Anomalous Spontaneous Emission Enhancement by Bound States in the Continuum in Nonlocal Metasurfaces

Conventional Purcell theory emphasizes high quality factors ( Q ) for spontaneous emission (SE) enhancement in cavities but overlooks collective Bloch mode effects in nonlocal periodic nanostructures like photonic crystal slabs. We introduce a unified temporal coupled-mode framework to evaluate Purcell and photoluminescence factors through momentum-space integration, revealing anomalous SE enhancement mediated by bound states in the continuum (BICs) in non-Hermitian systems. In nonlocal dielectric metasurfaces with comparable effective mode volumes, this yields substantial SE enhancement in low- Q regimes, defying the traditional high- Q paradigm and is inversely correlated with system Q, while emission rates are stably twice the photoluminescence, eliminating critical coupling requirements. Unique spectral profiles, contradicting Lorentzian/Fano-like assumptions, arise from collective mode interactions. Full-wave simulations confirm these challenges to conventional wisdom, with BICs outperforming high- Q designs across broad numerical apertures. This establishes a novel paradigm leveraging non-Hermiticity and topological protection for robust, bright emitters, redefining nanophotonic applications in lasers and light-emitting diodes.