Robust and high Q-factor quasi-BIC metasurface with polarization independence.

Conventional high-Q quasi-bound states in the continuum (qBICs) rely on precise symmetry breaking yet suffer polarization dependence and extreme structural susceptibility. Here, we propose a robust polarization-insensitive qBIC metasurface enabled by a Brillouin zone folding-unfolding strategy. This approach realizes a typical bound state-qBIC-bound state transition when the rotational angle of the nanopillars varies, enabling a high and stable Q factor of the metasurface with large fabrication tolerance. Additionally, polarization insensitivity is achieved via preserving the fourfold rotational symmetry of the unit cell since the rotational angle is introduced as the perturbation parameter. Both numerical simulations and measurements of fabricated samples verify the outstanding performance of our design with high Q (above 1700). This scheme greatly relaxes fabrication limitations and facilitates the application of high-Q qBIC metasurfaces in biosensing and miniature spectrometers.