Control and enhancement of optical nonlinearities in plasmonic semiconductor nanostructures.

The efficiency of nanoscale nonlinear elements in photonic integrated circuits is hindered by the physical limits to the nonlinear optical response of dielectrics, which cannot be engineered as it is a fundamental material property. Here, we experimentally demonstrate that ultrafast optical nonlinearities in doped semiconductors can be engineered and can easily exceed those of conventional undoped dielectrics. The electron response of heavily doped semiconductors acquires in fact a hydrodynamic character that introduces nonlocal effects as well as additional nonlinear sources. Our experimental findings are supported by a comprehensive computational analysis based on the hydrodynamic model. In particular, by studying third-harmonic generation from plasmonic nanoantenna arrays made out of heavily n-doped InGaAs with increasing levels of free-carrier density, we discriminate between hydrodynamic and dielectric nonlinearities. Most importantly, we demonstrate that the maximum nonlinear efficiency as well as its spectral location can be engineered by tuning the doping level. Crucially, the maximum efficiency can be increased by almost two orders of magnitude with respect to the classical dielectric nonlinearity. Having employed the common material platform InGaAs/InP that supports integrated waveguides, our findings pave the way for future exploitation of plasmonic nonlinearities in all-semiconductor photonic integrated circuits.