Efficient generation of quantum light using bound states in the continuum in silicon-nanowire slow-light waveguides.

On-chip quantum light sources are a fundamental component of integrated photonic quantum information systems. However, their performance is fundamentally limited by weak nonlinear interactions and high propagation losses, which hinder efficient generation. Here, we propose a fishbone-modulated grating structure on a silicon-nanowire waveguide to significantly enhance spontaneous four-wave mixing. The proposed structure achieves a quantum-light generation efficiency two orders of magnitude higher than conventional silicon-nanowire waveguides. Specifically, we exploit a Su-Schrieffer-Heeger-like topological interface to slow light in the waveguide, thereby enhancing the nonlinear coefficient. Moreover, we leverage the principles of bound states in the continuum to optimize the modulated gratings on the waveguide, achieving ultra-low waveguide propagation loss. The simultaneous realization of the slow-wave effect and ultralow loss effectively enhances the spontaneous four-wave mixing process in silicon. Our approach opens new possibilities for designing enhanced nonlinear nanophotonic devices using various nonlinear optical materials.