几何相位电磁超表面:从原理到应用 下载: 5203次特邀综述
1 引言
自由调控电磁波(光)一直是人类的梦想和追求,这不仅具有重要的科学意义,而且在信息、**、能源等领域有着广泛的应用。然而,自然材料有限的原子种类和晶格排列方式,导致电磁响应参数(如电响应参数——介电常数
随着这一领域的发展,各个国家的科研团队都在不停寻找新型超材料来提高对电磁波的操控能力。2011年,美国哈佛大学的Cappasso课题组[20]提出将不同人工微结构按照特定的排列方式组合构成非均匀超材料表面,简称“超表面”。在外界光照射下,超表面将所携带的“相位突变”加载到再辐射的电磁波上,由此可以实现人为设计的电磁辐射强度和相位分布,对电磁波波前进行调控。超表面概念提出以来,科研人员开展了大量的研究工作,并实现了一系列电磁波调控现象,比如平面波到表面波的转换[21]、定向散射[22-24]、光束聚焦[25-27]、圆偏振多功能[28-29]、频率多功能[30-31]、利用数字编码超表面控制波束[32]等。这类结构简单且易于加工,同时又保持奇异电磁调控能力的超表面,逐渐代替原始的三维结构,开始成为超材料研究的主要分支。
超表面从相位突变机理来看可以分为几何相位超表面和共振相位超表面两大类,如
图 1. 几何相位和共振相位电磁超表面的原理及应用[21,34,50,52-53]
Fig. 1. Principle of geometric-phase and resonance-based metasurfaces and their applications[21,34,50,52-53]
本文会对几何相位电磁超表面进行系统性的综述。从介绍庞加莱球和琼斯矩阵的相关概念出发,介绍几何相位的来源和分析方法。在此基础上,在第3节和第4节分别介绍几何相位超表面对于电磁波的远场和近场调控。最后,介绍几何相位超表面的几种典型应用,包括全息成像、特殊光束激发和探测、几何相位的主动调控、基于复合超表面的自旋不依赖调控及消色差超透镜。这些研究一方面将人们对于利用几何相位调控电磁波的理解和应用提升到了全新的高度,也为未来开发和研制基于几何相位超表面的新型光学或光电器件提供了新的思路和空间。
2 几何相位超表面基本原理
首先介绍几何相位的起源。如
图 2. 几何相位的来源。(a)微结构散射电磁波;(b)庞加莱球示意图
Fig. 2. Origin of geometric phase. (a) Electromagnetic waves scattered by microstructure; (b) schematic of Poincare sphere
数学上通过琼斯矩阵来分析几何相位。当电磁波照射超表面单元结构组成的周期性阵列时,反射/透射特性一般可由
当单元结构沿垂直于超表面的方向绕
由(2)式可以看到,当入射波为右旋圆偏振
反射场的行为基本与透射行为一致,唯一的区别是对反射电磁波圆偏振的定义不同。通过(2)式和(3)式,不但能像分析庞加莱球一样分析具有不同旋转角的微结构所具有的几何相位,而且可以表征偏振守恒散射和交叉偏振散射的幅度与微结构电磁响应之间的联系,有助于未来进一步设计高效的几何相位超表面。
3 基于几何相位超表面的光波前调制
根据几何相位超表面的原理,可通过设计单元结构的几何转动角度进行特定的宏观排列分布,来实现所需光波前的调制,从而获得各种各样的奇异现象。三种典型的相关现象如下:第一种是线性梯度几何相位超表面,可以使电磁波发生自旋依赖的奇异偏折,即光子自旋霍尔效应;第二种是抛物线梯度几何相位超表面,可使电磁波发生聚焦,用来设计平面超透镜;第三种是螺旋线梯度几何相位超表面,能使电磁波的自旋角动量转换为轨道角动量,即产生涡旋光。
3.1 线性梯度几何相位超表面
线性梯度超表面是一种很重要的超表面,其相位突变(透射相位或反射相位)呈线性排列,包括线性梯度共振相位超表面和线性梯度几何相位超表面。当电磁波入射到线性梯度共振相位超表面时,超表面会将携带的相位梯度
式中:
与线性梯度共振相位超表面不同,几何相位超表面的相位梯度
图 3. 线性梯度几何相位超表面。(a)线性梯度共振相位超表面和线性梯度几何相位超表面;(b)亚波长光栅制作的几何相位器件[36];(c)等离子体链中的光子自旋霍尔效应[37];(d)高效的自旋依赖波前调控几何相位超表面[56];(e)金纳米棒构成的超表面实现自旋依赖偏折[57];(f)几何相位超表面增强自旋霍尔效应[58]
Fig. 3. Linear gradient geometric-phase metasurfaces. (a) Left: linear gradient resonance-based metasurface, right: linear gradient geometric-phase metasurface; (b) geometric-phase optical elements with computer-generated subwavelength gratings[36]; (c) photonic spin Hall effect in plasmonic chains[37]; (d) efficient geometric-phase metasurface for spin-dependent wavefront control[56]; (e) geometric-phase metasurface composed of gold nanorods for beam-refraction[57]; (f) geometric-phase metasurface and
