相位不连续超材料表面的研究进展

近年来,超材料表面性能研究得到快速发展,在电磁波亚波长操纵等领域展现了独特的应用前景。超材料表面通常由许多周期性排列的金属结构单元组成,如果合适地设计结构单元的几何构型,就能够按照意愿得到拥有特定光学性质的材料。对近期利用相位不连续超材料表面对电磁波进行操控的研究进展进行了综述,包括:利用相位不连续表面控制电磁波出射方向;利用相位不连续表面结构合成圆偏振光;相位不连续应用于隐身;太赫兹波段相位不连续结构的应用;最后对该研究领域的发展进行了展望。     

Hollow‐Out Patterning Ultrathin Acoustic Metasurfaces for Multifunctionalities Using Soft fiber/Rigid Bead Networks

Acoustic metasurfaces that can manipulate and control sound waves at 2D subwavelength scales open new avenues to unusual applications, such as asymmetric transmission, super‐resolution imaging, and particle manipulation. However, the long‐standing goals of pushing frontier metamaterials research into real practice are still severely constrained by cumbersome configuration, large acoustic loss, and rigid structure of the existing metamaterials. An ultrathin metasurface (10–300 µm in thickness, up to ≈λ/650, λ the wavelength) that is capable of imparting sound wave with a nontrivial phase shift with high transmittance (>80%) in the range of 5–30 kHz is fabricated here. The metasurface is comprised of a porous network of soft polymer fiber/rigid beads that are physically equivalent to crosslinked spring‐mass resonators. Moreover, the traditional paper‐cutting art to carve the ultrathin metasurface into hollow‐out patterns is incorporated, resulting in a variety of remarkable functions, including acoustic vortex, focusing, and super‐resolution. The hollow‐out patterning approach innovates the traditional one‐step metadevice fabrication process into two separated steps: 1) fabrication of ultrathin metasurfaces; 2) hollow‐out patterning of metasurfaces. The strategy opens an avenue to mass production of acoustic metadevices, shedding light on the applications of the metamaterials in acoustic cloaking, acoustic positioning, and particle manipulation.     

Modular Design for Versatile Broadband Polarizing Metasurfaces with Freely Switching Functions

Polarization is a fundamental property of electromagnetic waves that plays a key role in many physical phenomena and applications. Schemes to manipulate it are revisited with the emergence of metasurfaces, which have brought multi-functionalities straightforwardly. However, this has come at the expense of design complexity that relies strongly on field theory. Here, an ingenious strategy of modular design is proposed to construct subwavelength multifunctional polarization control devices. Chiral metasurfaces with different handedness are first proposed and regarded as modules. The versatile polarization controller can thus be obtained with the combination of different modules. These experiments demonstrate that the well-designed polarization controller possesses reconfigurable functionality, and various broadband polarization and amplitude regulation functions with high efficiency including arbitrary linear polarization rotation, asymmetric transmission effect, neutral-density-like filter, polarization beam splitter, etc., can be readily realized just by changing the cascaded modules. The physical mechanisms of the versatile polarization controller and chiral metasurface modules are both guaranteed by the Fabry–Pérot-like resonances, which are theoretically verified via the transfer matrix method. It is envisioned that the modular concept will be of great benefit to designing compact multifunctional polarization controllers.     

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。     

太赫兹波前调制超表面器件研究进展

超表面是一种由人工微结构组成的超薄平面器件,能够实现对电磁波振幅、相位以及偏振态的调控,具有体积小、重量轻、集成度高、可灵活操控电磁波等优势,在电磁波谱、波前调制中发挥着巨大的作用。综述了近年来基于超表面的太赫兹波前调制器件的研究进展。总结了基于Pancharatnam-Berry相位、基于局域表面等离子体共振(LSPR)、基于Mie共振的三种超表面单元结构对电磁波的振幅、相位调控机理,并讨论了实现高效率超表面的方法。之后,介绍了用于设计波前调制超表面器件的纯相位调制方法和复振幅调制方法。综述了在太赫兹波段典型的超表面波前调制器,包括单一功能、复合功能以及可调谐功能的超表面波前调制器件。在早期的研究工作中,设计的超表面可实现波束偏转、波束聚焦、全息成像、以及涡旋光束、自聚焦光束、洛伦兹光束等特殊光束产生等功能。为提高太赫兹器件的利用率,波分复用、偏振复用等功能复用的太赫兹超表面器件被提出。随着对太赫兹波前动态调控需求的增长, 一些主动的太赫兹超表面器件被提出并在实验上被验证。共有两种主动的超表面器件。其中一种主动超表面是通过将超表面结构与半导体材料或相变材料结合形成的,另一种是通过光泵浦硅片形成的全光器件。全光超表面在不用重新加工的前提下能够被重复使用。通过调整投影在硅片上的超表面图像即可动态操控太赫兹波前。全光超表面具有动态控制波束扫描和波束聚焦的能力,将来可应用于太赫兹通信、太赫兹雷达等领域。最后,对太赫兹波前调制超表面器件的发展趋势与应用前景进行了展望。     

