Resonant harmonic generation and dynamic screening in a double quantum well

Second- and third-harmonic generation are observed in a semiconductor heterostructure which approximates a two-state system with an 11-meV level spacing. A resonance in the second- (third-) harmonic generation is found when the depolarization-shifted infrared absorption peak is Stark tuned through 2 (3) times the pump frequency. These resonances are thus associated with the depolarization-shifted, and not the bare, intersubband energy. Data are analyzed with a theory of second-harmonic generation including dynamic screening. Saturation is observed at pump intensities >10 kW/${\mathrm{cm}}^2$.