Band-folding quasi-bound states in the continuum supported by dimer nanobars with symmetry broken along z-direction

Symmetry-protected bound states in the continuum (BIC) can be transformed into quasi-BIC (QBIC) with an ultra-high quality factor so that the external excitation can couple with them. Typically, notches are commonly induced by introducing in-plane broken symmetry. We introduce a metasurface composed of dimer nanobars with out-of-plane broken symmetry. It is demonstrated that band-folding QBICs can be realized by introducing notches along z-direction. The effect of polarization on coupling between the QBIC mode and incident light is also studied. The polarization response of QBIC indicates that the linewidth of the resonance peak rather than the resonance intensity is regulated by the azimuth angle and shows an inverse square relationship, which is not mentioned in previous studies. Analysis of far-field contributions from multipole decomposition and electromagnetic field distribution reveal that the dominant contributors to the QBIC are the magnetic dipole moment and electric quadrupole. Our results may provide a route for enhancing light-matter interaction.