Observation of robust intrinsic C points in self-biased gyromagnetic photonic crystals

Abstract C point, singularities of circularly polarized states in momentum space, has recently sparked significant interest due to its relevance in fundamental physics and wide applications in chiral optics. However, conventional approaches for realizing intrinsic C points (chiral bound states in the continuum) through structural symmetry breaking suffer from high sensitivity to geometrical parameters and narrow incident angles, making them vulnerable to external perturbations and structural imperfections. Here, we report the first experimental observation of robust intrinsic C points in a self-biased gyromagnetic photonic crystal with time-reversal symmetry breaking while preserving structural symmetry. We show that the magneto-optical coupling between two initially degenerate dipole modes induces Zeeman splitting and generates a pair of chiral eigenstates with opposing handedness. We experimentally validate the superior robustness of these intrinsic C points against structural variations and demonstrate their switchable handedness and circular dichroism (CD) by reversing the magnetization direction. More intriguingly, we experimentally observe robust and switchable photonic chiral flat surfaces with high CDs over an unprecedentedly wide range of incident angles (±20°) along arbitrary directions. Our findings push the research frontier of C points and may have potential applications in chiral optics.