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摘要: 宽频带和动态可调谐的超构表面在太赫兹无线通信、传感和医学成像等应用中具有重要的价值。结合VO2薄膜的相变原理，本文设计了一种单层“台阶型”复合超构表面，能够实现宽频带四分之一波片和半波片之间的灵活开关功能。VO2薄膜为常温绝缘相时，超构表面可视为透射型双偏振的宽频带四分之一波片。在1.43~2.43 THz宽频带范围内，能够将垂直入射的x偏振光转换成左旋圆偏振光，椭圆率大于0.99，相对带宽为52%。VO2薄膜为高温金属相时，超构表面能够实现反射型半波片功能，垂直入射的x偏振光能够转换成y偏振光。此外，本文也详细地研究了波片性能随倾斜入射角度的变化情况，结果表明，随着入射角度的增加，四分之一波片能够实现宽频带和双频带的动态切换，半波片可以实现频率可调谐度为57%的频移。本文所提出的单层“台阶型”复合超构表面有望促进宽频带偏振转换器件，可调谐开关和紧凑型光学器件的发展。Abstract: Dynamically tunable and broadband control of polarization is important in terahertz applications such as wireless communication, sensing, and medical imaging. We propose a single-layered “stepped” hybrid metasurface based on wire resonator and VO2 phase transition, which enables the flexible switching between broadband quarter-wave plate and half-wave plate. The hybrid metasurface is a transmission-type broadband quarter-wave plate when VO2 film is insulating phase. At 1.43~2.43 THz, it can convert the normally propagating x-polarization to left-handed circular polarization with an ellipticity over 0.99 and 52% bandwidth of the central wavelength. The hybrid metasurface can realize x- to y-polarization conversion and act as a half-wave plate when VO2 is in a metallic phase. In addition, we study the wave plate performance at different oblique incident angles. The results show that the quarter-wave plate can achieve dynamic switching between broadband and dual-band properties and the half-wave plate can achieve a frequency tunability of 57% with the increase of the incident angle. The proposed switchable terahertz qurter-/half-wave plate is expected to promote the development of broadband polarization conversion components, tunable switches and compact optical components.
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Key words:
- switchable metasurface /
- quarter-wave plate /
- half-wave plate /
- broadband /
- terahertz
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图 1 基于相变原理的单层“台阶型”复合超构表面的工作原理和结构示意图。（a）VO2为绝缘相时，超构表面具有透射型四分之一波片功能；（b）VO2为金属相时，超构表面具有反射型半波片功能；（c）复合超构表面基本单元的结构参数图。
Figure 1. Schematics of structure and working principle of single-layered “stepped” hybrid metasurface based on VO2 phase transition. (a) The hybrid metasurface can act as a transmission-type quarter-wave plate when VO2 is in an insulating phase. (b) The hybrid metasurface is a reflection-type half-wave plate when VO2 is in a metallic phase. (c) Stereogram of a unit cell in the proposed metasurface.
图 2 常温下单层“台阶型”复合超构表面的透射偏振特性。（a）和（d）透射系数；（b）和（e）相位差；（c）和（f）归一化椭圆率和透射圆偏振光的能量。黄色区域带宽为1 THz，绿色区域带宽为0.22 THz。
Figure 2. Polarization performance of single-layered “stepped” hybrid metasurface when VO2 film is insulating phase. (a) and (d) Transmission coefficient; (b) and (e) phase difference between y- and x-polarized transmitted light; (c) and (f) calculated intensity S0 and ellipticity χ. The yellow area indicates the bandwidth of 1 THz, and the green area indicates the bandwidth of 0.22 THz.
图 3 平行于x轴方向线栅的（a）透射特性和（b）相位差。平行于y轴方向线栅的（c）透射特性和（d）相位差。
Figure 3. (a) Simulated transmission and (b) phase difference of the wire metasurface parallelled to the x-axis direction. (c) Simulated transmission and (d) phase difference of the wire metasurface parallelled to y-axis direction
图 4 温度为87°C时（x偏振光垂直入射），超构表面的反射特性和表面电流分布。（a）反射系数和偏振转换率；（b）和（c）“台阶型”金属谐振器和VO2薄膜在2.80 THz处的表面电流分布
Figure 4. At 87°C, reflection performance and surface current distribution of the hybrid metasurface under x-polarized light normal incidence. (a) Reflection coefficient and polarization conversion ratio. (b) and (c) Surface current distribution of "stepped" metal resonator and VO2 film at 2.80 THz
图 5 VO2相变过程中四分之一波片和半波片输出光偏振态的变化情况。（a）垂直入射x偏振光和（b）y偏振光激发超构表面产生的透射偏振态。（c）垂直入射x偏振光激发超构表面产生的反射偏振态
Figure 5. Polarization ellipse of the output wave with VO2 conductivity ranging from 10 S/m to 200000 S/m. Polarization ellipse of the transmitted wave (a) at 2.20 THz for x-polarized and (b) at 2.95 THz for y-polarized normal illumination of the hybrid metasurface. (c) Polarization ellipse of the reflected wave at 2.80 THz for x-polarized normal illumination of the hybrid metasurface
图 6 四分之一波片性能随入射角度的变化关系。（a）和（c）χ；（b）和（d）S0
Figure 6. Quarter-wave plate performance as a function of oblique incident angle. (a) and (c) ellipticity χ. (b) and (d) intensity S0
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