Polarization-Multiplexed Metalens Enabled By Adjoint Optimization

Polarization imaging technology has important application value in target detection, biomedicine, and other fields, but traditional polarization imaging systems suffer from complex structures, large volume, and heavy weight. The polarization imaging system based on metasurfaces can avoid these problems effectively, which is conducive to the development of miniaturized, lighter and easily-integrated optical systems. However, the traditional design method for metasurfaces ignores near-field electromagnetic coupling caused by the local aperiodicity, which will seriously affect the diffraction efficiency of metalenses, especially if they have a large numerical aperture. To solve this problem, a general method for designing polarization-multiplexed metalenses based on boundary optimization is proposed in this paper, and a polarization imaging metalens with a large numerical aperture (similar to 0.94) is designed, which can independently control x- and y-polarized light. For the optimization design with artificially optimal initial structures, the traditional design method of parameter scanning and manual selection was used to obtain the initial structure of the metalens, and then it was further optimized by the boundary optimization, resulting in about 20% improvement of the diffraction efficiency compared with that before optimization. For the optimization design with a uniform array as the initial structure, the diffraction efficiency can reach about 92% after about 20 iterations. The optimized design method proposed in this paper can effectively improve the efficiency of polarization-multiplexed metasurfaces, showing promising applications in polarization imaging, optical communication, and other fields.