Symmetry-protected bound states in the continuum supported by all-dielectric metasurfaces

Symmetry-protected bound states in the continuum (BICs) are nonradiative states with infinite lifetime and perfect confinement of energy even though lying in the radiation continuum due to the symmetry incompatibility. Herein, we study the symmetry-protected BIC supported by metasurfaces composed of silicon nanodisks. Through adding or removing parts of the nanodisks from the edge, a sharp Fano resonance emerges and demonstrates the excitation of quasi-BIC. Their $Q$ factors exhibit the same dependence on the asymmetry degree with these two opposite operations. Furthermore, from both qualitative and quantitative perspectives, analysis on far-field contributions from multipole moments along different directions combining with near-field distributions explains the evolution from BIC to quasi-BIC. The dominant contributor to the quasi-BIC is illustrated to be the electric quadrupole in the $x\ensuremath{-}y$ plane. Finally, the topological charge carried by the BIC is calculated to be $\ensuremath{-}1$, demonstrating the topological characteristics of our design. Such metasurfaces are robust in nanofabrication. Our results may provide a route for resonators with better performance applied in sensing, switching, nonlinear optics, and so on.