Terahertz vectorial focusing and multifocal modulation based on a reconfigurable metasurface

Metasurfaces can precisely control phase, amplitude, and polarization at subwavelength scales. They have become a versatile platform for terahertz applications such as polarization control, beam steering, and focusing. However, existing focusing devices still face challenges. Most support only single polarization or limited polarization operation. Achieving simultaneous reconfigurability, multiple polarization states, multi-focus capability, and broadband performance remains difficult. These limitations stem from inter-channel coupling, restricted modulation, and poor spectral stability. To overcome them, a reconfigurable vanadium dioxide (VO2) metasurface is developed. A bilayer metasurface with a spin-decoupling technique enables independent vectorial focusing in four channels. Using the phase transition of VO2, the device switches between two dual-focus modes near 1.0 THz. At high temperature, 0 deg/90 deg linear polarization channels are activated. At low temperature, 45 deg/135 deg polarization channels are enabled. Full-wave simulations show high focusing efficiency and stable broadband performance at 0.85 to 1.15 THz. This work offers a new strategy for highly integrated and multi-degree-of-freedom wavefront manipulation.