Mie‐Resonant Membrane Huygens' Metasurfaces

All‐dielectric metasurfaces have become a new paradigm for flat optics as they allow flexible engineering of the electromagnetic space of propagating waves. Such metasurfaces are usually composed of individual subwavelength elements embedded into a host medium or placed on a substrate, which often diminishes the quality of the resonances. The substrate imposes limitations on the metasurface functionalities, especially for infrared and terahertz frequencies. Here a novel concept of membrane Huygens' metasurfaces is introduced. The metasurfaces feature an inverted design, and they consist of arrays of holes made in a thin membrane of high‐index dielectric material, with the response governed by the electric and magnetic Mie resonances excited within dielectric domains of the membrane. Highly efficient transmission combined with the 2π phase coverage in the freestanding membranes is demonstrated. Several functional metadevices for wavefront control are designed, including beam deflector, a lens, and an axicon. Such membrane metasurfaces provide novel opportunities for efficient large‐area metadevices, whose advanced functionality is defined by structuring rather than by chemical composition.     

All‐Dielectric Programmable Huygens' Metasurfaces

Low‐loss nanostructured dielectric metasurfaces have emerged as a breakthrough platform for ultrathin optics and cutting‐edge photonic applications, including beam shaping, focusing, and holography. However, the static nature of their constituent materials has traditionally limited them to fixed functionalities. Tunable all‐dielectric infrared Huygens' metasurfaces consisting of multi‐layer Ge disk meta‐units with strategically incorporated non‐volatile phase change material Ge₃Sb₂Te₆ are introduced. Switching the phase‐change material between its amorphous and crystalline structural state enables nearly full dynamic light phase control with high transmittance in the mid‐IR spectrum. The metasurface is realized experimentally, showing post‐fabrication tuning of the light phase within a range of 81% of the full 2π phase shift. Additionally, the versatility of the tunable Huygen's metasurfaces is demonstrated by optically programming the spatial light phase distribution of the metasurface with single meta‐unit precision and retrieving high‐resolution phase‐encoded images using hyperspectral measurements. The programmable metasurface concept overcomes the static limitations of previous dielectric metasurfaces, paving the way for “universal” metasurfaces and highly efficient, ultracompact active optical elements like tunable lenses, dynamic holograms, and spatial light modulators.     

基于电介质超表面的光场复振幅调制及应用

超表面作为一种人工设计的二维阵列纳米结构,能够在亚波长尺度上实现光场波前振幅、相位和偏振态的灵活调控,为现代光学器件的小型化、集成化提供了全新的实现途径。随着光学成像、显示等应用的发展,在可见光波段具有高工作效率的微型光学器件的需求日益凸显。近年来,由高折射率、低损耗电介质材料制备的光学超表面得到了极大地发展,在消色差光学超透镜、偏振相关全息显示等方面展现出广泛的应用前景。文中围绕电介质超表面的相关研究,首先介绍广义斯涅耳定律及电介质超表面结构调控光场振幅、相位和偏振态的基本原理,在此基础上,重点回顾近年来关于光场波前单一参量调控和多参量联合调控在全息显示、结构光场产生等方面的研究进展,最后讨论电介质超表面发展的可能挑战与前景。     

Polarization and Holography Recording in Real- and k-Space Based on Dielectric Metasurface

The diverse design freedom and mechanisms of metasurfaces motivate the manipulation of polarization in an ultrashort distance with subwavelength resolution and make metasurfaces outperform conventional polarization optical elements. However, in order to enhance the information capability and encryption security of metasurface holograms, polarization manipulation together with multiplexing technologies are still highly desired. Here, a birefringent dielectric metasurface with the capability of encoding a grayscale image in real-space based on Malus's law by utilizing the inhomogeneous polarization distribution and realizing the reconstruction of a vectorial holographic image in k-space with the help of the phase profiles of different polarization components of output light is demonstrated. This novel functionality is realized by exploiting the manipulation of polarization and phase of output light simultaneously offered by the dielectric metasurface. The proposed method may enhance the information capability and security level of applications such as the anticounterfeiting and encryption.     

基于电介质超表面的双频带双偏振通道波前调控

随着微纳加工技术的发展,超表面在亚波长尺度对电磁波的多维度调控展现出传统光学器件难以比拟的优势。基于电介质硅纳米柱结构构建了具有双频带响应的超表面,利用微结构对不同偏振入射光反射系数的差异,通过构建梯度几何相位实现了双波长下的异常反射;同时设计了超表面灰度成像阵列,在近红外波段实现了对正交偏振态和双波长入射具有不同响应的正负灰度图像。文中提出的超表面设计为基于超表面的多功能集成技术的发展奠定了基础。     

Multistate Tuning of Third Harmonic Generation in Fano-Resonant Hybrid Dielectric Metasurfaces

