Chiral topological bound states in the continuum within photonic crystal cavities

We propose a magneto-optically tunable platform for chiral topological bound states in the continuum (BICs) within photonic crystal cavities. By integrating magneto-optical (MO) materials into topological bulk-state cavities, we demonstrate dynamic control over circular polarization states through external magnetic fields, a capability unattainable with conventional symmetry-breaking approaches. Initially, doubly degenerate topological quadrupole BICs are formed in a hexagonal lattice photonic crystal. Applying a vertical magnetic field breaks the time-reversal symmetry, lifting the degeneracy and generating paired chiral BICs with opposite pseudo-spin and orbital angular momentum. These chiral BICs manifest as circularly polarized beams in the far-field, with handedness directly controlled by the magnetic field direction. The synergy of topological BICs and MO materials achieves a high quality factor (<i>Q</i>∼10⁹) and tunable polarization states, enabling applications such as reconfigurable lasers and polarization-sensitive sensors. This work bridges topological photonics and magneto-optics, offering a paradigm for dynamically controlled photonic devices in quantum optics and telecommunications.