早在超表面概念出现之前,以色列理工学院的Hasman课题组[36-37]已经完成了一系列利用这种机制设计自旋依赖光学器件的开创性工作,如
3.2 抛物线梯度几何相位超表面
透镜几乎是所有复杂光学系统中最关键的部件,在光学中起着重要作用。传统透镜通常由透明介电材料(例如玻璃)制成。为实现所需的功能,传统的透镜必须具有一定的弯曲形状,以便不同局部位置处的透射波(或反射波)可以获得所需的相位。这种要求使传统透镜尺寸庞大,难以制造且功能受限,限制了其在现代集成光学中的应用。利用超表面进行平面聚焦可以克服这些问题,其关键思想是以特定的方式排列单元结构,使得表面上的透射/反射相位分布满足
式中:
图 4. 基于超表面的平面超透镜。(a)共振相位超透镜聚焦示意图,几何相位超透镜聚焦和发散示意图;(b)可见光频段下的等离子体超透镜[61];(c)基于Si纳米柱制造的超透镜[62];(d)超透镜及其单元TiO2纳米柱[63];(e)悬链线阵列平面透镜[64]
Fig. 4. Flat metalens based on metasurfaces. (a) Left: focusing schematic of resonance-based metalens, right: focusing and defocusing schematic of geometric-phase metalens; (b) plasmonic metalens component of single layer nanorod array in visible band[61]; (c) metalens based on Si nanobeam array[62]; (d) metalens and its building block, TiO2 nanofin[63]; (e) one-dimensional flat lens based on catenary array[64]
3.3 螺旋线梯度几何相位超表面
涡旋光是一种具有螺旋相位波前,带有轨道角动量(OAM),能够携带不同拓扑荷数
式中:
图 5. 基于几何相位超表面的涡旋光发生器。(a)通过螺旋板测得的l=±1涡旋光束[68];(b)基于几何相位超表面的涡旋光束产生[57];(c)基于“V”结构的涡旋光束发生器[69];(d)基于悬链线阵列的OAM发生器[74];(e)用于光学涡旋生成的平面手征超表面[75];(f)用于矢量涡旋光束生成的单个等离子体超表面[76]
Fig. 5. Vortex beam generators based on geometric-phase metasurfaces. (a) l=±1 vortex beam measured from diffraction q-plate using spiral wave plate[68]; (b) vortex beam generation based on geometric-phase metasurface[57]; (c) V-shaped vortex beam generator[69]; (d) OAM generators based on catenary arrays[74]; (e) planar chiral metasurface for optical vortex generation[75]; (f) single plasmonic metasurface for vortex beam generation[76]
3.4 几何相位超表面的高效调控
上述研究都是通过设计空间的几何相位分布来实现奇异光束偏折、光束聚集、特殊光束产生等波前调控现象。然而,在早期的几何相位超表面研究中,上述波前调控现象的效率都很低,这是由于早期的几何相位超表面只利用单层人工微结构来构建,理论上讲这样的单层微结构工作效率无法高于25%[77]。这在新加坡国立大学Ding等[78]的实验中得到证实,他们在微波体系中证明透射式几何相位超表面的效率限制在25%左右,如
反射体系中有金属背板,所以超表面能够达到很高的工作效率。但是在透射体系中要实现高效率则较为困难。 2011年,复旦大学周磊课题组[83]研究发现只要单元微结构琼斯矩阵元素满足
就可得到100%效率的超表面。显然,仅表现出电响应的单层微结构不能满足(8)式条件。如果微结构同时表现出电响应和磁响应,则不会存在这种约束。因此,适当地调整电响应和磁响应,再进一步增加适当各向异性,就可获得满足(8)式条件的反射(透射)微结构。根据以上原理,复旦大学Luo等[84]采用ABA结构设计了一个100%效率的几何相位超表面,如
图 6. 高效几何相位超表面。(a)效率大约为25%的透射式几何相位超表面[78];(b)高效贝塞尔光束发生器[79-80];(c)(d)在微波和太赫兹频段工作的反射体系中效率几乎为100%的光子自旋霍尔效应[81-82];(e)透射体系中效率几乎为100%的光子自旋霍尔效应[84]
