Qian Ziheng, Tian Shengnan, Zhou Wei, Wang Junwei, Guo Hanming
Opt Express. 2022 Mar 28;30(7):11203-11216. doi: 10.1364/OE.450601.
Metasurfaces provide a remarkable platform to manipulate over phase, amplitude, and polarization flexibly and precisely. Bifocal metalens draws great research interest due to its ability of converging wavefronts to different focal positions horizontally and longitudinally. However, as wavelength of incident light changes, chromatic aberration will cause the focal lengths reliance on the incident wavelength, which will affect the performance of metasurface, especially for longitudinal bifocal metalens. In this work, a broadband achromatic longitudinal bifocal metalens (BALBM) based on single nanofin unit cell is demonstrated. Pancharatnam-Berry (PB) phase is used to converge the incident light. Cross commixed sequence distribution (CCSD) is introduced to control the positions of focal points Fand Fwhen left-handed circularly polarized (LCP) and right-handed circularly polarized (RCP) incident. Propagation phase is used to compensate the phase difference caused by chromatic aberration. Simulation results show that in the continuous wavelength range from 500 nm to 700 nm, the focal point shifts caused by chromatic dispersion are reduced 81% for F and 83% for F, respectively. The focal length variations are stabilized to 6.21% for Fand 4.8% for Fcomparing with the focal lengths at the initial wavelength 500 nm. The proposed BALBM brings advances to bifocal metasurfaces in versatile application areas including machine vision, optical computed tomography and microimaging.
超表面提供了一个卓越的平台，可灵活且精确地操纵相位、幅度和偏振。双焦点超透镜由于其能够在水平和纵向将波前汇聚到不同的焦点位置而引起了极大的研究兴趣。然而，随着入射光波长的变化，色差会导致焦距依赖于入射波长，这将影响超表面的性能，尤其是对于纵向双焦点超透镜。在这项工作中，展示了一种基于单个纳米鳍单元的宽带消色差纵向双焦点超透镜（BALBM）。潘查拉特纳姆 - 贝里（PB）相位用于汇聚入射光。引入交叉混合序列分布（CCSD）来控制左旋圆偏振（LCP）和右旋圆偏振（RCP）入射时焦点F和F的位置。传播相位用于补偿色差引起的相位差。仿真结果表明，在500 nm至700 nm的连续波长范围内，色散引起的焦点偏移对于F分别减少了81%，对于F减少了83%。与初始波长500 nm处的焦距相比，F的焦距变化稳定在6.21%，F的焦距变化稳定在4.8%。所提出的BALBM为双焦点超表面在包括机器视觉、光学计算机断层扫描和微成像等多种应用领域带来了进展。
