非线性光学超构表面：基础与应用

光学超构表面是一种由亚波长尺度的超构单元在面内排布而构成的准二维人工结构材料。研究人员可以通过选择超构单元的材料组成、几何形状对光的振幅、偏振、相位和频率等光场自由度进行灵活调控。聚焦于超构表面在非线性光场调控领域的原理与应用。首先,概述了非线性晶体到非线性超构表面的发展历程。然后,讨论了对称性和几何相位在非线性光学超构表面中的重要作用。最后,介绍了非线性光学超构表面在波前调控、量子信息处理和太赫兹波的产生与调控等领域中的应用。     

光学超构表面中的复合相位调控

超构表面是一种二维的超构材料,能在平面上实现对光波相位、振幅、偏振等参数的灵活调控。相位型超构表面可突破经典折反射定律,使得光场调控不再依赖于曲面光学元件,为实现光学系统的平面化、集成化和多功能化提供了有效途径。特别地,通过对传输相位和几何相位协同调控,能够有效解决传统超构表面存在的功能单一、带宽受限、可调谐性差等原理性问题。文章介绍了复合相位调控的实现原理和方法,以及复合相位超构表面的典型应用及特点,最后对复合相位超构表面的未来研究方向进行了总结和展望。     

非线性光学超构表面

在线性光学范畴内,人们已经通过亚波长尺度的超薄超构表面成功实现了对光的众多新颖特性的调控功能.其主要理念是通过对具有亚波长尺度且空间方向变化的超构功能基元进行特定的排列,从而实现对光的偏振、相位和振幅的有效控制.近来,超构表面上的非线性光学特性也引起了大家的广泛关注.在本综述中,我们对非线性光学超构表面的设计、超构功能单元的材料和对称性选择、非线性手性光学、非线性贝里几何相位和非线性波前整形等内容进行了总结;最后对非线性光学超构表面在调控光与物质的相互作用中面临的挑战和前景进行了展望.     

非线性光学超构表面

非线性光学超构表面是一类由空间变化的超构功能基元组成的超薄非线性光学器件。通过合理选择超构功能基元的材料组成、空间对称性,人们可以在亚波长尺度上对超构表面上产生的非线性光波的偏振、相位、振幅等自由度进行多维度光场调控。文章介绍了在非线性光学超构表面上实现谐波、四波混频、太赫兹波辐射的设计与原理,并讨论了如何在非线性光学超构表面上实现非线性光的波束调控、全息成像、光学图像加密等功能。     

超构表面的振幅调控及其功能器件研究进展

超构表面是一种具有亚波长特征尺寸的人工平面结构材料,可以在亚波长尺度上对入射电磁波的振幅、相位、偏振、频率、光谱等参量进行精密且灵活的调控,近年来备受关注。振幅是光波的基本参量之一,本文将从振幅调控的角度出发,对基于超构表面材料的振幅调控机理进行分析,主要包括通过改变纳米结构的尺寸和方向角对振幅进行调控,同时对基于振幅调控超构表面的功能器件的研究现状及其应用场景进行总结和讨论。研究表明,振幅调控超构表面具有设计灵活、加工简单、功能强大、可与其他参量调控相融合等特点,其在高分辨率图像显示、高密度信息存储、信息加密、信息复用、光束整形、光信息处理和安全防伪等诸多领域具有重要的研究价值和广阔的应用前景。     

基于伴随仿真的偏振复用超构透镜

偏振成像技术在目标探测、生物医学等领域具有重要应用价值,基于超构表面设计的偏振成像系统可以有效避免传统偏振成像系统存在的结构复杂、体积和质量大等问题,有利于实现光学系统微型化、轻量化和集成化。然而,传统超构表面设计方法忽略了超构表面结构的局部非周期性引起的近场电磁耦合,在大数值孔径的条件下会严重影响器件的衍射效率。为了解决这个问题,本文提出了一种基于边界优化的偏振复用超构透镜设计方法,并由此设计了一种能对x和y偏振光独立调控的大数值孔径(～0.94)偏振成像超构透镜。在基于人工择优初始结构的优化设计中,通过参数扫描、人工择优的传统设计方法得到超构透镜初始结构,然后通过边界优化方法对超构透镜进行进一步的优化,其衍射效率相比于优化前可以提高20%左右;在基于均匀阵列初始结构的优化设计中,通过20次左右的迭代,超构透镜衍射效率可以达到92%左右。本文提出的优化设计方法可有效提高偏振复用超构表面器件效率,并且能够简化多功能超构表面的设计步骤,在偏振成像、光通信等领域具有应用前景。     

光学超构材料专题编者按

正超构材料(metamaterials)自21世纪初被提出以来,已经经历了20年的发展,目前,它仍然是非常活跃的前沿领域.超构材料的研究覆盖非常广泛,涉及光、电磁波、太赫兹波、声波、热、力学和弹性波等.超构材料的基本思想是,利用人工结构单元作为人造原子,来构造宏观连续的介质,通过结构单元的设计,来调控介质的材料参数,实现对波的传播性质的控制.最早的超构材料结构单元是1999年英国科学家John Pendry提出的金属开口环结构,它可以与电磁场相互作用,产生磁共振,从而实现负磁导率系数.后来,人们又提出了很多其他的人工结构单元,实现各种应用.超构材     