Fig. 6. Highly efficient geometric-phase metasurfaces. (a) About 25% efficiency geometric-phase metasurface in transmissive geometry[78]; (b) highly efficient vector Bessel beams generator[79-80]; (c)(d) photonic spin Hall effect with nearly 100% efficiency in reflective geometry working in microwave and terahertz regions[81-82]; (e) photonic spin Hall effect with nearly 100% efficiency in transmissive geometry[84]
利用以上介绍的几何相位超表面对光波前调制的奇异现象仅仅是冰山一角,还有许多其他的现象,比如利用圆锥面相位梯度超表面可以实现贝塞尔光束激发[85-87],利用随机无序的编码超表面可以实现漫散射[88]等。相信随着对几何相位超表面的不断研究,相关应用会越来越多,越来越高效。
4 基于几何相位超表面的等离激元调控
表面等离激元(SPPs)是一种局域在金属/介质分界面处,可沿界面传播的电磁波本征态[89]。SPPs由金属表面自由电子的运动产生,是一种很好的光子和电子相互作用的媒质。SPPs最重要的两个性质是局域场增强和亚波长特性,这使其在光子学[90]、增强非线性[91-92]、增强拉曼效应[93]、化学与生物[94-95]、等离子体集成元件[96]和光波导[97-98]等领域都有很大的应用价值,因此,对SPPs的调控显得非常重要。但SPPs的波矢和空气中的波矢不匹配,因此,SPPs的激发和调控都比较困难。电磁超表面的出现,提供了新的自由度来调控SPPs。
4.1 单个散射体调控表面等离激元
在利用几何相位激发和调控SPPs方面,最早可以追溯到以色列理工学院Hasman课题组[99-100]用单个散射体对SPPs调控的一系列研究工作。如
图 7. SPPs产生及调控。(a)圆形纳米缝隙激发自旋依赖的出射表面波,右图为两种不同圆偏振入射下表面波与平面波之间的干涉条纹[99];(b)聚焦表面波的半圆形纳米缝隙,右图为焦点的自旋分裂[100];(c)阿基米德螺旋结构示意图;(d)金属板上产生的SPPs电场分布[102];(e)SPPs轨道角动量的近场测量[103];(f)自旋依赖的滤波器[104]
Fig. 7. SPPs generation and control. (a) Outgoing spin-dependent surface waves generated by circular nanoslot, and interference fringes between surface and plane waves for two circular polarization incidences are shown in the right panel[99]; (b) surface waves focused by semicircular nanoslot, and spin-splitting of focal spot is shown in the right panel[100]; (c) schematic of Archimedes spiral; (d) electric-field profile of generated surface plasmon on metal surface[102]; (e) near-field measurement of O
具有手征特性的散射体可以实现更复杂的SPPs激发[102-109]。
式中:
4.2 几何相位超表面调控表面等离激元
相比单个散射体,几何相位超表面提供了更丰富的SPPs激发和调控手段。最简单的调控SPPs的体系是在金膜上刻蚀一系列矩形缝隙。当缝隙远小于波长时,它的辐射可由一个偶极子的辐射来描述[110]。在这种情况下,一个圆偏振入射波(
式中:2
图 8. 几何相位超表面产生的SPPs相关轨迹。(a)通过金属表面的偶极子源产生SPPs的示意图;(b)通过纳米缝隙实现SPPs单向传播;(c)在一个单元结构中使用单个纳米缝隙实现SPPs单向传播[111, 114];(d)等离子体自旋霍尔效应的灵活相干调控示意图(左图:两种入射自旋分别产生的局部轨道;右图:写入字母“b”时通过旋转入射线偏振方向而产生的动态图像)[120]
Fig. 8. SPPs trajectories enabled by geometric-phase metasurfaces. (a) Schematic of SPPs generated by dipole source on metal surface; (b) unidirectional propagation of SPPs through nanoslots[111]; (c) unidirectional propagation of SPPs, where nanoslots are used in the unit cell[114]; (d) schematic of flexible coherent control of plasmonic spin Hall effect (left: local orbitals produced by two incident spins; right: dynamic images produced by rotating linear polarization of incidence when letter ‘b’ is w
