The bifocal metalenses for independent focusing of orthogonally circularly polarized light

Compared with traditional optical lenses, metalenses have obvious advantages of ultra-thin and easy-integration. In this paper, resonant phase and geometric phase are combined to design a metalens that can independently control the focus of right-handed circular polarization (RCP) and left-handed circular polarization (LCP) light. This solves the disadvantage that resonant phase or geometric phase alone cannot focus the RCP and LCP light at any two different positions simultaneously and independently. Two types of metalenses have been designed. One is off-axis metalens, which can focus RCP and LCP light in symmetric and asymmetric positions, respectively. The other is on-axis metalens, which can realize the bifocal effect of RCP and LCP light at different positions along the axis. Furthermore, by increasing the numerical aperture, the maximum electric field energy intensity and full width at half maximum of two focal points of the same metalens can gradually approach each other. This provides a new way for the regulation of polarization-dependent imaging, information detection, as well as the possibility of realizing multi-functional metasurface devices. (paper)