Topological Transition for Bound States in the Continuum via Brillouin Zone Folding in Non‐Hermitian Metasurfaces

ABSTRACT Bound states in the continuum (BICs) support high quality ( Q ) resonances and nontrivial topological charges. However, achieving even higher and more robust ultrahigh‐ Q factors while flexibly controlling their topological properties across different energy bands remains a significant challenge. Here, we report a versatile approach for manipulating the topological charges of BICs in non‐Hermitian metasurfaces by combining Brillouin‐zone folding (BZF) physics. By tuning the resonator thickness, Dirac‐point (DP)‐mediated band inversions are induced, enabling the merging and topological charge flipping of BZF‐induced BICs (BZF‐BICs) and Friedrich–Wintgen BICs (FW‐BICs). Furthermore, by introducing a gap‐perturbation parameter, non‐Hermitian topological transitions via exceptional‐point (EP) Fermi arcs are realized, in which fractional topological charges mediate BZF‐BIC annihilation and re‐emergence across photonic bands. These two complementary mechanisms allow the topological charge to evolve from +1 to −1 and back to +1, achieving flexible control of BZF‐BIC topology across multiple bands while preserving global charge conservation. This work establishes a unified framework for topological charge control of ultrahigh‐ Q photonic states and offers new opportunities for non‐Hermitian topological photonics.