An efficient and miniaturized ultra-thin tunable UWB graphene metasurface absorber for terahertz gap regime

This work introduces an ultra-thin tunable ultra-wideband (UWB) metasurface absorber (MSA) for the terahertz (THz) gap. The polarization-insensitive MSA provides an absorptivity ($A\left(f\right)$) ≥ 90% from 0.1 to 11.5 THz, corresponding to 196.6% fractional bandwidth. The usage of resonant slots engraved on top patterned graphene sheet ($G_{pat}$) and strong plasmonic coupling in the Fabry-Perot cavity formed between top $G_{pat}$ and bottom continuous graphene ($G_{cont}$) in bilayer stack configuration ensures absorptivity over a UWB THz spectrum. An equivalent circuit model (ECM) closely follows the $A\left(f\right)$ response of the proposed MSA. The proposed DC-biasing mechanism can regulate the chemical potential (${\mu }_c$) of the connected $G_{cont}$ efficiently. A DC bias voltage of 0 to 6.1 V is adequate to vary ${\mu }_c$ of $G_{cont}$ from 0 to 0.6 eV for achieving tunable $A\left(f\right)$. The structure maintains its ultra-thin nature and has a thickness of only ${\lambda }_0$/1500, where ${\lambda }_0$ is the free space wavelength calculated at 0.1 THz. In addition, the periodicity is only ${\lambda }_0$/300. The MSA also provides stable absorption response from 0.1 to 11.5 THz with $A\left(f\right)$ ≥ 80% for incidence angle (θ) up to ${60}^{\circ }$ under both transverse magnetic (TM) and transverse electric (TE) polarization.