Transmissive-reflective broadband terahertz multifunctional wave plate based on VO₂-metal hybrid Huygens' metasurface

We present a switchable broadband terahertz multifunctional wave plate with VO₂-metal hybrid Huygens' metasurface, which enables the switching between a quarter-wave plate and a half-wave plate. In the insulating phase of the VO₂ film, the hybrid Huygens' metasurface acts as a broadband reversible quarter-wave plate, capable of reverting normally incident linearly polarized light into circularly polarized light and vice versa. The maximum efficiencies of transmitted circular and linear polarizations are 90.97% and 91.03%, respectively, within the frequency range of 0.81-1.26 THz. The hybrid Huygens' metasurface functions as a half-wave plate when VO₂ is in its metallic phase, exhibiting a bandwidth of 0.9 THz. Furthermore, typical parameters that determine the polarization properties are analyzed to examine their accuracy in describing circularly polarized light. The proposed broadband terahertz multifunctional wave plate exhibits full-space tunability, high efficiency, and broad bandwidth, showing great potential to advance the development of broadband polarization converters and compact optical systems.