Multiband quasibound states in the continuum engineered by space-group-invariant metasurfaces

Bound states in the continuum (BICs) enabling resonances with high quality ($Q$) factor allow for high-efficiency light-matter interactions and boost various research fields such as nonlinear photonics and optical detecting. However, further improvement of the $Q$ factor of the resonances is challenging since the accurate fabrication of the tiny symmetry defects of meta-atoms is limited by the nanofabrication technology, which is critical for transition from BICs to quasi-BICs in previous works. Here we propose a different approach to trigger the transition of quasi-BICs that does not need to change the space group symmetry of nanostructures. By modulating the periods of the nanostructures, our approach enlarges the unit cell and enriches the ideal BICs eigenstates, achieving multiband quasi-BICs in metasurfaces. By tuning the asymmetry parameters, the selective excitation of multiband quasi-BICs is demonstrated. Thanks to the multi-narrow-band resonances obtained by the unit cell perturbation, a multivariable sensing method is proposed to simultaneously obtain the birefringence property and thickness of a nanoscale layer. The proposed approach exciting multiband quasi-BICs offers a possible solution for ultra-high-$Q$ devices without complex fabrication techniques and may found realistic application in many fields such as harmonic generation, optical detecting, and compact low-threshold lasers.