Universal Polarization‐ and Angle‐Independent Quasi‐Bound States in the Continuum via Degeneracy Protection in Metasurfaces

Bound states in the continuum (BICs) feature high quality ( Q ) factors and strong electromagnetic field confinement. These characteristics make them highly attractive for applications in light–matter interactions, nonlinear optics, and optical sensing. However, most quasi‐BICs rely on structural symmetry breaking, which inevitably introduces polarization sensitivity and angle dependence, limiting their practical use. Here, a design strategy is demonstrated to achieve polarization‐independent and angle‐independent quasi‐BICs in all‐dielectric metasurfaces, enabled by degeneracy protection. By precisely tuning nanopore diameters, secondary folding of the Brillouin zone is induced, generating mode‐degenerate quasi‐BICs that naturally eliminate polarization dependence in far‐field radiation. Furthermore, the band folding leads to flat photonic bands that strongly suppress angular dispersion, resulting in resonances that are highly stable across a wide range of incidence angles. This degeneracy‐enabled mechanism allows for the construction of quasi‐BICs that are intrinsically robust against variations in polarization and angle. This work establishes a generalizable platform for realizing symmetry‐preserving quasi‐BICs, offering new opportunities for high‐resolution spectral imaging, low‐loss filtering, and high‐sensitivity optical sensing.