Ultrabroadband coherent perfect absorption with composite graphene metasurfaces.

We investigate the design and performance of a new multilayer graphene metasurface for achieving ultrabroadband coherent perfect absorption (CPA) in the THz regime. The proposed structure comprises three graphene patterned metasurfaces separated by thin dielectric spacer layers. The top and bottom metasurfaces have crossed shape unit cells of varying sizes, while the middle graphene metasurface is square-shaped. This distinctive geometrical asymmetry and the presence of multiple layers within the structure facilitate the achievement of wideband asymmetric reflection under incoherent illumination. This interesting property serves as a crucial step towards achieving near-total absorption under coherent illumination across a broad frequency range. Numerical simulations demonstrate that the absorption efficiency surpasses 90% across an ultrabroadband frequency range from 2.8 to 5.7 THz, i.e., a bandwidth of 2.9 THz. The CPA effect can be selectively tuned by manipulating the phase difference between the two incident coherent beams. Moreover, the absorption response can be dynamically adjusted by altering the Fermi level of graphene. The study also examines the influence of geometric parameters on the absorption characteristics. The results of this research work offer valuable insights into the design of broadband graphene metasurfaces for coherent absorption applications, and they contribute to the advancement of sophisticated optical devices operating in the THz frequency range.