High-efficiency terahertz metasurface with independently controlled and switchable function in transmission and reflection modes

Metasurfaces exhibit flexible functional tunability by importing phase change materials. However, in most cases, these metasurfaces work in either transmission mode or reflection mode, which limits the application in function integration. Aiming at functions integration, a novel metasurface is proposed to realize the invertible transition between transmission and reflection modes in terahertz (THz) frequencies by utilizing a structured vanadium dioxide (VO2). The wavefronts of the transmission and reflection modes can be independently manipulated according to the insulator-metal phase transition induced simply by the temperature, allowing the integration of expanded functions. As an example of the functionality, a highly efficient, polarization-independent, and bifunctional metasurface with a lens in the transmission mode and a half-wave plate in the reflection mode has been designed and numerically demonstrated. The focusing efficiency of the meta-lens reaches to 68% with focal spot size exceeding the calculated diffraction limit at frequency of 1.4 THz, and the polarization conversion rate of the half-wave plate is higher than 90% within frequency width of 0.44 THz. The proposed tunable metasurface provides a simple way to develop high-performance multifunctional metadevice.