  This article presents a novel approach for designing dual-band absorbers
based on graphene and metallic metasurfaces for terahertz and mid-infrared
regimes, respectively. The absorbers are composed of a two-dimensional (2D)
array of square patches deposited on a dielectric film terminated by a metal
plate. Using an analytical circuit model, we obtain closed-form relations for
different parameters of the structure to achieve the dual-band absorber. Two
absorption bands with obtained absorptivity of 98% at 0.55 and 1.54 THz for the
graphene-based structure and 7 and 25 THz for the metallic-based case are
achieved. We demonstrate that the graphene-based absorber remains as dual-band
for a wide range of the Fermi level. Furthermore, the proposed dual-band
absorbers have a polarization-insensitive characteristic remaining over a wide
range of incident angles. The most important advantage of this device is its
simplicity compared to previously reported structures.
