the incorporated geometric anisotropy, the designed structure exhibits distinct responses under x- and y-polarized light, revealing either a color image, or a holographic projection in the far-field. Furthermore, inserting upconversion nanoparticles (UCNPs) into the dielectric gaps of the structures, the designed metasurface is able to generate a third luminescent image upon illumination with the near-infrared light. The stochastic distribution of the UCNPs constitutes a unique “fingerprint”, achieving a physically unclonable function (PUF) layer. Crucially, our triple-mode metasurface requires only readily attainable equipment such as a macro-lens/camera and a laser pointer to read most of the channels, thus paving the way towards highly secure and easy-to-authenticate metasurface-driven OVDs (mOVDs).     

Phototunable Dielectric Huygens' Metasurfaces

Conventional dielectric metasurfaces achieve their properties through geometrical tuning and consequently are static. Although some unique properties are demonstrated, the usefulness for realistic applications is thus inherently limited. Here, control of the resonant eigenmodes supported by Huygens' metasurface (HMS) absorbers through optical excitation is proposed and demonstrated. An intensity transmission modulation depth of 99.93% is demonstrated at 1.03 THz, with an associated phase change of greater than π/2 rad. Coupled mode theory and S‐parameter simulations are used to elucidate the mechanism underlying the dynamics of the metasurface and it is found that the tuning is primarily governed by modification of the magnetic dipole‐like odd eigenmode, which both lifts the degeneracy, and eliminates critical coupling. The dynamic HMS demonstrates wide tunability and versatility which is not limited to the spectral range demonstrated, offering a new path for reconfigurable metasurface applications.     

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.     

Multifunctional Hyperbolic Nanogroove Metasurface for Submolecular Detection

Metasurface serves as a promising plasmonic sensing platform for engineering the enhanced light–matter interactions. Here, a hyperbolic metasurface with the nanogroove structure in the subwavelength scale is designed. This metasurface is able to modify the wavefront and wavelength of surface plasmon wave with the variation of the nanogroove width or periodicity. At the specific optical frequency, surface plasmon polaritons are tightly confined and propagated with a diffraction‐free feature due to the epsilon‐near‐zero effect. Most importantly, the groove hyperbolic metasurface can enhance the plasmonic sensing with an ultrahigh phase sensitivity of 30 373 deg RIU^?1 and Goos–Hänchen shift sensitivity of 10.134 mm RIU^?1. The detection resolution for refractive index change of glycerol solution is achieved as 10^?8 RIU based on the phase measurement. The detection limit of bovine serum albumin (BSA) molecule is measured as low as 0.1 × 10^?18m (1 × 10^?19 mol L^?1), which corresponds to a submolecular detection level (0.13 BSA mm^?2). As for low‐weight biotin molecule, the detection limit is estimated below 1 × 10^?15m (1 × 10^?15 mol L^?1, 1300 biotin mm^?2). This enhanced plasmonic sensing performance is two orders of magnitude higher than those with current state‐of‐art plasmonic metamaterials and metasurfaces.     

太赫兹超表面计算全息

全息术是一种三维成像技术,它已经被应用于多种实际场景。随着计算机科学与技术的迅猛发展,计算全息由于其方便和灵活的特性,已经成为一种广泛应用的全息成像方法。本文回顾了我们近期基于超表面的太赫兹计算全息研究进展。其中,作为全息板的超表面展示出了超越传统光学器件的独特性能。首先,利用超表面实现了对于全息板每个像素的相位振幅同时且独立的调控,进而实现了高质量全息成像。这种新的电磁波操控能力也带来了新的全息成像效果,如利用介质超表面实现了全息像沿传播方向上的连续变化。其次,对超表面在不同偏振态下的响应进行设计,分别实现了线偏振态与频率复用、圆偏振态复用、以及基于表面波的偏振复用超表面全息术。此外,本文提出了依赖于温度变化而主动可控的超表面全息术,为今后计算全息术的设计与实现提供了新的方案,也推动了超表面在实际应用方面的发展。     

