Control of the nonlinear response of bulk GaAs induced by long-wavelength infrared pulses.

The nonlinear optical response of GaAs is studied using extremely nonresonant 10 mu m laser pulses with peak intensities greater than 2 GW/cm(2). We observe over an order of magnitude enhancement in the four-wave mixing efficiency by decreasing the CO2 laser beat-wave frequency. This enhancement is attributed to currents of photoexcited unbound carriers modulated at the beat frequency, confirmed by measurements of nonlinear absorption at this long wavelength as well as a fully microscopic analysis of the excitation dynamics. Modeling of such nonperturbative semiconductor-laser interactions predicts that further decreasing the beat frequency can increase the nonlinear response and allow for its control over two orders of magnitude. (C) 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement.