▎ 摘　　要

Metasurfaces offer tremendous opportunities in controlling wave propagation in unusual ways that cannot be achieved with conventional optical devices. Common approach in designing metasurfaces has been the use of spatially varying, subwavelength-thick metallic and/or dielectric nanostructures for obtaining required phase change locally that yields desired optical performance. Here, we theoretically demonstrate an alternative metasurface based on time-varying resonant elements rather than space-varying ones. Our metasurface design utilizes graphene microribbon arrays that exhibit resonant behavior at terahertz wavelengths. By controlling the Fermi level, and therefore the doping of graphene, one can induce time-varying changes in the complex refractive indices of graphene, resulting in active control of the reflection amplitude and phase. Time-varying phase changes that can be achieved by applying a 1 GHz alternative current signal has been shown to change the frequency of a reflected terahertz wave on the order of 5 X 10(-2) to 5 GHz depending on the operation wavelength. Time-varying metasurfaces provide an additional knob for controlling the wave propagation and facilitating active control with temporal modulation that cannot be achieved with passive space-varying metasurfaces. Time-varying metasurfaces will enable Doppler-like shifts and novel methods of actively controlling and manipulating light-matter interactions in ultrathin optical devices.