Skyrmion Quasi‐Bound States in the Continuum for 3D Light Trapping in Arbitrarily Large Volumes

Abstract Real‐world resonances face surface‐scaling radiation leakage, hindering light confinement in optically large systems. Conventional bound states in continuum (BICs) challenge this by enabling states with theoretically infinite lifetimes in 2D periodic structures. However, when fabricated, the truncation of these systems to finite sizes inevitably re‐introduces leakage, uncontrollably downgrading their quality factors. Here, a novel form of finite‐size 3D quasi‐BICs (QBICs) is demonstrated that leverage a skyrmion field topology of Bloch modes in 3D cubic photonic crystals. The associated finite systems exhibit highly suppressed radiation in all three spatial directions, with a remarkable exponential volume‐scaling of quality factors. With an unprecedentedly large 3D prototype of 24 cm³ and great scalability, the existence of skyrmion QBICs is experimentally proven through unique far‐field polarization wrapping and spectral singularity. These demonstrations enable to have omnidirectional topological radiation suppression, 3D vortex generation, and access to arbitrarily large volume and volumetric interfaces. 3D QBIC may empower extreme 3D light‐trapping, enhanced volumetric sensing, and topological skyrmion emitters.