各种自旋依赖的相反偏移与不同的几何相位有关,包括光束中心的横向偏移、入射光束线动量或OAM的偏移、圆形光栅焦点的偏移。
5 应用
本文介绍了几何相位超表面调控电磁波的机理,包括对于远场光波前的调制和近场SPPs的调控。基于这些机理,研究者们提出一系列新奇的应用,在此之前,先简单介绍超表面加工和制备通常使用的方法。目前最常用的样品加工方法有光刻、电子束曝光、聚焦离子束刻蚀、干涉光刻、自组装和纳米压印光刻等[121-123]。利用这些工艺和技术,不仅可以加工出单层金属超表面结构、介质超表面结构,还能加工一些结构和功能复杂的多层结构。这些微纳加工工艺和技术的发展对超表面走向应用起到了非常关键的作用。
5.1 高效全息成像
全息成像技术通过记录某个物体的物光波的相位和幅度信息,再现该物体的立体图像,有望在信息存储、干涉度量、遥感等各个方面获得广泛应用。然而现有的全息技术在记录有相位信息的全息片时通常存在效率低、加工制备困难等问题,几何相位超表面的出现有助于解决这一全息领域的技术瓶颈。
2013年,德国帕德博恩大学Zentgraf课题组[39]首先提出利用几何相位来产生所需的复杂波前,从而有效地实现二维或三维全息图,如
图 9. 几何相位在全息图像中的应用。(a)超表面三维全息图[39];(b)基于几何相位超表面的高效全息图像[40];(c)利用手性超表面产生的全息图[41];(d)基于全硅介质的超表面在三个不同平面产生全息图像的示意图[42];(e) 生成不同全息图像的复合超表面[43];(f)多色全息图[44]
Fig. 9. Applications of geometric-phase metasurface in holograms. (a) Three-dimensional hologram enabled by metasurface[39]; (b) highly efficient holographic images based on geometric-phase metasurface[40]; (c) hologram generated by chiral metasurface[41]; (d) holographic images at three separate planes based on silicon metasurfaces in broad visible band[42]; (e) multiplexed metasurface for generating holographic images[43]; (f) multicolor hologram[44]
5.2 轨道角动量发生器和探测器
由于光的自旋角动量和本征轨道角动量具有不易测量的特性,传统的方法通常是使用大量的自由空间组件来进行光角动量的产生和探测,例如波片和偏振分析仪、衍射光栅[129-130]、空间光调制器[131-132]及干涉测量仪[133-134]。几何相位超表面在生成和检测光角动量方面具有很强的能力[135-138],所以开始逐渐取代传统方法来检测光的自旋角动量和本征轨道角动量。2012年,哈佛大学Capasso课题组[137]提出一种基于等离子体光电二极管超表面对光的OAM进行全息检测的方法,如
图 10. OAM发生器和探测器。(a) OAM的全息检测[137];(b)(c)几何相位超表面对不同偏振波束的折射[135-136];(d)超表面产生光学OAM[139]
Fig. 10. OAM generators and detectors. (a) Holographic detection of OAM[137]; (b)(c) refraction of different polarized beams by geometric-phase metasurfaces[135-136]; (d) optical OAM generated by metasurface[139]
5.3 几何相位超表面的主动调控
上述各种奇异电磁波调控现象都是基于被动的几何相位超表面,一旦制备完成,其效果就被锁定,不能改变,这在实际应用中受到非常大的限制,因此研究人员一直致力于构造动态可调的超表面。研究人员基于共振相位超表面已经开展了很多探索和研究。其调控原理是通过可调元件改变每个单元的相位,从而动态调控电磁波波前。基于这样的方案,人们已经做了很多的工作,如动态可切换超透镜[140]、可编程微波超表面[141]、可重构惠更斯超透镜[142]。
对于几何相位超表面来说,动态调控的机理与共振相位超表面具有很大的差别。由 (2)式可知,几何相位超表面的电磁特性主要分为两部分,强度(
图 11. 超表面的主动调控。(a)基于完整相图调节超表面功能[143];(b) PIN二极管的可调几何相位超表面[144];(c)石墨烯超表面的振幅调制[145];(d)基于微流体通道的可调超透镜[146]
Fig. 11. Active control of metasurfaces. (a) Tailor functionalities of metasurfaces based on complete phase diagram[143]; (b) tunable geometric-phase metasurface with PIN diodes[144]; (c) amplitude modulation with gated-graphene metasurfaces[145]; (d) metalens with tunable phase gradient by using random access reconfigurable metamaterial[146]
