Bound states in the continuum (BICs) provide a powerful route to optical resonances with vanishing radiation loss and enhanced light-matter interactions. Of particular interest are chiral BICs, whose resonant states exhibit intrinsic circular polarization and spin-selective radiation. Magneto-optical photonic crystals were recently predicted to host such states through time-reversal-symmetry breaking. Here, we experimentally demonstrate chiral BICs induced by time-reversal-symmetry breaking in a magneto-optical photonic crystal, where an out-of-plane magnetic field lifts a degenerate BIC pair into two nondegenerate resonances with opposite circular polarizations. Numerical simulations reveal the associated chiral phase vortices and strong spin-selective radiation, while microwave measurements confirm the predicted behavior through angle-resolved transmission spectra and pronounced broadband circular dichroism. Our results establish an experimental platform for exploring chiral BIC physics in magneto-optical photonic structures, opening opportunities for spin-selective photonics.

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• Metasurfaces
• Photonic crystals
• Topological metamaterials