Coherent Control of Wave Beams Via Unidirectional Evanescent Modes Excitation

Conventional coherent absorption occurs only when two incident beams exhibit mirror symmetry with respect to the absorbing surface, i.e., the two beams have the same incident angles, phases, and amplitudes. This study proposes a more general metasurface paradigm for coherent perfect absorption with impinging waves from arbitrary asymmetric directions. By exploiting excitation of unidirectional evanescent waves, the output can be fixed at one reflection direction for any amplitude and phase of the control wave. It shows theoretically and confirm experimentally that the relative amplitude of the reflected wave can be tuned continuously from zero to unity by changing the phase difference between the two beams, i.e., switching from coherent perfect absorption to full reflection. It hopes that this study will open up promising possibilities for wave manipulation via evanescent waves engineering with applications in optical switches, optical computing, one-side sensing, photovoltaics, and radar cross-section control.     

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。     

Radiation-Type Metasurfaces for Advanced Electromagnetic Manipulation

Manipulating the phase, polarization, and energy distribution of electromagnetic (EM) waves has facilitated numerous applications. Nowadays, metasurface provides an innovational scenario to carry out more promising and advanced control of EM waves. However, it is a great challenge to manipulate polarization, phase, and energy distribution simultaneously with a low profile. Herein, a class of single-layer radiation-type metasurfaces to achieve advanced EM manipulation is proposed. Desired EM functions can be achieved based on the geometric phase and resonant phase. Such metasurfaces enable the capability to manipulate arbitrary phases and linear polarization states simultaneously. Moreover, arbitrary energy distributions can be controlled. As examples of potential applications, three advanced EM functional devices are presented: a novel multiple-input multiple-output antenna with efficient crosstalk suppression and information encryption, an energy-controllable router, and a metasurface holographic imaging based on power transmission algorithm, respectively. The proposed strategy may open up an alternative way of controlling EM waves with advanced performance and minimalist complexity. Moreover, it may lead to advances in information encoding and cryptography.     

2比特激励编码超构表面的辐射与散射调控

超构表面(metasurface, MS)是当前电磁领域研究的热点和前沿,它能够有效调控电磁波. 2比特数字编码MS由于不同形态单元个数的增加,增强了散射和辐射控制的灵活性,为更好的隐身效果、更宽的带宽以及多功能的实现提供了可能. 文中将分形磁电偶极子天线结构和极化控制MS单元有机融合,先构建出激励MS单元,再通过结构变换和数字编码,提出了2比特可激励编码MS,并设计了拓扑结构. 仿真和实验验证了辐射与散射调控的多功能特征. 设计的2比特激励编码MS在天线雷达散射截面(radar cross section, RCS)减缩和未来无线通信系统领域具有潜在的应用价值.     

基于异常反射型超表面的天线反射面设计

在实际应用中有许多场合需要对电磁波的传播进行人工调控, 传统的光学器件或者人工电磁介质都是采用三维结构, 但传统的三维结构介质很难与其他器件或设备进行集成.利用相位非连续人工电磁表面则可以在传播路径的介质分界面上引入相位突变, 进而实现人工调控电磁波.文章在此基础上在微波波段利用构造分界面相位梯度提出了一种异常反射型超表面的设计, 用作天线反射面可将圆极化波高效地转化为交叉极化波, 为微波段的电磁波人工调控提供了新的手段.根据广义反射定律及斜入射时相位突变修正设计的天线反射面, 在X波段可以实现对入射电磁波的人工调控, 并通过仿真分析验证了该设计方法的准确性, 为人工电磁表面作为天线反射面提供了一种设计思路.     

基于狭槽天线超表面的复振幅光瞳滤波器

为了产生具有特殊性质的聚焦矢量光场,根据狭槽天线与线偏振片的相似性,设计了刻蚀有多个不同方向狭槽天线环形阵列的超表面,作为离散型复振幅光瞳滤波器使用。以径向偏振入射光为例,经该滤波器并经大数值孔径透镜紧聚焦后,可获得衍射受限的超长3维光管场。结果表明,由于狭槽天线具有局域调控入射光场振幅和相位的特性,各个环带上具有各自特定的透射率,同时具有特定的二元相移（0/π）分布,可以用来实现具有特殊性质的矢量光场。这种结构的超表面对矢量光束的产生与调控提供了新途径。     

