Unified Topological Dynamics of Merging Bound States in the Continuum for High‐Order Topological Charges

Abstract

Bound states in the continuum (BICs) are polarization singularities in momentum space whose topological charges (TCs) govern advanced light‐matter interactions. While lattice symmetry protects the existence of robust BICs at the Γ‐point (SP‐BICs), it also restricts their TCs to low‐order values. Achieving high‐order TCs in common crystal lattices, such as ‐symmetric systems, has therefore remained an open question. Here, we systematically demonstrate that high‐order TCs that surpass fundamental symmetry bounds can be created through the rich dynamics of a parameter‐driven merging process of off‐Γ BICs. We introduce a unified geometric framework based on the interplay between Fabry–Pérot interference and guided resonances, which uncovers different types of merging BICs dynamics, including near‐isotropic, anisotropic, and cross‐merging. Leveraging this mechanism, we realize unconventional TCs of up to at either a symmetry‐protected BIC or a degeneracy point in a simple ‐symmetric photonic crystal slab. We further show that this high‐order topology enables the generation of high‐quality Bessel OAM beams, providing a physically transparent route toward engineering high‐order topological photonics.