Stimulated Emission Tomography of Spontaneous Four-Wave Mixing in Plasmonic Nanoantennas.

We used stimulated emission tomography (SET) to assess the efficiency of spontaneous four-wave mixing (SFWM) from a plasmonic nanoantenna under pulsed excitation. We characterize the SFWM photon generation rate by measuring stimulated degenerate four-wave mixing. We produce a map of the SFWM joint spectral density that characterizes the biphoton state, which we find has a broad bandwidth due to the absence of phase matching. The joint spectral density retrieval via SET is fast and straightforward compared to traditional coincidence measurements. By calculating the number of stimulating and generated photons along with the frequency mixing efficiency, we have determined the power-independent intrinsic SFWM generation rate to be on the order of 10 photon pairs per second per mW squared per particle, while the power-independent extrinsic generation rate is approximately 1 photon pair per second per mW squared per particle. There is scope to increase the nonlinear response by scaling up to large area metasurfaces using low-loss dielectric materials that would allow the produced photon pairs to exceed background fluorescence. Such an SFWM metasurface could be a potential alternative to parametric down conversion, which requires rarer second-order nonlinear materials that are also challenging to integrate with photonic structures.