FAR-INFRARED 2ND-HARMONIC GENERATION IN GAAS/ALXGA1-XAS HETEROSTRUCTURES - PERTURBATIVE AND NONPERTURBATIVE RESPONSE

We report measurements of far-infrared (FIR) harmonic generation from GaAs/AlxGa1-xAs heterostructures. The samples studied were a modulation-doped Al0.3Ga0.7As/GaAs heterojunction and a sample with ten modulation-doped half-parabolic quantum wells. The samples were driven with intense far-infrared radiation from a molecular gas laser at 29.5 cm-1 and the University of California-Santa Barbara free-electron laser at 51.3 cm-1. The FIR radiation was polarized parallel to the growth direction. Second harmonics of the FIR were detected from both the semi-insulating GaAs substrate and from the confined electrons. For the heterojunction sample, the second-harmonic power generated by the electrons depended quadratically on fundamental power at low power, as expected from time-dependent perturbation theory. However, this dependence became subquadratic at higher powers, indicating a nonperturbative response. At high FIR powers, electrons were also ionized from the heterojunction and half-parabolic wells. For the heterojunction at f = 29.5 cm-1 in the perturbative regime, the surface second-order susceptibility was computed to be CHI(S)(2) = 1.0+/-0.75 X 10(-8) esu-1 cm3. This value agrees, within experimental error, with a simple model of the heterojunction as a triangular quantum well. The second-order polarizability of a conduction electron in the heterojunction is nine orders of magnitude larger than that of a valence electron in pure GaAs.