Symmetry-protected dual quasi-bound states in the continuum with high tunability in metasurface

We propose a split ring metasurface supporting simultaneously symmetry-protected dual bound states in the continuum (BIC) under normal excitation. Such two BICs can be classified as symmetry protected toroidal dipole (TD) BIC and magnetic dipole (MD) BIC, and the asymmetrically distributed holes can convert these two BICs into TD quasi-BIC and MD quasi-BIC with ultrahigh Q-factors, respectively. Two eigenmodes reveal the existence of symmetry protected BICs with an infinite lifetime and exhibit the intrinsic TD and MD characters by multipole analysis. In addition, it is shown that the two quasi-BICs can be switched on and off at orthogonal incident polarization orientations simultaneously. Further, such two quasi-BICs can be efficiently modulated by varying the Fermi energy of the graphene covered on the dielectric metasurface, indicating that the TD and MD quasi-BICs in the proposed metasurface have potential applications in designing novel quasi-BIC-based optical modulator and switching.