Degenerate spontaneous parametric down-conversion in nonlinear metasurfaces

We propose a simple scheme of degenerate spontaneous parametric down -conversion (SPDC) in nonlinear metasurfaces or photonic crystal slabs with quasi -guided modes. It employs a band crossing between even- and odd -parity quasi -guided mode bands inside the light cone (above the light line) and a selection rule in the conversion efficiency of the SPDC. The efficiency can be evaluated fully classically via the inverse process of noncollinear second -harmonic generation (SHG). As a toy model, we study the SPDC and SHG in a monolayer of noncentrosymmetric spheres and confirm that the scenario works well to enhance the SPDC. (c) 2024 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement