Tailoring terahertz wavefront with state switching in VO2 Pancharatnam-Berry metasurfaces

Metasurface provides a novel approach to control electromagnetic wave, and it attracts great attention. But, it is still challenging to realize wavefront tailoring with dynamically switchable functions. In this work, dynamically switchable metasurfaces are presented based on vanadium dioxide, and they realize state switching of reflected wavefront for circularly polarized wave in terahertz band. The designed meta-atom consists of vanadium dioxide strip, silica spacer, and gold substrate. Based on Pancharatnam-Berry phase, 360 degrees phase modulation is achieved by axially rotating vanadium dioxide strip. Through phase transition of vanadium dioxide between metallic and insulating states, three applications are designed to demonstrate state switching of metasurfaces at 1.0 THz. First of all, a gradient metasurface is designed and anomalous reflection is formed. Anomalous reflection for metallic vanadium dioxide and specular reflection for insulating vanadium dioxide are dynamically switched. Secondly, vortex beam generators are implemented for metallic vanadium dioxide. Incident waves with different chirality achieve orbital angular momentums with different mode numbers, and it is switched to plane wave when vanadium dioxide is insulator. Finally, a reflective lens is presented with switchable focusing function. Our work may be applied in fields such as terahertz switching, wireless communication, and planar lens.