Mixing of Surface and Bulk Optical Nonlinearities via Surface Plasmon Polaritons

We present controlled interplay between the surface second-order and bulk third-order nonlinearities of gold, utilizing wave mixing between surface plasmons on the metal-air interface and free-space photons. By tuning the nonlinear interactions, we demonstrate experimentally that photons emerging from these inherently different mechanisms can interfere, while both their relative amplitude and phase are controlled solely by the pump beam. These results provide a foundation for engineering plasmonic nonlinearities and open new possibilities for quantum photonic applications.