Hybrid dielectric metasurfaces have emerged as a promising approach to enhancing near field confinement and thus high optical nonlinearity by utilizing low loss dielectric rather than relatively high loss metallic resonators. A wider range of applications can be realized if more design dimensions can be provided from material and fabrication perspectives to allow dynamic control of light. Here, tunable third harmonic generation (THG) via hybrid metasurfaces with phase change material Ge₂Sb₂Te₅ (GST) deposited on top of amorphous silicon metasurfaces is demonstrated. Fano resonance is excited to confine the incident light inside the hybrid metasurfaces, and an experimental quality factor (Q-factor ≈ 125) is achieved at the fundamental pump wavelength around 1210 nm. Not only the switching between a turn-on state of Fano resonance in the amorphous state of GST and a turn-off state in its crystalline state are demonstrated, but also gradual multistate tuning of THG emission at its intermediate states. A high THG conversion efficiency of η = 2.9 × 10⁻⁶% is achieved, which is 32 times more than that of a GST-based Fabry–Pèrot cavity under a similar pump laser power. Experimental results show the potential of exploring GST-based hybrid dielectric metasurfaces for tunable nonlinear optical devices.     

Nonvolatile Reconfigurable Phase‐Change Metadevices for Beam Steering in the Near Infrared

The development of flat, compact beam‐steering devices with no bulky moving parts is opening up a new route to a variety of exciting applications, such as LIDAR scanning systems for autonomous vehicles, robotics and sensing, free‐space, and even surface wave optical signal coupling. In this paper, the design, fabrication and characterization of innovative, nonvolatile, and reconfigurable beam‐steering metadevices enabled by a combination of optical metasurfaces and chalcogenide phase‐change materials is reported. The metadevices reflect an incident optical beam in a mirror‐like fashion when the phase‐change layer is in the crystalline state, but reflect anomalously at predesigned angles when the phase‐change layer is switched into its amorphous state. Experimental angle‐resolved spectrometry measurements verify that fabricated devices perform as designed, with high efficiencies, up to 40%, when operating at 1550 nm. Laser‐induced crystallization and reamorphization experiments confirm reversible switching of the device. It is believed that reconfigurable phase‐change‐based beam‐steering and beam‐shaping metadevices, such as those reported here, can offer real applications advantages, such as high efficiency, compactness, fast switching times and, due to the nonvolatile nature of chalcogenide phase‐change materials, low power consumption.     

Hybrid State Engineering of Phase-Change Metasurface for All-Optical Cryptography

Chalcogenide material Ge₂Sb₂Te₅ (GST) has bistable phases, the so-called amorphous and crystalline phases that exhibit large refractive index contrast. It can be reversibly switched within a nanosecond time scale through applying thermal bias, especially optical or electrical pulse signals. Recently, GST has been exploited as an ingredient of all-optical dynamic metasurfaces, thanks to its ultrafast and efficient switching functionality. However, most of these devices provide only two-level switching functionality and this limitation hinders their application to diverse all-optical systems. In this paper, the method to expand switching functionality of GST metasurfaces to three level through engineering thermo-optically creatable hybrid state that is co-existing state of amorphous and crystalline GST-based meta-atoms is proposed. Furthermore, the novel hologram technique is introduced for providing the visual information that is only recognizable in the hybrid state GST metasurface. Thanks to thermo-optical complexity to make the hybrid state, the metasurface allows the realization of highly secured visual cryptography architecture without the complex optical setup. The phase-change metasurface based on multi-physical design has significant potential for applications such as all-optical image encryption, security, and anti-counterfeiting.     

动态可调的太赫兹超构表面

近年来,采用人工设计金属阵列的超构表面以实现对太赫兹波的调制受到越来越广泛的关注。设计了2种互补的亚波长金结构阵列超构表面,正、反结构2个超构表面对太赫兹波均有共振响应。利用光泵浦太赫兹时域透射光谱系统,通过控制泵浦光实现对太赫兹波的谱调制。仅需28 mW的外加泵浦光,反结构超构表面在0.91 THz处的振幅调制深度可达到95%。利用该反结构超构表面对太赫兹波的开关作用,进一步设计了太赫兹振幅全息图,希望利用该结构实现太赫兹波前的动态调控。初步的理论模拟验证了这一方法的可行性,可较好地实现对太赫兹波的动态调控。     

Recent advances in metasurface hologram technologies (Invited paper)

Since Leith and Upatnieks demonstrated the first optical hologram in 1964, hologram technology has attracted a great deal of interest in a wide range of optical fields owing to its potential use in future optical applications such as holographic imaging and optical data storage. Although there have been considerable efforts to develop holographic technologies using conventional optics, critical issues still hinder future development. Recently, metasurfaces composed of artificially fabricated subwavelength structures have been considered as novel holographic devices that show an unprecedented ability to control electromagnetic waves. In this review, we outline the recent progress in metasurface holography. A general introduction to several types of metasurface holography categorized based on their physics and application is provided. Then, our personal perspective on the future of this field is discussed.     

