An Ultra-Broadband Tunable Terahertz Absorber Based on a Patterned Graphene Metasurface with a Near-Perfect Absorption Plateau

The numerical model uses a compact three-layer unit cell. Patterned graphene lies on a silica spacer, with a gold film underneath. The patterned graphene is the resonant part of the unit cell and accounts for both absorption loss and electrical tuning. A silica layer separates it from the gold reflector, while the bottom gold film keeps the transmitted power nearly zero. At E f = 1.0 eV, the optimized structure shows absorption above 90% between 2.53 and 6.67 THz, giving a bandwidth of 4.14 THz. From 3.61 to 6.19 THz, the spectrum forms a nearly flat plateau, and the absorption is close to 0.99 over most of this interval. The impedance curve helps explain why incident waves couple efficiently into the absorber. Over most of the operating band, the normalized impedance has a real part of about 1 and an imaginary part close to 0. Little reflection therefore occurs at the upper interface. The field maps show where the energy is concentrated in the graphene pattern. Near the low-frequency edge, the strongest fields appear around the annular features and the connecting bridges. The main field concentration then shifts inward as the frequency increases. This migration indicates that the broadband response is not produced by a single isolated resonance. Instead, neighboring localized modes from different graphene regions overlap, and this overlap, together with the near-matched impedance condition, maintains the broad absorption band. Changing the graphene Fermi energy offers a direct route for tuning the absorption spectrum. The broadband high-absorption response is also retained for both TE- and TM-polarized waves at moderate oblique incidence. These characteristics may be useful for tunable THz absorption and polarization-robust THz wave-control applications.