A Compact Three‐Layer VO 2 ‐Integrated Reflective THz Metasurface With Switchable Half‐Wave‐Plate and Quarter‐Wave‐Plate Responses

This paper presents a VO 2 ‐integrated reflective switchable polarization conversion metasurface (SPCM) that exploits the insulator‐to‐metal phase transition of vanadium dioxide (VO 2 ) to achieve binary switching between two distinct polarization conversion modes within a single device. The unit cell consists of a square split‐ring resonator (SSRR) incorporating gold and VO 2 on a 50 μm polyimide substrate, backed by a continuous gold ground plane. When VO 2 is in the insulating state, the metasurface operates as a linear‐to‐circular (LTC) converter with ellipticity approaching + 1 and an axial ratio below 2 dB across 600–685 GHz. In the metallic state, the device switches to linear‐to‐linear (LTL) cross‐polarization converter with a polarization conversion ratio (PCR) exceeding 0.8 over the same band. Unlike prior multifunctional designs that rely on auxiliary graphene biasing or photoexcited semiconductors to access both conversion types, the present design maps each VO 2 phase directly and independently to one conversion mode. Angular stability tests show PCR > 0.60 for incidence angles up to 55° in the LTL state and axial ratio < 3 dB for θ i < 30° in the LTC state.