Full‐State Synthesis of Electromagnetic Fields using High Efficiency Phase‐Only Metasurfaces

A metasurface is a thin array of subwavelength elements with designable scattering responses, and metasurface holography is a powerful tool for imaging and field control. The existing metasurface holograms are classified into two types: one is based on phase‐only metasurfaces (including the recently presented vectorial metasurface holography), which has high power efficiency but cannot control the phases of generated fields; while the other is based on phase‐amplitude‐modulated metasurfaces, which can control both field amplitudes and phases in the region of interest (ROI) but has very low efficiency. Here, for the first time, it is proposed to synthesize the field amplitudes and phases in ROI simultaneously and independently by using high‐efficiency phase‐only metasurfaces. All points in ROI may have independent values of field amplitudes and phases, and the requirements for X and Y components may be different in achieving spatially varied polarization states. To this end, an efficient design method based on equivalent electromagnetic model and gradient‐based nonlinear optimization is proposed. Full‐wave simulations and experimental results demonstrate that the phase‐only metasurface designed by the method has 10 times higher efficiency than the phase‐amplitude‐modulated metasurface. This work opens a way to realize more complicated and high‐efficiency metasurface holography.     

基于π型结构双折射超表面的设计与应用

为了实现结构简单、透射率高的双折射超表面, 采用广义薄板跃迁条件分析了该超表面结构与其周围入射场、反射场和透射场的关系, 并利用介质空间域的表面极化率、磁化率等描述了相应超表面的等效特性, 设计出一种基于π型金属结构单元的双折射超表面。通过将具有梯度透射相位的7个单元按照顺序排列, 形成具有对垂直入射x, y极化电磁波双折射性能的超表面。结果表明, 在波束折射和偏振分束超表面中, 损失均低于-8dB; λ/4波片中达到全透射; 所设计的双折射超表面对垂直入射的电磁波具有高透射特性, 并且能够分离x极化电磁波和y极化电磁波的波束, 实现双折射。该研究结果对高性能超表面的设计与实现具有一定的指导意义。     

Flexible Plasmonic Metasurfaces with User‐Designed Patterns for Molecular Sensing and Cryptography

Flexible plasmonic metasurfaces have garnered considerable attention because the material's mechanical flexibility enables new functionalities and integrated applications. Here, by adopting low‐cost materials and simple techniques, we demonstrate a method of fabricating large flexible metasurfaces with arbitrary user‐designed iridescent patterns. These naked‐eye recognizable patterns together with their excellent plasmonic activities have yielded new functionalities and novel applications. Demonstrations include plasmonic sensing, reflective displays, developing new encryption strategies and integrated devices, etc. Moreover, the low fabrication cost (?2) would enable the practical use of the material. The metasurface can even be fashioned into an innovative, multifunctional medical ID bracelet. We believe our flexible plasmonic metafilm will inspire the fabrication of many novel applications and open up new horizons in various fields.     

All-Dielectric Crescent Metasurface Sensor Driven by Bound States in the Continuum

Metasurfaces based on quasi-bound states in the continuum (quasi-BICs) constitute an emerging toolkit in nanophotonic sensing as they sustain high quality factor resonances and substantial near-field enhancements. It is demonstrated that silicon metasurfaces composed of crescent shaped meta-atoms provide tailored light-matter interaction controlled by the crescent geometry. Significantly, this metasurface not only exhibits a fundamental quasi-BIC resonance, but also supports a higher-order resonance with tunable electromagnetic field enhancement and advantageous properties for sensing. The higher-order resonance shows twice the sensitivity of the fundamental one for bulk refractive index sensing. It is further demonstrated that both the fundamental and higher-order resonances can be exploited for sensing ultrathin layers of biomolecules in air and buffer solutions. Specifically, when measuring in buffer solution, the figure of merit of the sensor, defined as the change in the spectral position of the resonance normalized to its full width at half maximum, is a factor of 2.5 larger for the higher-order resonance when compared to the fundamental one. Due to its high sensitivity and potential for straightforward microfluidic integration, the silicon crescent metasurface is ideally suited for real-time and in situ biosensing, enabling compact sensing devices for a wide range of diagnostic applications.     

