High-efficiency all-silicon metasurfaces with 2π phase control based on multiple resonators

Flat all-silicon metasurfaces have significant potential as ultrathin optical devices and systems that offer the advantages of mature fabrication technology and complementary metal–oxidesemiconductor compatibility. However, previously reported all-silicon metasurfaces suffered from the drawback of low transmission owing to the large intrinsic reflection loss induced by high-index silicon substrates. To achieve high transmission, silicon-based metasurfaces are usually composed of low-index dielectric substrates and silicon pillars. It is still a substantial challenge for an all-silicon metasurface to realize high transmission. In this paper, we present a design to overcome this drawback by integrating two-dimensional periodic arrays of subwavelength silicon cylinders as perfect antireflection resonators on the bottom and top surfaces of the silicon substrate. Using this design, a uniform array of all-silicon unit cells comprising a metasurface can exhibit high transmission, up to 99.5%, at the target wavelength of 10.6 µm, which indicates a significant improvement over the design without the antireflection scheme. Furthermore, as an example of a metasurface application, an all-silicon polarization-insensitive metalens is built based on a series of all-silicon unit cells with high transmission and 2π phase control, which indicates subwavelength-focusing capability. The transmission of the entire metalens reached 81.9%, and the focusing efficiency was 71%. This study not only recommends a means to develop ultrathin all-silicon optical devices with high efficiency but also proposes a general approach to achieve high-quality metasurfaces based on a single high-index dielectric.