Optically Programmable GST Metasurface for Coded Terahertz Wavefront Control

Compact and programmable wavefront control is a central task for advancing terahertz (THz) wave spectroscopy, imaging, and wireless communications. Although electrically programmable metasurfaces have exhibited remarkable versatility and significantly promoted THz dynamic device development, realizing two-dimensional (2D), nonvolatile, broadband, and high-resolution wavefront control remains a critical objective. Here, we present an optically programmable metasurface method that potentially overcomes these difficulties by leveraging the reversible phase change of the chalcogenide material Ge₂Sb₂Te₅ (GST). The core innovation lies in the use of selective optical excitation to locally address and induce phase changes in constituent GST patches, enabling reconfigurable and nonvolatile reversal of the meta-atom symmetry. This unique mechanism yields a robust and broadband 0/π phase-switching capability at the meta-atom level, operating with subwavelength resolution and without the need for complex integrated electrodes. By employing spatially patterned optical pumping with predesigned masks as examples, we experimentally demonstrate two distinct 2D coded functionalities: controllable beam steering and tunable beam focusing. Our method establishes a new paradigm for programmable THz metasurfaces, offering a promising pathway for active and flexible THz wavefront engineering critical for systems requiring long-term, stable functionalities.