High-Q Polarization-Independent Dual-BIC Metasurface via Weak Perturbation

Quasi-bound state in the continuum (BIC) with ultra-high quality factor (Q) is of paramount significance for promoting the development of photonic devices. However, the design strategies of introducing small dimensional asymmetry or perturbation pose great challenge for its actual realization. Here, we propose a totally symmetric metasurface with relatively large perturbation parameters that supports two high-Q quasi-BICs induced by Brillouin zone folding (BZF), which can avoid tiny structural asymmetry or positional change. The BZF is triggered by periodically introducing circular nanorods (i.e., perturbations) into the weak eigenfield regions of the lattice, enabling two high-Q quasi-BICs at $\Gamma $ point even when the radius of the circular nanorods is relatively large. Moreover, the C4 symmetry of the unit cell remains undisturbed, making it insensitive to the incident polarization. The BICs also exhibit inconspicuous dependence on the shape of the lattice, further enhancing the manufacturing tolerances. Numerical simulations show that an ultra-high Q of up to 16,390 can be accessible even when the circular nanorods has a 60-nm radius. Such design will help facilitate the implementation of high-performance photonic devices in a variety of application fields such as optical sensing and nonlinearity.