Far-infrared nonlinear optics. II. χ ( 3 ) contributions from the dynamics of free carriers in semiconductors

TLDR: The first bulk frequency tripling in the far-infrared was reported in this paper, where the power conversion efficiency of Ge, Si, and GaAs was investigated at 20 GHz.

Abstract: We report the first bulk frequency tripling in the far-infrared. The experiments are carried out at 20 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ in the doped semiconductors Ge, Si, and GaAs. Good power conversion efficiency is obtained (${10}^{\mathrm{\ensuremath{-}}3}$). Our arrangement allows an absolute determination of the nonlinear susceptibility ${\ensuremath{\chi}}^{(3)}$(3\ensuremath{\omega},\ensuremath{\omega},\ensuremath{\omega},\ensuremath{\omega}). We show that in the far-infrared, ${\ensuremath{\chi}}^{(3)}$ is dominated by free-carrier contributions. Hence, our measurement gives new insight into nonlinear transport properties such as the momentum dependence of the effective mass and of the relaxation time. Furthermore strong changes of ${\ensuremath{\chi}}^{(3)}$ (as well as of the absorption) are found to occur at high laser intensity above 100 kW/${\mathrm{cm}}^{2}$, which we attribute to carrier heating and hot carrier transfer into higher energy-band minima.