Plasmonic Mirrorless Optical Parametric Oscillator

We analyze a nondegenerate four-wave mixing process on a metal-dielectric interface in which the signal and idler waves are counter-propagating surface plasmons and the process is pumped by a free wave propagating in air. We show that, similarly to mirrorless parametric oscillations, above a certain threshold intensity of the pump, the system exhibits instability, which results in the spontaneous generation of plasmons propagating on the metal surface. Interestingly, we find that, in such a plasmonic nonlinear interaction, phase-matching is naturally fulfilled by the geometry of the process, allowing a broadband tunability of the output frequencies by varying the angle of incidence of the pump.