Quasi-bound states in the continuum induced by C4v structure

We propose a method to achieve symmetry-protected bound states in the continuum (BICs) while maintaining the C4v structural symmetry, utilizing a periodically arranged array of topological Dirac semimetals (TDS) squares. By numerically controlling the metamaterial using four identical square films of TDS, we can manipulate both the resonance position and number. This phenomenon arises due to the inherent permittivity asymmetry of the metamaterial, causing the creation of quasi-BICs. When all four squares have the same Fermi energy, a BIC is formed. Different Fermi energies are assigned to the four squares, resulting in quasi-BICs with narrow resonances. The electric analysis in the x-y plane identified the dominant contributors as the electric quadrupole and octupole. Our findings provide a direct route to achieve high-quality (high-Q) quasi-BICs without any introduction of symmetry-breaking elements, such as transverse, in-plane, and out-of-plane components in the structure, or the use of oblique incidences within a non-breaking system.