Ge2Sb2Se4Te1-based spin-decoupled metasurface for multidimensional and switchable focusing in the mid-infrared regime

Metalens is an ultrathin flat optical device with a great potential for applications in spin-controlled nanophotonics. However, most advanced metalenses typically work at a single wavelength or one specific spin state of incident light, hindering their applications in practice. In the present work, we report three distinct spin-dependent metalenses based on the novel phase-change material of Ge2Sb2Se4Te1 (GSST), enabling multidimensional splitting and switchable manipulation capabilities. Such schemes are successfully realized due to synergizing propagation phase and specific PB phase together with deliberately selecting several anisotropic GSST nanofins that behave as half-wave plates for their constituent elements. In particular, the transverse spin-dependent splitting metalens (TSSM) is demonstrated to enable the left-circularly polarized (LCP) and right-circularly polarized (RCP) incidence to focus at opposite transverse offsets within a broadband width from 3750 to 4600 nm (20.2% of the bandwidth). The longitudinal spin-dependent splitting metalens (LSSM) realizes the longitudinal spin-dependent splitting performance along z-axis upon RCP and LCP incidence in a considerable bandwidth (from 3900 to 4900 nm, 23.8% of the bandwidth). While for the hybrid design, it is implemented to obtain transverse and longitudinal spin-dependent splitting simultaneously for LCP and RCP light. It should be emphasized that all of the proposed metalenses are proved to ensure continuous tunability and the switching of "ON" and "OFF" states in terms of focusing and splitting features upon lambda(0)= 4200 nm by gradually converting Ge2Sb/Se4Te1 from the amorphous to crystalline state. Our results lay a solid basis for versatile potential applications such as optical imaging and communications, optical sensors, machine vision and so forth. (C) 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement