Design and equivalent circuit model extraction of a broadband graphene metasurface absorber based on a hexagonal spider web structure in the terahertz band.

In this paper, an ultra-wideband terahertz absorber is designed utilizing a graphene-based metasurface. The absorber is composed of three layers including the graphene metasurface, Topas-cyclic olefin copolymer dielectric substrate, and a gold ground layer. The particle swarm optimization algorithm and interpolate quasi-Newton optimization are utilized to find an optimized structure with the widest bandwidth. Full-wave simulations verify achieving absorbance of more than 90% in an extremely wide frequency band within the range of 1 THz to 3.5 THz (fractional bandwidth = 111%) under illumination of a normal incident wave. The proposed structure is polarization insensitive up to a polarization angle of 75°, while the performance of the absorber (absorbance level and bandwidth) is almost fixed for incident angles $ \theta $θ up to 60°. Moreover, the switching capability of the structure from reflection ($ {\gt} 92\% $>92%) to absorption ($ {\gt} 90\% $>90%) is investigated. The equivalent circuit model is extracted for the designed absorber, and the corresponding result is compared to that of the full-wave simulation, which confirms the validity of the extracted circuit.