Transmissive metasurface with switchable dual-phase gradients for wavefront manipulation of vortex beams

To expand terahertz wave applications, metasurface technology with subwavelength 2D array structures emerged, enabling precise wavefront manipulation and polarization conversion. However, most reported metasurface devices have single, non-tunable functions, limiting practical flexibility and diversity. Addressing this, we designed a dynamically tunable metasurface with optical responses, capable of converting circularly polarized light to cross-polarized light. It utilizes photosensitive silicon excellent optical response, with light excitation adjusting its material properties to achieve reversible insulator-to-metal phase transition, enabling precise terahertz control. By changing illumination to adjust silicon's conductivity, the metasurface exhibits two phase distributions at the same target frequency. Using Pancharatnam-Berry (PB) phase principles, rotating two units' azimuths achieves these distributions easily. PB phase configured metasurfaces show conjugate symmetric responses under opposite circular polarization excitation, due to theoretical conjugate symmetry in PB phase modulation, causing opposite phase effects for left and right rotation. To verify its terahertz wavefront manipulation ability, we developed a metasurface generating vortex beams with different orbital angular momentum (OAM) via corresponding light excitation, with orders switching from ±1 to ±2. In summary, combining polarization states and PB phases achieves 360° phase modulation in terahertz reconfigurable metasurfaces, applied successfully to circularly polarized light. Notably, this tunable technology enables diverse dual-function combinations, including switchable designs for computational holography, multi-channel information multiplexing, and dynamic wavefront control.