We have observed an extremely large second-order susceptibility for second-harmonic generation of 9.6–10.8 μm radiation due to intersubband transitions in electric-field-biased GaAs quantum wells. For 92-Å GaAs wells with 309-Å ${\mathrm{Al}}_{0.48}$${\mathrm{Ga}}_{0.52}$As barriers under a bias of 36 kV/cm, the peak value of the susceptibility was 28 nm/V, 73 times larger than bulk GaAs. The magnitude and sign of the susceptibility depend on the bias field, and are in accord with theoretical predictions.

• Received 28 December 1988

DOI:https://doi.org/10.1103/PhysRevLett.62.1041