  The development of nanoscale nonlinear elements in photonic integrated
circuits is hindered by the physical limits to the nonlinear optical response
of dielectrics, which requires that the interacting waves propagate in
transparent volumes for distances much longer than their wavelength. Here we
present experimental evidence that optical nonlinearities in doped
semiconductors are due to free-electron and their efficiency could exceed by
several orders of magnitude that of conventional dielectric nonlinearities. Our
experimental findings are supported by comprehensive computational results
based on the hydrodynamic modeling, which naturally includes nonlocal effects,
of the free-electron dynamics in heavily doped semiconductors. 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. As a result, the value of
maximum nonlinear efficiency as well as its spectral location can now be
controlled by tuning the doping level. 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.
