High-order photonic degeneracy enables topological reconfiguration in bound states in the continuum

Abstract

Abstract Extreme suppression of free-space radiation in open planar photonic systems remains a long-standing goal in nanophotonics. Conventional bound states in the continuum (BICs), constrained by symmetry-protected or accidental singularities, restrict experimental Q - k scaling exponents to n  = 2-8 because ordinary bands lack sufficient degrees of freedom to host, redistribute or merge additional radiation-suppressing singularities. Here we demonstrate a degeneracy-driven topological-reconfiguration mechanism: an E 2 -type high-order degeneracy at Γ acts as a multi-charge reservoir, enabling controlled release, inversion and merging of topological charges under symmetry breaking. This unlocks a Q ∝ k −10 radiation-suppression regime in a single-layer all-dielectric metasurface. Experimentally, we realize ultrahigh free-space Q-factors with 3% geometric robustness and a maximum Q of 1.1 × 10 6 . A 100-pixel array exploiting these resonances demonstrates picometre-scale spectral discrimination by resolving ultranarrow H 13 C 14 N absorption lines. These results establish high-order-degeneracy-enabled BIC physics for robust ultrahigh-Q photonics and precision free-space spectroscopy.