5.4 基于复合超表面的自旋不依赖调控
绝大多数几何相位调控现象的结果都与入射波的自旋状态相关,即表现出自旋依赖性,这是几何相位的一个重要特征。本质上这是因为对于左旋和右旋偏振光来说,几何相位的符号是相反的。这种自旋依赖特性使得控制左右旋偏振光变得困难,导致对相反自旋分别赋予不同的信息受到限制。为解决这个问题,研究者们提出了各种各样的方法,例如Xiao等[120]将银膜上纳米缝隙的几何相位和两个自旋的内、外向SPPs特定目标场分布相匹配,在左右旋入射下分别产生不同的局部轨道图案。赫瑞-瓦特大学Wen等[43]通过控制入射光的螺旋度,实现了左右旋入射下不同的全息图像。
此外,科学家们还提出融合几何相位超表面和共振超表面的复合超表面,来克服几何相位超表面的自旋依赖带来的问题。2017年,Capasso团队[147]利用复合超表面使得对于正交偏振态(线偏振、圆偏振、椭圆偏振)的入射波能够产生任意和独立的相位分布,并通过实验展示了手性全息图,在左旋和右旋偏振的入射波激励下,产生了不同的远场图案,如
图 12. 复合超表面的应用。(a)左右旋入射波产生不同的全息图像[147];(b)相反自旋的不同OAM[148] ;(c)左图:复合单元结构中的不对称传输,右图:左旋和右旋入射下透射场和反射场的衍射图案[149]
Fig. 12. Applications of composite metasurfaces. (a) Different holographic images generated by LCP and RCP incident waves[147]; (b) different OAMs for two opposite spins[148]; (c) left: asymmetric transmission in composite unit cells, right: diffraction patterns in transmission and reflection field illuminated by LCP and RCP waves[149]
5.5 消色差平面超透镜
复合超表面可用来打破几何相位的自旋依赖性,事实上这种复合超表面的功能并不仅限于此。近年来,各科研团队开始探索将几何相位超表面和共振超表面相结合来设计消色差平面透镜。色差又称色像差,是透镜成像的一个严重缺陷,就是指不同波长的光焦点存在微小的区别,原因是光在介质中传播时具有色散现象。色差校正的传统方法是通过叠加多个透镜来实现,但这会造成体积庞大、无法简单集成等问题。
2015年,哈佛大学Capasso课题组[49]提出一种多波长消色差超器件(包括超透镜)的方案,如
图 13. 几何相位在平面光学上的应用。(a)多波长消色差全介质超器件[49];(b)宽频带反射型消色差超透镜[50] ;(c)宽频带透射型消色差超透镜[51]
Fig. 13. Applications of geometric-phase metasurface in planar optics. (a) Multiwavelength achromatic dielectric meta-devices[49]; (b) schematic of broadband reflective achromatic metalens[50]; (c) broadband transmissive achromatic metalens[51]
6 结束语
首先简要介绍几何相位超表面的原理,用来帮助理解几何相位的来源,接着回顾了近些年来人们在几何相位超表面用于光波前调制、近场SPPs产生和调控等方面的研究工作,最后列举了几何相位超表面的典型应用。这些研究工作大大加深了人们对于几何相位超表面的理解和认识,为利用几何相位来设计相关器件提供了依据和指导。可以看出,几何相位超表面对于电磁波的静态调控功能已经相对完整,但需指出的是,几何相位本身的产生机理,导致很难对几何相位超表面进行快速主动的调控,这也是目前几何相位超表面发展的一个瓶颈。事实上,几何相位超表面能够充分地利用电磁波的极化空间这个自由度,如果能够做到对几何相位进行快速动态的调控,那么将意味着可以实现任意极化和任意波前的电磁波。与此同时,几何相位和共振相位复合的超表面在打破自旋依赖、实现消色差及多功能等方面已经显示出新奇的现象和重要的应用。但是也要指出,这种复合超表面原则上打开了对极化、频谱和波前三个空间调控的自由度,而现已实现的现象距离这些自由度的完全探索还很远,未来复合超表面将有可能完全利用这些自由度,从而实现对电磁波更加自由的调控。
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Article Outline
胡中, 徐涛, 汤蓉, 郭会杰, 肖诗逸. 几何相位电磁超表面:从原理到应用[J]. 激光与光电子学进展, 2019, 56(20): 202408. Zhong Hu, Tao Xu, Rong Tang, Huijie Guo, Shiyi Xiao. Geometric-Phase Metasurfaces: from Physics to Applications[J]. Laser & Optoelectronics Progress, 2019, 56(20): 202408.