基于复合超构表面的宽带圆极化双功能器件设计

多功能器件的设计是推动新一代电磁系统发展的重要力量,而超构表面因其对电磁波的幅度、相位和极化等特性的灵活调控在多功能器件领域备受关注.传统的多功能超构表面是利用各向异性单元对相互正交的线极化波具有不同响应的特性,从而设计出适用于线极化的多功能器件.本文提出了一种缝隙加载的环I形复合超构表面单元,通过单元臂长和旋转角度的调整实现了对圆极化电磁波传输和几何相位的独立控制.利用上述两种相位的共同作用,打破了左旋和右旋圆极化电磁波操控中存在的固有关系,为圆极化双功能器件的设计提供了新的思路.在此基础上,利用复合超构表面分别设计了异面偏折器和定向/涡旋光束产生器,实验结果表明,本文设计的两种反射型圆极化双功能器件在9—13 GHz的宽频带范围内均能良好工作.     

超构透镜的色差调控应用

超构表面是由精心设计和排布的亚波长纳米单元组成的平面元件,其可以在微观尺度下调制电磁场,从而实现波前的任意调控。目前,它已被用来灵活地操纵相位、偏振、振幅等各种光学参数。超构透镜是超构表面中相当重要且非常活跃的一个研究方向,由于其厚度在波长量级,与传统光学透镜相比,能够显著增加光学器件的集成度并且降低结构复杂度。但是,单元结构材料的固有色散以及结构几何形状衍射效应导致的色差会严重影响超构透镜的成像质量,从而限制了其在光电子器件中的潜在应用。本文首先讨论超构透镜控制色差的原理。随后回顾了几种重要的成像应用,包括分立波长消色差,宽带聚焦成像,光场成像等重要的成像系统。最后,本文对超构透镜未来的发展方向和应用前景做出展望。     

超构天线:原理、器件与应用

自从电磁波被发现和应用以来,利用各种材料或者结构调节电磁波的辐射行为、构造高性能的电磁辐射器件一直是研究人员的追求目标.经过百余年的发展,电磁辐射器件的方向性提高、带宽拓展等技术逐渐达到瓶颈.受自然材料电磁特性的限制,微带天线、喇叭天线等传统电磁辐射器件存在体积重量大、工作带宽窄、无法快速动态调控等缺陷,难以满足日益发展的通信技术的需求.近年来出现的亚波长结构可在深度亚波长尺度下调控电磁波的传输行为,出现了多种奇异的电磁现象,完善了传统的电磁学理论,在一定程度上突破了传统材料电磁特性的限制,形成全新的电磁辐射技术,有效解决了传统天线存在的口径大、厚度高、带宽窄等难题,促进了电磁学、光子学、材料学等领域的发展.这种基于超构材料的新型天线可以被称为超构天线,具有高方向性、低副瓣、超宽带、可重构等传统天线难以实现的功能.本文主要回顾了近年来基于亚波长超构材料的超构天线技术的发展现状和取得的成果,介绍了超构材料在亚波长尺度下对电磁波振幅、相位、偏振态等的衍射调控机理,以及在此基础上形成的新型辐射器件,例如相控阵天线、高方向性天线、低雷达散射截面天线,基于亚波长结构的多种偏振调控器件及其在天线中的应用等.在衍射极限尺度下,这种亚波长结构的调控行为可有效提升电磁辐射器件的方向性、带宽,并可重构天线的工作频率、偏振态等性能.     

基于电磁超表面的透镜成像技术研究进展

电磁超表面属于超材料的一种,是由许多亚波长纳米结构单元组成的二维功能性平面结构.根据惠更斯原理,超表面阵列可以任意调控光波的相位、振幅和偏振.与传统器件相比,基于这种超材料设计的光学功能器件最大的优势是其具有极薄的厚度.本文首先介绍了广义斯涅耳定律以及纳米单元结构调控相位的基本原理,重点归纳了电磁超表面在透镜成像技术方面的研究进展,包括等离子体超表面、全介质超表面以及金属/介质混合式超表面在成像方面的应用,最后指出了超表面在成像方面尚未解决的前沿问题以及与实际应用接轨的重要问题,希望能为以后的深入研究提供一定的参考和借鉴.     

Manipulate light polarizations with metamaterials: From microwave to visible

Polarization is an important characteristic of electromagnetic (EM) waves, and efficient manipulations over EM wave polarizations are always desirable in practical applications. Here, we review the recent efforts in controlling light polarizations with metamaterials, at frequencies ranged from microwave to visible. We first presented a 4 × 4 version transfer matrix method (TMM) to study the scatterings by an anisotropic metamaterial of EM waves with arbitrary propagating directions and polarizations. With the 4 × 4 TMM, we discovered several amazing polarization manipulation phenomena based on the reflection geometry and proposed corresponding model metamaterial systems to realize such effects. Metamaterial samples were fabricated with the help of finite-difference-time-domain (FDTD) simulations, and experiments were performed to successfully realize these ideas at both microwave and visible frequencies. Efforts in employing metamaterials to manipulate light polarizations based on the transmission geometry are also reviewed.     