Full‐Color Complex‐Amplitude Vectorial Holograms Based on Multi‐Freedom Metasurfaces

Phase, polarization, amplitude, and frequency represent the basic dimensions of light, playing crucial roles for both fundamental light–material interactions and all major optical applications. Metasurfaces have emerged as a compact platform to manipulate these knobs, but previous metasurfaces have limited flexibility to simultaneous control them. A multi‐freedom metasurface that can simultaneously and independently modulate phase, polarization, and amplitude in an analytical form is introduced, and frequency multiplexing is further realized by a k‐space engineering technique. The multi‐freedom metasurface seamlessly combines geometric Pancharatnam–Berry phase and detour phase, both of which are frequency independent. As a result, it allows complex‐amplitude vectorial hologram at various frequencies based on the same design strategy, without sophisticated nanostructure searching of massive geometric parameters. Based on this principle, full‐color complex‐amplitude vectorial meta‐holograms in the visible are experimentally demonstrated with a metal–insulator–metal architecture, unlocking the long‐sought full potential of advanced light field manipulation through ultrathin metasurfaces.     

一种具有多种谐振模式的太赫兹超表面器件

设计了一种中心对称的分裂环形状超表面结构,该结构具有偏振不敏感和高品质因子的特性。通过理论和实验研究,深入分析了其谐振点的频谱特性,并确定了谐振峰的模式,包括LC、偶极和高阶谐振等。其中,几种高阶谐振模式表现出较高的高品质因数Q(约230),并且对超表面衬底材料的介电常数变化高度敏感。此外,还研究了具有不对称超表面结构的电磁性质,发现通过分别增加超表面结构沿水平轴(x轴)和垂直轴(y轴)的不对称性,可以产生和增强0.332 THz和0.210 THz的谐振。     

可调谐超表面的生物分子检测研究进展

超表面作为一种超薄微纳器件具有优良的性能,利用超表面检测生物分子具有越来越广泛的前景。本文调研了可调谐超表面检测生物分子的研究进展,并分别从太赫兹波段、中红外波段、光波段阐述当前的研究成果。本文重点介绍了当前超表面利用外加电压、改变环境温度等实现可调谐的方法,并对超表面检测不同生物分子的性能进行了总结分析,最后给出了可调谐超表面在生物分子检测方面未来发展的机遇与挑战。     

光学超构表面的微纳加工技术研究进展

超构表面由二维平面内精心排布的亚波长单元组成,为设计超紧凑型光学元件提供了新的范式,在微型化光学系统方面显示出了极大的潜力。在不到十年时间里,超构表面由于具有超轻、超薄且能够操纵光波的各种参量以实现多功能集成的优势在多学科领域引起了广泛的关注。然而,在光学波段,高自由度、非周期、排列密集的超构单元对其加工制备提出了很多极端的参数要求,如极小尺度、极高精度、高深宽比、难加工材料、跨尺度等,这使得超构表面从实验室走向实际应用面临极大的挑战。文中总结了近些年用于超构表面各类微纳加工方法的各类方法的原理、特点和最新进展,包括小面积直写方法、大面积模板转移方法以及一些新兴的加工方法。最后,针对超构表面在加工方面的目前的挑战和未来的发展趋势进行了总结和展望。     

利用石墨烯-金属复合结构实现太赫兹电磁诱导透明超表面主动调控

近年来,在超表面中实现对电磁诱导透明的主动式调控引起了越来越多的研究兴趣。采用石墨烯-金属复合结构,设计并研制了一种新颖的调制策略,通过同时施加光泵和偏置电压改变石墨烯的电导率,在太赫兹波段实现了一种主动式电磁诱导透明超表面,其在透射窗口频率处的振幅调制深度可达73%。模拟和理论分析表明,其内在物理机理在于石墨烯对金属谐振结构的短接作用,石墨烯的电导率越大,短接效果越明显,谐振强度也越弱。该石墨烯-金属复合超表面为设计紧凑的主动式太赫兹光开关器件提供了一种实现途径,在太赫兹通信中具有潜在的应用前景。     

Digital Nonlinear Metasurface with Customizable Nonreciprocity

Recently, investigation of metasurface has been extended to nonreciprocity through breaking time‐reversal symmetry. Among a number of magnetless strategies, nonlinearity is an important nonreciprocal principle amenable to the metasurface. As passive analog‐based devices, most of the existing nonlinear nonreciprocal metasurfaces are inherently characterized by the relatively unchangeable performance, sharp frequency response, and hysteresis loops. Here, an analog–digital–analog mechanism is proposed to realize the nonlinear nonreciprocity, which provides a digitally reconfigurable solution for a family of nonreciprocal performance within a shared hardware architecture. This concept is validated by a metasurface prototype with the integration of a digital module at microwave frequencies. Based on the proposed mechanism, the properties can be customized as demanded, ranging from a normal reciprocal response to a variety of nonreciprocal functions, including electromagnetic (EM) diode and EM unidirectional limiting functions, which have been experimentally demonstrated with direction reversibility and threshold tunability. The proposed metasurface is also underpinned by the nonhysteretic performance and wide operating bandwidth, thereby potentially making it an inexpensive and stable candidate for advanced manipulations of EM waves.