基於液晶的高效率相位可調之超穎表面

Abstract

動態可調的超穎表面近年來逐漸受到重視，由於其可調製任意位置相位之特性，可應用於自駕車、相機及遙測等領域。我們藉由次波長三明治型光柵理論，提出一個電控可調的液晶超穎表面以控制光束方向和偏振特性。此理論有效率地預測一個可引發共振疊加的結構以達到2π 的相位改變。我們亦以自建之平行化三維時域有限差分法電磁模擬器搭配各向異性材料演算法驗證該理論。我們設計之超穎表面是以內含液晶的矽柱為最小單元組成之陣列，隨著施加偏壓以改變約0.2的液晶分子折射率變化範圍，此設計針對1550 奈米波長的近紅外線達到1.96π 相位調製範圍及超過99% 的反射率。藉著控制反射波的相位，我們個別演示了光束偏轉器及偏振轉換器之應用。該可調式光束偏轉器的最大的偏轉角度為40.23 度。該偏振轉換器能將線偏光轉換成與之互相垂直之線偏光、左右旋極化光，甚至是橢偏光，並具有接近全反射的效率及超過30 dB 的優異消光比。

Dynamic metasurfaces with arbitrary phase manipulation are favorable for numerous applications in the fields of self-driving cars, cameras, and sensing. We propose an electrically tunable liquid crystal (LC) metasurfaces for beam steering and polarization manipulation by exploiting the modal method of subwavelength sandwich gratings. This method efficiently predicts a resonance overlapping structure for 2π phase tuning. The in-house parallelized three-dimensional finite-difference time-domain (FDTD) electromagnetic numerical solver for anisotropic materials is established to verify the correctness of modal method. The delicate metasurface consisting of embedded LC silicon pillar resonator array achieves 1.96π phase tuning and ultrahigh reflectance over 99% with a small LC refractive index change of approximately 0.2 for a single wavelength 1550 nm. By controlling the phase of reflected wave, the metasurface realizes a tunable beam deflector or a polarization converter. The proposed beam deflector has a maximum deflected angle of 40.23◦. The proposed polarization converter can transform linear to crossed linear, circular, or elliptical polarization with near total reflection and excellent polarization extinction ratios over 30 dB.