Numerical study of a tunable graphene terahertz absorber with an asymmetric ring-comb metasurface

A tunable terahertz absorber based on a comb-integrated asymmetric ring graphene metasurface is numerically investigated. The device has a compact trilayer configuration consisting of monolayer graphene, a SiO₂ spacer, and an Au back reflector. When the graphene Fermi level increases from 0.5 to 1.0 eV, the main absorption peak moves from 1.052 to 1.492 THz, corresponding to a 0.440 THz shift; the peak absorptance remains above 90% and ranges from 91.54% to 99.52% for the five sampled states. The magnetic-field distributions show a coupled response of the asymmetric outer ring and the inner comb-like branches. The outer ring provides the main resonant path, whereas the comb branches introduce local coupling and field confinement. The Au back reflector suppresses transmission, and the SiO₂ spacer contributes the cavity phase required to reduce reflection. Parametric simulations show that changes in w_line, R_in, and R_out mainly alter the resonance frequency and spectral line shape. Under oblique incidence, identifiable absorption bands remain for both TE and TM excitation, although their angular evolution differs. These results show that a single patterned graphene layer can electrically reposition a high-absorption resonance within 1-2 THz.