Broadband THz Absorbers With Graphene-Based Anisotropic Metamaterial Films

We introduce a broadband THz absorber with an array of graphene-dielectric multilayered frustum pyramids on a metal sheet. The multilayered graphene-dielectric structure can be considered an effectively homogeneous metamaterial with an hyperbolic dispersion and anisotropic permittivity. Surface plasmonic waves are excited on graphene layers and the incident waves of different frequencies are absorbed at different levels of the stacked pyramid, due to the squeezing effect of the slow waves at the tapered waveguide. An absorption dip is observed and explained physically, and finally removed by adding a rectangular portion to the pyramid unit cell. High absorption with an extremely broad bandwidth from 8 THz to over 100 THz is achieved. The absorption spectrum of the present structure can be scaled down to a lower frequency (e.g. 2 THz) by increasing the size of the unit cell.