Giant bandwidth tailoring of perfect light absorbers: Empowered by quasibound states in the continuum

The ability to adjust the absorption bandwidth of an optical absorber with high efficiency can facilitate the development of novel photonic and optoelectronic devices. In this work, based on the temporal coupled-mode theory, we establish a connection between the absorption bandwidth of an absorber and radiation engineering with the symmetry-broken quasi-BIC and propose a BIC-inspired strategy to tailor light absorption at critical coupling with a wide range of functionality. Based on this, the manipulation of the broadband or narrow-band quasi-BIC-governed graphene perfect absorber is realized, and the corresponding absorption bandwidth at critical coupling can be drastically adjusted by more than three orders of magnitude. Moreover, we predict and verify the quadratic scalability of the absorption bandwidth on the asymmetry parameter of the quasi-BIC structure, essentially arising from the powerful physics of BIC in radiation engineering. Present work not only provides a profound physical insight into the tailoring functionalities of radiation and in particular absorption by BIC, but also affords new possibilities for research into active optical devices.