基于几何相位的三层透射型等离子编码超表面

本文提出一种由多纳米棒与圆形金属缺口组合而 成的三层几何相位编码超表面,本结 构在0.737 THz处对透射波具有完美的几何相位调制效果且很大程度 提高了散射光的效率和 带宽,进而扩展纳米棒的应用领域。此外,针对几何相位超表面,我们引入数字信号处理中 的傅里叶卷积计算理论,实现对编码超表面的加法或减法运算,从而达到透射散射角度的自 由操控。通过对不同的编码序列进行多次傅里叶卷积运算即可扩展到控制光波的所有方向, 为可见光的自由、灵活的操作提供了更大的自由度。本文提出的几何相位编码超表面也可以 应用于微波与太赫兹频段,为电磁波的实时控制及波束扫描打开新的途径。     

宽带消色差红外光学超构透镜研究进展（特邀）

超构透镜是由亚波长散射单元结构排列而成的具有聚焦功能的平面二维超构表面。超构表面能够在亚波长尺度上操控光场的振幅、相位、色散和偏振态,是近年来迅速发展起来的新型光场调控载体。亚波长共振纳米结构使得高阶衍射被抑制,入射光场可以完美地被调制到设计的衍射级次上,从而确保了超构表面器件具有高的光子调控效率。同时,超构单元在设计上的灵活性及其特定的电磁响应使得超构表面可以实现对光场多个维度的定制化操控。不同于传统光学透镜依赖光传播的相位累积效应,宽带消色差超构透镜通过对光场相位和相位色散的同时独立调控解决了传统通过级联多个透镜修正色差造成的光学系统复杂和体积庞大限制,为发展小型化片上集成光学提供了全新的思路。文中围绕超构透镜的相关研究,首先介绍了超构表面调控光场振幅、相位和偏振态的基本原理,在此基础上,重点回顾了近年来关于超透镜的研究发展,包括通过单一参量调控的单波长超透镜的实现,以及通过对光场偏振、相位及相位色散的多参量联合调控的多功能宽带消色差超构透镜的发展现状,最后讨论其进一步发展的可能挑战与应用前景。     

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.     

圆极化复用型多功能超构表面研究进展

文中回顾并总结了课题组近期关于圆极化复用型多功能超构表面的研究进展. 从几何相位在正交圆极化波作用下的共轭对称响应出发,基于超构表面的等效琼斯矩阵提出了交叉极化双通道复用的独立调控方法,突破了几何相位的交叉极化耦合限制. 进一步,基于同极化与交叉极化分量间的幅相关系,提出了三通道复用的多元波前集成方法,实现了对透射圆极化电磁能量的全场调控. 在此基础上,通过引入手性诱导相位提出了圆极化四（全）通道复用的波前调控方法,有效提高了对圆极化电磁能量的利用效率,为提高现代通信系统信道容量及信息传输速率打下理论基础.     

多维度超表面及其在信息加密防伪上的应用

超表面是一种厚度在亚波长或波长量级的人工层状材料。可通过调控超表面单元结构上的大小、形状、转角、位移量等自由度，实现对电磁波频率、振幅、相位、偏振等特性的灵活有效调控。超表面具有超薄、宽带、低损耗、易加工、灵活设计，功能强大等特点。文中综述了具有单维度、双维度、多维度光场调控功能的超表面及其在外部激励作用下具有主动可调特性超表面的发展历程，并特别介绍了这些超表面用于信息加密防伪领域的实现方式与优势特点。相比于传统的信息加密防伪技术，超表面信息加密防伪术具有亚波长像素，精密控制，安全系数高等特点，展现了全新视角，拥有广阔的发展前景。     

基于信息超表面的无线通信（特邀）

信息超表面由于其强大的处理空间电磁波的能力,成为国内外物理和信息领域的研究热点之一。文中主要介绍信息超表面在无线通信领域的一系列研究进展。信息超表面能实时操控电磁波及直接处理数字编码信息,并进一步对信息进行感知、理解,甚至记忆、学习和认知,这使其在无线通信领域展现出巨大潜能。文中首先介绍信息超表面在承担无线中继职能时所涉及的信道建模研究进展、以及其对信道的改善作用;其次介绍信息超表面在新体制发射机中的应用,通过对入射到信息超表面上的载波进行幅度或相位调制,实现了多种简化的发射机架构。此外,文中还介绍了利用信息超表面近场、远场以及散射场等信息,实现了多种新型无线通信系统。最后,文章对基于信息超表面的无线通信进行了总结和展望。     

基于3D打印技术的任意曲面共形超表面隐身衣

超表面因其具有强大的电磁操控能力以及损耗小、剖面低和易加工等优势,在电磁工程领域受到了研究人员的广泛关注.然而,由于受加工技术的限制,目前所报道的超表面地毯隐身衣大多由二维平面结构拼接而成,研制能与任意目标曲面共形的地毯隐身衣仍然是一个挑战.为此,本文基于超表面和3D打印技术设计并验证了一款具有任意曲面的共形超表面隐身...     

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.