Topological Expansion of the High‐ Q Region in Momentum Space via Dirac‐Point‐Induced BICs

ABSTRACT Bound states in the continuum (BICs) are an intriguing phenomenon as they are accompanied by theoretically infinite quality factors ( Q factors), but restricted by the moderate wavevector range, thus impeding the physical realization and utilization. Although merging BICs provides an effective strategy to broaden the high‐ Q region in momentum space, it requires fine tuning of two optical singularities to approach each other closely, and the achievable high‐ Q bandwidth remains limited due to momentum mismatch. To overcome this contradiction, we demonstrate a kind of general BICs off the high‐symmetry lines in honeycomb‐lattice photonic crystal slabs, which are induced by the Dirac points regulated by topological charge conservation. These general BICs ensure high‐ Q resonances over an expanded momentum‐space range, effectively mitigating fabrication imperfections. Furthermore, distinct from the interactions between circular points (C points) of opposite chirality with identical topological charges or of the same chirality with opposite charges, we uncover another case involving C points with opposite chirality and topological charge by breaking the C 6 z rotational symmetry. These findings reveal the rich interplay among symmetry, topology, and degeneracy in photonic crystals, offering deeper insights into chiral optics and facilitating high‐ Q photonic devices.