Magnetically Decoupled Janus Bound States in the Continuum for Chirality–Topology Control

ABSTRACT Bound states in the continuum (BICs) with infinite Q factors and topological polarization charges have emerged as a pivotal platform in quantum information. The Janus BICs carrying asymmetric topological charges exhibit great potential for nonreciprocal photonics devices. Here, the topological properties of the Janus BICs are preserved while decoupled chirality is induced, which enhances the system's advantages by providing an extra degree of freedom. In contrast to previous Janus BICs, where chirality is coupled to topological charges, we achieve decoupled control through time‐reversal symmetry breaking. The theory of modified nonreciprocal temporal coupled mode theory (TCMT) with perturbation is developed to describe our system, which consists of the bi‐layer photonic crystals (PhCs) with symmetry and magneto‐optical (MO) symmetry slabs. When time‐reversal symmetry is broken, the degenerate BICs in the MO slab split into opposite chiral modes, whose chiralities are controllable through the direction and strength of the external magnetic field. By combining the simulation method, we observe the independent modulation of topological charges and chiralities in the polarization of the far field and evolution of C points with the change of separation. Our findings open pathways for applications in chirality‐dependent devices and asymmetric light–matter interaction.