▎ 摘　　要

Graphene-integrated metasurfaces enable a new paradigm to control dynamic manipulation of electromagnetic waves from terahertz (THz) to mid-infrared. Despite the rapid growth, simultaneously achieving real-time continuous reconfigurability and high efficiency is still very challenging. To demonstrate this potential, an efficient gate-tunable tri-layer reflective metasurface (metamirror) performing like a Fabry-Perot cavity is designed for focusing linearly polarized THz radiation at 0.75 THz. The high efficiency (up to 30.4%) tuning mechanism is investigated in detail. By comparing the performance of the metamirror and a tunable bi-layer transmissive metalens, it is found that the metamirror is more compact, flexible, and efficient. Our finding is anticipated to be an ultrathin and flat platform to efficiently manipulate THz waves, which can stimulate further studies related to reconfigurable flat optics, real time imaging, and sensing.