Study on double-layer adjustable metalens based on phase change material Sb2S3

Based on double-layer Sb2S3 material, a method is proposed to realize adjustable metalens at 1.31 μm using geometric phase to regulate the incident beam. Different functions are realized by changing different states of Sb2S3. In design 1, the lower layer of phase change material is set as a half-wave plate in the amorphous and crystalline states, and the upper layer can be switched between half-wave plate and full-wave plate when it is in the two states so that the focal length of the metalens can be switched between F1 and F2. The full width at half maximum (FWHM) is close to the diffraction limit, and the focus efficiency can reach 69%. In design 2, the upper layer is always in the amorphous state with the high transmittivity. An optical switch is realized when the lower layer is used as a half-wave plate with the transmittivity close to 0 in the crystalline state. At the same time, a bifocal metalens with the focusing efficiency of 62% can be realized when the lower layer is used as a half-wave plate with the high transmittivity in the amorphous state. By adjusting the material crystallization rate, bifocal metalens with various intensities are realized. Our design has great potential in optical imaging, scanners that combine beam switches and lenses, and dual-function devices.