▎ 摘　　要

In this article, we theoretically study the optical properties of graphene metasurfaces consisting of layered graphene ribbons and their potential applications as multifunctional optical devices in the long-wavelength infrared region. By engineering the plasmonic resonance in graphene ribbons with different widths, the phase of reflected light can be tuned over a range of nearly 2 pi, while the reflectivity is kept relatively high at the target frequencies. Owing to the weak light graphene interaction in the off-resonance region, independent control of the reflected light can be achieved by stacking multiple layers of graphene ribbons. Since the interlayer coupling between the ribbons is negligible, we have developed a transmission line-based uncoupled model as a physical interpretation of the stacked metasurface. The modeled results show excellent agreement with numerical simulations. As a proof-of-principle demonstration, we have designed and demonstrated graphene metasurfaces as flat, dual-band focusing reflectors operating at 25 and 16 THz. Our work provides a general design scheme for multiband, multifunctional metasurfaces with various potential applications, including beam steering, optical communication, and information processing.