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摘要: 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超表面时，会被分解为左旋圆偏振(LCP)波和右旋圆偏振(RCP)波，通过外部环境改变介质的介电常数和磁导率，超表面光器件可以动态地控制各种偏振光特别是圆偏振光的响应特性如折射率、二色性、旋光性、不对称传输等。按照可调谐手征超表面所控制的负折射率、圆二色性和旋光性、不对称传输性质，对其最新的研究进展进行了综述。最后，对可调谐手征超表面这一快速发展领域未来可能的发展方向和存在的挑战提出了自己的看法。Abstract: Chiral metasurfaces are ultra-thin metamaterials composed of planar chiral cell structures with specific electromagnetic responses. They have attracted great attention due to their singular ability to control electromagnetic waves at will. With tunable materials incorporated into the metasurfaces design, one can realize tunable/reconfigurable metadevices with functionalities controlled by external stimuli, opening a new platform to dynamically manipulate electromagnetic waves. In this paper, we introduce some theoretical foundations of the electromagnetic properties of tunable/reconfigurable chiral metasurfaces. When a linearly polarized light enters a tunable chiral metasurface, it can be decomposed into left-handed circularly polarized (LCP) wave and right-handed circularly polarized (RCP) wave. By changing the dielectric constant and magnetic permeability of the medium through the external environment, the metadevices can dynamically control the response characteristics to various polarized lights, especially circularly polarized lights such as refractive index, dichroism, optical rotation, asymmetric transmission, etc. According to the properties of negative refractive index, circular dichroism, optical rotation, and asymmetric transmission controlled by the tunable chiral metasurfaces, we review the latest research progress. Finally, we put forward our own opinions on the possible future development directions and existing challenges of the rapidly developing field of the tunable chiral metasurface.
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图 1 (a) 楔形棱镜的模拟结构[17]；(b) 楔形结构在两种不同的频率0.6 THz和1.1 THz处的电场分布；(c) 可调NIMs的单元结构[19]；(d) NIMs的负折射
Figure 1. (a) The wedge-shaped prism simulation structure[17]; (b) The electric field distributions of the wedge structure at two different frequencies of 0.6 THz and 1.1 THz; (c) Schematic illustration of a unit cellof the tunable NIMs[19]; (d) The negative refraction of the NIMs
图 3 (a) 工作原理图[24]；(b) θ=φ=45°时非晶态和晶态的CDtran光谱；(c) 主动可调手征的相变超材料[25]；(d) 模拟和测量的透射率和CD光谱
Figure 3. (a) Schematic of the operation concept[24]; (b) The CDtran spectra for both amorphous and crystalline states under θ=φ=45°; (c) A phase transition metamaterial with actively adjustable chirality[25]; (d) Simulated and measured transmittance and CD spectra
图 5 (a) 单元结构[28]；(b) CD和OA；(c) 手征超材料的三维示意图[29]；(d) 具有不同费米能级的混合结构的CD谱；(e) 石墨烯超表面示意图[33]
Figure 5. (a) Illustration of the unit cell[28]; (b) CD and OA; (c) 3D schematic view of the chiral metamaterial[29]; (d) CD spectra of the hybrid structure with different Fermi energies; (e) A schematic illustration of a graphene metasurface[33]
图 6 (a) 圆偏振波在直角坐标系中以斜入射方式入射到无图案单层黑磷(BP)膜的示意图[34]；(b) 圆二色性光谱；(c) 将CDPL作为手征超表面CDEXT的函数输出；(d) 示意图显示了在不改变激发的CP态情况下，通过MMs耦合操纵PL极化[7]；(e) 手征超表面示意图[35]；(f) LC集成等离子体手征超表面在开、关条件下的模拟反射和CD光谱
Figure 6. (a) Schematic of circularly polarized waves impinge at a film of unpatterned monolayer black phosphorus (BP) at an oblique incidence in a Cartesian coordinate system[34]; (b) Circular dichroism spectra; (c) Output CDPL as a function of the CDEXT of chiral metasurfaces; (d) Schematic diagrams indicate the manipulation of the PL polarization through the coupling to MMs without switching the CP state of the excitation[7]; (e) Schematic of the chiral metasurface[35]; (f) Simulated reflection and CD spectra of the LC-integrated plasmonic chiral metasurface at 'ON' and 'OFF' conditions
图 7 (a) 与微流体系统结合的手征超表面示意图[36]；(b) 不同折射率混合溶液的CD谱；(c) 手征超表面示意图[39]；(d) 在不同折射率的情况下，SCMM-BLT沿x轴拉伸10%的OC光谱
Figure 7. (a) Schematic view of the chiral metasurface integrated with a microfluid system[36]; (b) The CD spectrum as a function of the refractive index of the mixed solution; (c) Schematic view of the chiral metasurface[39]; (d) OC spectra of SCMM-BLT stretched along x-axis at the level of 10% with different surrounding refractive indext
图 8 (a) 具有G形孔的石墨烯手征超表面[44]；(b) 无衬底时相对与总透射的传输差；(c) 石墨烯手征超表面[46]；(d) 结构的正反传播方向的圆转换二色性(CCD)光谱示意图；(e) 单层石墨烯平面手征超表面[47]；(f) 不同费米能级下非对称透射与波长的关系
Figure 8. (a) The graphene chiral metasurface with G-shaped holes[44]; (b) The relative enantiomeric difference in the total transmission without a substrate; (c) Schematic view of the graphene chiral metasurface[46]; (d) Circular conversion dichroism (CCD) spectra of the structure for forward and backward propagation directions; (e) The schematic diagram of the monolayer graphene-based planar chiral metasurface[47]; (f) The relation between the asymmetric transmission and the wavelength under different fermi energies
图 9 (a) 混合金属-石墨烯超表面单元示意图[48]；(b) 不同费米能级石墨烯的不对称传输参数；(c) 超表面三维视图[52]；(d) 不同μc向前传播的CCD光谱；(e) 装置原理图[53]；(f) y极化(实线)和x极化波(虚线)的AT参数
Figure 9. (a) Schematic diagram of a unit cell of the proposed hybrid metal-graphene metasurface[48]; (b) Asymmetric transmission parameters with different Fermi energies of graphene; (c) Three dimensional view of the metasurface array[52]; (d) CCD spectra of the structure for forward propagation directions with different values of μc; (e) Schematic diagram of the device[53]; (f) AT parameters of y-polarized (solid line) and x-polarized waves (dashed line)
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