Tapered metasurfaces for eliminating up-down asymmetry of quasi-bound states in the continuum resonance

Either the bound states in the continuum (BIC) or quasi-BIC structure can be used as Hermitian system when it has up-down symmetry (Cartesian coordinates in the out-plane or z direction), so that BICs is stable. However, out-plane symmetry is not guaranteed in all cases. We take the quasi-BIC structure to enhance the absorption of two-dimensional materials as an example, but not limited to the use of other BIC structures, to solve the problem of the asymmetric quasi-BIC metasurfaces in the z direction. Previously reported works have proposed to control critical coupling light absorption in two-dimensional materials such as monolayer graphene by quasi-BIC metasurfaces. However, in order to apply the quasi-BIC structure in practice, the substrate needs to be added and it will greatly destroy the out-plane symmetry, resulting in the reduction of the original high absorption rate of graphene. To solve the above problems, the tapered structure designed in this paper can recapture the electromagnetic field into the quasi-BIC structure and avoid the leakage of the field into the substrate. This structure maintains the high absorption rate of the original structure on graphene, and the calculated results show that our tapered structure is applicable to different structures and substrates. © 2023