Observation of Degenerate Merging Bound States in the Continuum in All-Dielectric Metasurfaces

Merging bound states in the continuum (BICs) provide a pathway to ultrahigh-Q resonances in free-space, yet their realizations have so far been restricted to single-mode scenarios, leaving the creation of degenerate merging BICs elusive due to stringent symmetry-imposed topological constraints. Here, we overcome this barrier by designing a C _4v -symmetric all-dielectric metasurface that supports two degenerate guided resonances, each hosting multiple accidental BICs. We show that these BICs can be topologically steered to coalesce at the Γ-point with identical eigenfrequencies, thereby forming robust degenerate merging states beyond the reach of conventional metasurface designs. Furthermore, by introducing controlled symmetry breaking, the degeneracy is deterministically lifted, giving rise to non-degenerate merging BICs with tunable eigenfrequencies while retaining ultrahigh-Q scaling. We experimentally verify both regimes on a silicon-on-insulator platform, achieving Q-factors consistently above 3×10 ⁴ and a record value of 3.3×10 ⁵ in substrate-supported nanostructures, exceeding all previously reported BIC-based devices. Our results establish the first versatile route to controllable degenerate and non-degenerate merging BICs, bridging symmetry, topology, and high-Q nanophotonics, and enabling new opportunities for nonlinear optics, quantum light sources, and ultrasensitive photonic devices.