▎ 摘　　要

Two types of broadband and polarization independent graphene metamaterial absorbers are designed. One is a composite four-ring structure with azimuthally symmetry, the other is a nested multi-ring structure. The nested structure is more compact and has a broader band of absorption. The bandwidth with over 90% absorption is 7.1 THz for the composite four-ring structure. The bandwidth increases to 8.7 THz and 11.9 THz for the nested dual-ring and three-ring absorber, respectively. The wide bandwidth and polarization independent absorption owe to the combination of symmetric multi-resonators with different sizes in unit cell. Physical mechanisms of the broadband absorbers are given by the impedance matching theory as well as the surface current distributions. Instead of one single layer graphene as papers reported before, multilayers of graphene are coated on the dielectric surface to form the metal resonant-multilayer graphene-dielectric-metal ground structure. It is found that both the absorbing width and magnitude increase greatly as the graphene layers increase while a large dip appears when the number of graphene layers is large enough. Moreover, by investigating the absorptions under different graphene permittivities, we fmd that the reduced graphene permittivity is better to simplify the calculation.