Multispectral tunable symmetry-protected bound states in the continuum in all-dielectric split-ring resonator metasurfaces

Symmetry incompatible bound states in the continuum (BICs) are the non-radiating states that remain decoupled from the radiation continuum due to the symmetry mismatch. However, it has always been challenging to observe multiple symmetry-protected BICs in single resonator dielectric metasurfaces. This work reports multiple symmetry-protected BICs for both linear polarizations in all-dielectric metasurfaces made up of split-ring resonators (SRRs). Intensive numerical simulations predict that SRR can support a high-quality factor (Q) of 106 for standalone and with substrate in presence of asymmetry. However, for a realistic and practical scenario, a Q-factor of asymptotic to 8000 for standalone structures is achieved, which reduces by a factor of 1.25 in the presence of silica substrate. Even if the out-of-plane asymmetry is introduced either in the geometry or variation in the angle of incidence, BICs are transformed into quasi-BICs possessing Fano resonance. Eigenmode and multipole decomposition analysis confirm multiple BIC resonances; notable dominant contributions are from magnetic dipole, toroidal dipole, and electric and magnetic quadrupoles. Integrating monolayer graphene adds tunability to the metasurface, where distinct quasi-BIC modes can be employed to facilitate an excellent switching.

Automated summary

This study reports the existence of multiple symmetry-protected bound states in the continuum (BICs) in all-dielectric metasurfaces made up of split-ring resonators (SRRs). Numerical simulations show that SRRs can support a high-quality factor (Q) of 106 without asymmetry, and around 8000 for realistic standalone structures, reducing to 1.25 times in the presence of a silica substrate. Introduction of out-of-plane asymmetry transforms BICs into quasi-BICs with Fano resonance. Eigenmode and multipole decomposition analysis confirm the presence of multiple BIC resonances, with dominant contributions from magnetic dipole, toroidal dipole, and electric and magnetic quadrupoles. Integration of monolayer graphene adds tunability to the metasurface, allowing for excellent switching using distinct quasi-BIC modes.