Sandwich nano-fin to reduce the aspect ratio requirement of metasurface

The current high-aspect-ratio Pancharatnam–Berry (PB)-phase metasurfaces in which vertical dimensions are much larger than the lateral dimensions are challenging to fabricate. Therefore, reducing the aspect ratio requirement has become a research priority. We propose a sandwich nano-fin consisting of GaN/amorphous silicon (a-Si)/GaN on a silica substrate to reduce the strict aspect ratio requirement of current PB-phase metasurfaces. The phase modulation ability and polarization conversion efficiency (PCE) of sandwich nano-fins were investigated. Compared with a conventional GaN nano-fin, the aspect ratio of sandwich nano-fin decreased from 6 to 4 under the same geometric dimension. The PCE of the sandwich nano-fin was up to 68.27%, 1.5 times higher than that of a GaN nano-fin under the same aspect ratio of 4. Moreover, after adding the a-Si layer, the overall PCE improved from 32.17 to 40.54% for the sandwich nano-fin. The sandwich structure simultaneously reduces the challenge of manufacturing and improves efficiency compared to current PB-phase metasurfaces.