Tunable high-<i>Q</i> perfect absorber by employing doubly degenerate quasi-bound states in the continuum

Quasi-bound states in the continuum (QBICs) with strong field enhancement can effectively enhance the interaction between light and graphene, which is important for the development of high-performance perfect absorbers. Traditional absorbers often rely on back reflectors or dual-side illumination to surpass the 50% absorption limit, posing practical design challenges. In this study, we propose a tunable high-Q perfect absorber based on doubly degenerate QBICs, where two distinct QBIC modes coexist at the same resonance wavelength. This degeneracy plays a pivotal role in enabling perfect absorption by allowing simultaneous excitation of orthogonal magnetic dipole modes, thereby eliminating the need for asymmetric illumination or reflective backing. The absorber's unit cell comprises a graphene layer and two silicon split-rings, where two QBICs can be selectively excited by tuning the distance and rotation angle between the split-rings. It is demonstrated that they are degenerate at a wavelength of 1562 nm, thereby enabling a narrow-band perfect absorption with a Q value exceeding 450. Furthermore, a maximum absorption intensity difference of 99.1% could achieve by dynamically modulating the Fermi energy of graphene.