Quasi-symmetry-protected BICs in a double-notched silicon nanodisk metasurface

Bound states in the continuum (BICs) hold great promise in enhancing light-matter interaction as they have an infinite Q-factor. To date, the symmetry-protected BIC (SP-BIC) is one of the most intensively studied BICs because it is easily found in a dielectric metasurface satisfying certain group symmetry. To convert SP-BICs into quasi-BICs (QBICs), structural symmetry shall be broken so that external excitation can access them. Usually, the unit cell's asymmetry is created by removing or adding parts of dielectric nanostructures. The QBICs are usually excited only by s-polarized or p-polarized light because of the symmetry-breaking of the structure. In this work, we investigate the excited QBIC properties by introducing double notches on the edges of highly symmetrical silicon nanodisks. The QBIC shares the same optical response under the s-polarized and p-polarized light. The effect of polarization on coupling efficiency between the QBIC mode and incident light is studied, and the highest coupling efficiency occurs at a polarization angle of 135 ∘, which corresponds to the radiative channel. Moreover, the near-field distribution and multipole decomposition confirm that the QBIC is dominated by the magnetic dipole along the z direction. It is noted that the QBIC covers a wide spectrum region. Finally, we present an experimental confirmation; the measured spectrum shows a sharp Fano resonance with a Q-factor of 260. Our results suggest promising applications in enhancing light-matter interaction, such as lasing, sensing, and nonlinear harmonic generation.