Reconfigurable GSST-based metasurfaces for simultaneously demultiplexing optical spin and orbital angular momenta

Light beams carrying spin angular momentum (SAM) and orbital angular momentum (OAM) have garnered significant interest owing to their potential in optical communications, imaging, and micromanipulation. Although simultaneous detection of multiple SAM and OAM modes through demultiplexing is highly necessary, it remains challenging due to the static optical responses of conventional metasurfaces after device fabrication. Here, we propose a multifunctional metasurface demultiplexer utilizing the phase-change material Ge₂Sb₂Se₄Te₁ (GSST) for reconfigurable, simultaneous detection of multiple SAM and OAM modes. When GSST is amorphous, the coaxial beams carrying SAM and OAM modes are spatially demultiplexed into distinct vortex beams with defined topological charges, and each propagates to a predetermined position. The information of SAM and OAM of the incident beam can be determined by observing the position of the solid spot. Upon transition of GSST into the crystalline state, demultiplexing ceases due to the loss of phase modulation capability, thereby preventing mode retrieval. This reconfigurable detection strategy offers a promising platform for dynamically switchable devices in applications such as encrypted optical communications and dynamic holography.