Subcubic power dependence of third-harmonic generation for in-plane, far-infrared excitation of InAs quantum wells

Large third-order, free-carrier nonlinear susceptibilities, X⁽³⁾ (to approximately 0.2 esu), and subcubic dependence of the third-harmonic power on the incident intensity, have been observed between 19 cm^-1 and 23 cm^-1 for InAs/AlSb quantum wells with electron sheet densities between 2.5*10¹² cm^-2. and 8*10¹² cm^-2. We find that the transmission of the fundamental, and the samples' DC conductivity, decrease with increasing incident intensity, indicating a large rise in the scattering rate. Using the intensity-dependent transmission to account for absorption in the sample is not sufficient to recover a cubic power law for the third-harmonic intensity. In addition, given the increased scattering rate indicated by the conductivity data, the bulk free-carrier X⁽³⁾ due to non-parabolicity should decrease dramatically with increasing fundamental intensity, contrary to our results. Thus, non-parabolicity alone cannot account for the observed third-harmonic response.