Chiral Janus bound states in the continuum in photonic crystal slab with in-plane symmetry

Janus bound states in the continuum (Janus BICs) are a class of photonic states characterized by asymmetric topological charges in the upward and downward radiation channels, offering opposite circular polarizations and direction-dependent radiation properties under a single resonance. However, existing realizations of Janus BICs exhibit a fundamental limitation: although the BIC itself typically resides at the Γ point, the pronounced chiral response predominantly arises from off-Γ circularly polarized (C) points in momentum space. Consequently, the Γ-point mode remains intrinsically non-chiral, preventing the realization of a truly chiral Janus BIC. By introducing a controlled vertical dielectric asymmetry in a bilayer photonic crystal slab, we realize a Γ-point chiral Janus BIC that simultaneously exhibits asymmetric radiation channels and robust central chirality while preserving full in-plane symmetry. Our approach overcomes the absence of central chirality in existing Janus BICs, enabling a tunable and fabrication-tolerant platform for high-Q chiral photonics, polarization-controlled emission, and highly asymmetric light–matter interactions.