Spin-multiplexing phase-driven varifocal metalenses for multidimensional beam splitting and binary switching

Abstract

The Pancharatnam-Berry (PB) phase, subject to geometric phase, is currently utilized to implement spin-dependent optical functions. Simultaneously achieving spin-multiplexing and varifocal performances via a single-cell-designed metasurface that purely relies on PB phases has been scarcely proposed due to the quite sophisticated degrees of freedom to be taken into account. Here, by virtue of pure PB phases composed of convex and concave phases, spin-multiplexing and varifocal metalenses based on a single-cell-designed approach are developed. The unit cell consists of Ge2Sb2Se4Te1 (GSST) nanopillar pairs setting on CaF2 substrate. By adopting the specified pure PB phases, the proposed metalenses can focus the left-handed and right-handed circularly polarized (LCP and RCP) incidences into different positions. By ingenious design, the Ge2Sb2Se4Te1-based unit cell can work like a half-wave plate at two wavelength bands depending on the Ge2Sb2Se4Te1 phase states, enabling the proposed metalenses phase-driven zoom focusing. Particularly, the unique phase transition of GSST empowers the proposed schemes with binary switching focusing merit and large switching contrast ratio. The new design strategy constitutes a novel avenue in designing spin-multiplexing and varifocal optical devices, showing great potential in imaging, optical storage and optical interconnections.