Manipulate light polarizations with metamaterials: From microwave to visible

Polarization is an important characteristic of electromagnetic (EM) waves, and efficient manipulations over EM wave polarizations are always desirable in practical applications. Here, we review the recent efforts in controlling light polarizations with metamaterials, at frequencies ranged from microwave to visible. We first presented a 4 × 4 version transfer matrix method (TMM) to study the scatterings by an anisotropic metamaterial of EM waves with arbitrary propagating directions and polarizations. With the 4 × 4 TMM, we discovered several amazing polarization manipulation phenomena based on the reflection geometry and proposed corresponding model metamaterial systems to realize such effects. Metamaterial samples were fabricated with the help of finite-difference-time-domain (FDTD) simulations, and experiments were performed to successfully realize these ideas at both microwave and visible frequencies. Efforts in employing metamaterials to manipulate light polarizations based on the transmission geometry are also reviewed.     

Anisotropic Plasmonic Metasurfaces with Linear Polarization-Dependent Focusings

As a burgeoning approach to modulate electromagnetic waves, metasurfaces fascinate many researchers. However, it is a persistent problem to realize independent wavefront tailoring in different polarization states. In this work, anisotropic plasmonic metasurfaces are presented with linear polarization-dependent focusings. The designed plasmonic meta-atoms consist of cross-shaped gold, silica spacer, and gold substrate. Phase modulation in different polarization channels is independently achieved by adjusting the dimensions of cross-shaped gold in x- and y-directions. Three metasurfaces are presented to verify linear polarization-dependent focusings at 200 THz. First, a focusing metasurface is designed with focal lengths of 5.2 µm under x-polarized incidence and 6.5 µm under y-polarized incidence. Second, by applying convolution operation, the second metasurface exhibits different focusings with different deflection angles at opposite directions under orthogonally polarized incidences. Finally, a multi-focal metasurface is demonstrated, which switches between dual- and quad-focal points depending on polarization state. The work may provide a novel platform for near-infrared integrated photonics.     

Functional and nonlinear optical metasurfaces

Optical metasurfaces are thin‐layer subwavelength‐patterned structures that interact strongly with light. Metasurfaces have become the subject of several rapidly growing areas of research, being a logical extension of the field of metamaterials towards their practical applications. Metasurfaces demonstrate many useful properties of metadevices with engineered resonant electric and magnetic optical responses combined with low losses of thin‐layer structures. Here we introduce the basic concepts of this rapidly growing research field that stem from earlier studies of frequency‐selective surfaces in radiophysics, being enriched by the recent development of metamaterials and subwavelength nanophotonics. We review the most interesting properties of photonic metasurfaces, demonstrating their useful functionalities such as frequency selectivity, wavefront shaping, polarization control, etc. We discuss the ways to achieve tunability of metasurfaces and also demonstrate that nonlinear effects can be enhanced with the help of metasurface engineering.

     

Theory,experiment and applications of metamaterials

In this review article, a brief introduction on the theory, experiments and applications of metamaterials is presented. The main focuses are concentrated on the composing meta-atoms, the method of transformation optics, the experimental demonstration of negative refraction, and the realizations of invisibility cloaks and electromagnetic black hole. At the end of this review, some typical applications of metamaterials, including high-performance antennas made of zero-refractive-index materials, inhomogeneous metamaterial lenses, and planar metasurfaces, are introduced in details.     

Waves in magnetic metamaterials with strong coupling of elements

The properties of magnetic metamaterials composed of microresonators have been studied. Coupling of meta-atoms induces magnetoinductive waves, which determine the features of propagation of electromagnetic radiation in metamaterials. The complication of the coupling mechanism due to the radiation when the element sizes decrease as frequencies tend to optical ones is discussed.     

衍射极限尺度下的亚波长电磁学

作为波的本性之一,衍射是现代物理学的重要研究内容.衍射导致自由空间中波的能量不能被无限小地聚集,从而为成像、光刻、光存储、光波导等技术设定了一个原理性的障碍——衍射极限.对于电磁波和光波而言,尽管通过提高介质的折射率可以压缩衍射效应,但由于自然界中材料的折射率有限,该方法存在很大限制.近年来,随着表面等离子体光学的兴起,表面等离子体在超越传统衍射极限方面的能力和应用前景受到了学术界的关注.本文从亚波长电磁学的角度出发,介绍衍射极限研究的历史,综述了突破衍射极限的理论方法.首先,利用金属介质表面等离子体激元的短波长特性,可将等效波长压缩一个数量级以上,在纳米尺度实现光波的聚焦或定向传输;更进一步,通过人为设计超构材料和超构表面,利用结构化金属和介质中的局域谐振、耦合等特殊电磁响应,可实现亚波长局域相位调制、超宽带色散调控、近完美吸收、光子自旋轨道耦合等,从而突破传统理论的诸多局限,为下一代电磁学和光学功能器件奠定重要基础.