Generation of photon pairs through spontaneous four-wave mixing in subwavelength nonlinear films.

Pairs of entangled photons are crucial for photonic quantum technologies. The demand for integrability and multi-functionality suggests "flat" platforms-ultrathin layers and metasurfaces-as sources of photon pairs. Despite the success in the demonstration of spontaneous parametric downconversion (SPDC) from such sources, there are almost no works on spontaneous four-wave mixing (SFWM)-an alternative process to generate photon pairs. Meanwhile, SFWM can be implemented in any nanostructures, including ones made of isotropic materials, which are easier to fabricate than crystalline SPDC sources. Here, we investigate photon pair generation through SFWM in subwavelength films of amorphous silicon nitride (SiN) with varying nitrogen content. For all samples, we demonstrate two-photon quantum correlations, indicated by the normalized second-order correlation function g⁽²⁾(0): it exceeds 2 and decays as the pump power increases. By observing two-photon interference between SFWM from the SiN films and the fused silica (FS) substrate, we find the third-order susceptibilities of films with different nitrogen content.