Imaging through Nonlinear Metalens Using Second Harmonic Generation

The abrupt phase change of light at metasurfaces provides high flexibility in wave manipulation without the need for accumulation of propagating phase through dispersive materials. In the linear optical regime, one important application field of metasurfaces is imaging by planar metalenses, which enables device miniaturization and aberration correction compared to conventional optical microlens systems. With the incorporation of nonlinear responses into passive metasurfaces, optical functionalities of metalenses are anticipated to be further enriched, leading to completely new application areas. Here, imaging with nonlinear metalenses that combine the function of an ultrathin planar lens with simultaneous frequency conversion is demonstrated. With such nonlinear metalenses, imaging of objects with near infrared light while the image appears in the second harmonic signal of visible frequency range is experimentally demonstrated. Furthermore, the functionality of these nonlinear metalenses can be modified by switching the handedness of the circularly polarized fundamental wave, leading to either real or virtual nonlinear image formation. Nonlinear metalenses not only enable infrared light imaging through a visible detector but also have the ability to modulate nonlinear optical responses through an ultrathin metasurface device while the fundamental wave remains unaffected, which offers the capability of nonlinear information processing with novel optoelectronic devices.     

Energy‐Tailorable Spin‐Selective Multifunctional Metasurfaces with Full Fourier Components

Compact integrated multifunctional metasurface that can deal with concurrent tasks represent one of the most profound research fields in modern optics. Such integration is expected to have a striking impact on minimized optical systems in applications such as optical communication and computation. However, arbitrary multifunctional spin‐selective design with precise energy configuration in each channel is still a challenge, and suffers from intrinsic noise and complex designs. Here, a design principle is proposed to realize energy tailorable multifunctional metasurfaces, in which the functionalities can be arbitrarily designed if the channels have no or weak interference in k‐space. A design strategy is demostrated here with high‐efficiency dielectric nanopillars that can modulate full Fourier components of the optical field. The spin‐selective behavior of the dielectric metasurfaces is also investigated, which originates from the group effect introduced by numerous nanopillar arrays. This approach provides straightforward rules to control the functionality channels in the integrated metasurfaces, and paves the way for efficient concurrent optical communication.     

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.     

Tailoring MoS2 Valley‐Polarized Photoluminescence with Super Chiral Near‐Field

Transition metal dichalcogenides with intrinsic spin–valley degrees of freedom hold great potentials for applications in spintronic and valleytronic devices. MoS₂ monolayer possesses two inequivalent valleys in the Brillouin zone, with each valley coupling selectively with circularly polarized photons. The degree of valley polarization (DVP) is a parameter to characterize the purity of valley‐polarized photoluminescence (PL) of MoS₂ monolayer. Usually, the detected values of DVP in MoS₂ monolayer show achiral property under optical excitation of opposite helicities due to reciprocal phonon‐assisted intervalley scattering process. Here, it is reported that valley‐polarized PL of MoS₂ can be tailored through near‐field interaction with plasmonic chiral metasurface. The resonant field of the chiral metasurface couples with valley‐polarized excitons, and tailors the measured PL spectra in the far‐field, resulting in observation of chiral DVP of MoS₂‐metasurface under opposite helicities excitations. Valley‐contrast PL in the chiral heterostructure is also observed when illuminated by linearly polarized light. The manipulation of valley‐polarized PL in 2D materials using chiral metasurface represents a viable route toward valley‐polaritonic devices.     

Ultrafast Coherent Absorption in Diamond Metamaterials

Diamond is introduced as a material platform for visible/near‐infrared photonic metamaterials, with a nanostructured polycrystalline diamond metasurface only 170 nm thick providing an experimental demonstration of coherent light‐by‐light modulation at few‐optical‐cycle (6 fs) pulse durations. “Coherent control” of absorption in planar (subwavelength‐thickness) materials has emerged recently as a mechanism for high‐contrast all‐optical gating, with a speed of response that is limited only by the spectral width of the absorption line. It is shown here that a free‐standing diamond membrane structured by focused ion beam milling can provide strong, spectrally near‐flat absorption over a visible to near‐infrared wavelength range that is wide enough (wider than is characteristically achievable in plasmonic metal metasurfaces) to facilitate coherent modulation of ultrashort optical pulses comprising only a few oscillations of electromagnetic field.