SUBCUBIC POWER DEPENDENCE OF 3RD-HARMONIC GENERATION FOR INPLANE, FAR-INFRARED EXCITATION OF INAS QUANTUM-WELLS

Large third-order, free-carrier nonlinear susceptibilities, chi(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 a nd 23 cm-1 for InAs/AlSb quantum wells with electron sheet densities b etween 2.5 x 10(12) cm-2 and 8 X 10(12) cm-2. We find that the transmi ssion of the fundamental, and the samples' DC conductivity, decrease w ith increasing incident intensity, indicating a large rise in the scat tering rate. Using the intensity-dependent transmission to account for absorption in the sample is not sufficient to recover a cubic power l aw for the third-harmonic intensity. In addition, given the increased scattering rate indicated by the conductivity data, the bulk free-carr ier chi(3) due to non-parabolicity should decrease dramatically with i ncreasing fundamental intensity, contrary to our results. Thus, non-pa rabolicity alone cannot account for the observed third-harmonic respon se.