Optically Reconfigurable Spin-Decoupled Terahertz Metasurface Based Solely on the Curvature-Induced Geometric Phase

Spin-decoupled phase control is essential for independently manipulating opposite circular-polarization channels in terahertz (THz) metasurfaces. However, existing strategies generally require multiple phase degrees of freedom, increasing the complexity of phase engineering. Here, we propose an optically programmable reflective THz metasurface that realizes spin-decoupled phase control solely through curvature-induced geometric phase (CIGP), without overall meta-atom rotation or hybrid phase mechanisms. The proposed metasurface incorporates two independently controllable photosensitive silicon elements, each of which can be assigned either a low- or high-conductivity state to independently reconfigure the effective surface-current paths and corresponding CIGP of the two spin channels. At 1.1 THz, all four coding states (00, 01, 10, and 11) maintain reflection amplitudes above 0.8, while the left and right silicon elements independently provide 0/π phase control of the LCP and RCP channels, respectively. By assigning different conductivity states to the photosensitive silicon elements across the metasurface, the desired spin-dependent phase profiles can be encoded for reconfigurable focusing, multi-angle focal-spot steering, and independent dual-channel near-field imaging. As a proof-of-concept, full-wave numerical simulations show good agreement between the preset and simulated steering angles and distinct image reconstruction in the two spin channels with negligible mutual interference. This work establishes a single-geometric-phase route to optically programmable spin decoupling, providing a simple and flexible approach to multifunctional THz wavefront manipulation.