基于几何相位的亚太赫兹波宽带平面透镜

Wideband flat lens of sub-THz beams by geometric phase

几何相位,又被称为Pancharatnam-Berry相位,因设计简单、相位延迟可覆盖2π范围且与频率无关等优势在光学超表面中被广泛用于波前操控.几何相位超表面的效率和带宽往往受限于其结构单元实现正交偏振变换的能力.当工作频率偏离设计频率时,结构单元往往不能完全改变入射圆偏振光的手性,导致仅被改变手性的部分光波波前被调控,而其余光波未被调控.为此,本工作提出了一种几何相位超表面透镜,得益于其结构单元的高效率、宽频带的正交偏振变换能力,该透镜可实现亚太赫兹波束的宽带聚焦.其结构单元为一维低折射率介质光栅,可在130-165 GHz的范围内实现正交偏振变换.通过旋转、截断和拼接此光栅构成二维平面透镜,数值仿真表明该透镜在60-160 GHz的宽带范围内,均保持在40%以上的聚焦效率.超表面透镜的带宽远大于其结构单元的偏振变换带宽,这是由光栅不同的截断长度导致的.这款透镜可以采用3D打印进行加工,制作方便,成本低廉,对发展紧凑型亚太赫兹波束操控元件具有重要意义.

Geometric phase,also known as Pancharatnam-Berry phase,is widely employed in optical metasurfaces for wavefront manipulation,benefiting from the simple design procedure and 2πdispersion-free phase coverage.The efficiency and bandwidth of geometric-phase-based metasurfaces are usually constrained by the capability of the constituent inclusions in orthogonal polarization conversion.When operating at a frequency deviated from the designed one,the inclusions cannot fully alter the handedness of the circular polarization,leading the wavefront to be partially reshaped by the geometric phase.In this study,we propose a geometric phase metasurface for a wideband beam focusing that benefits from the wideband polarization-rotation capability of the constituent inclusions.The inclusions are one-dimensional(1 D)low-index dielectric gratings featuring orthogonal polarization conversion and high transmission efficiency from 130 to165 GHz.Below 130 GHz,the polarization conversion fails due to the undesired interactions of two waveguide modes inside the grating layer.Then,the gratings are rotated,cut off,and reconstructed to focus the circular polarized sub-THz waves.Numerical simulation results show that we obtain a focusing efficiency of>40%in the frequency range of60-160 GHz.The focusing bandwidth is much wider than that of the constituent 1 D gratings as a polarization convertor.This is explained by the different cutoff lengths of the gratings in various zones of the metasurface.The proposed design can be manufactured using cost-effective three-dimensional printing techniques,thus paving the way for compact and wideband sub-THz functional devices.