▎ 摘　　要

In this paper, a new type of broadband tunable metamaterial absorber is proposed. The structure consists of a single-layer ring graphene and a metal ground plane separated by a dielectric spacer. The graphene rings are connected each other to be easily tuned by the voltage. In addition, this absorber is polarization-independent at normal incidence due to the symmetric pattern of the unit cell. Results reveal the bandwidth of 90% absorption reaches to 2.25 THz for a single-ring structure. The bandwidth of 90% absorption increases to 3.2 THz by combining two similar rings with different sizes in one unit cell. The physical mechanism of the single-ring and dual-ring absorbers are given based on the impedance matching theory. (c) 2018 Author(s).