Double-layer metalens with a reduced meta-atom aspect ratio.

A dielectric metalens usually has a rather high meta-atom aspect ratio, causing difficulty in fabrication. In this Letter, we develop a kind of metalens constituting two different materials, which can lead to a much lower meta-atom aspect ratio for a full 0-2 phase modulation. The designed metalens is composed of double-layer circular nanopillars, and the pillars are constructed by a polycrystalline silicon layer and a titanium dioxide () layer. The experimental results demonstrate that the required aspect ratio can be much smaller than that of a single material, and the metalens can realize a focusing spot close to the diffraction limit. Numerical simulations show that the double-layer metalens is also achromatic due to the different dispersion characteristics of the two materials as compared with its only counterpart. Furthermore, it is polarization-independent with a focusing efficiency exceeding 22%.