Tailoring spin-sensitive focusings by optical metasurfaces

Abstract

Metalens is composed of artificially subwavelength metal or dielectric meta-atoms arranged according to phase distribution of lens, and it can focus wave in a compact and flexible manner. However, conventional geometric phase-based metalens cannot focus spin-sensitive splitting beams, hindering the simultaneous control of two spins. Here, spin-sensitive plasmonic metasurfaces are presented with independent phase modulation capability for circularly polarized channels. These metasurfaces encode phase information by integrating propagating phase and geometric phase to achieve arbitrarily independent control of focal length and focal position for circularly polarized waves (CPWs). Three metasurfaces, including longitudinally splitting focusing, multidimensional spin-sensitive splitting focusing, and multifocal metasurface, are numerically displayed to confirm these predictions. Firstly, a longitudinally splitting focusing is designed with focal length 3 μm under left-handed circular polarization (LCP) incidence and focal length 5 μm under right-handed circular polarization (RCP) incidence. Secondly, two anomalously reflected converging beams are formed by superimposing different gradient phases on a multidimensional spin-sensitive splitting focusing under LCP and RCP illuminations. Finally, a multifocal metasurface is presented, and it has bifocal points and four focal points under LCP and RCP incidences. This work provides novel methods for flexible control of circular polarization-sensitive focusing.