DOUBLY RESONANT 2ND-HARMONIC GENERATION OF 2.0 MU-M LIGHT IN COUPLED INGAAS ALAS QUANTUM-WELLS

We demonstrate intersubband absorption and second harmonic generation (SHG) in asymmetric coupled In0.6Ga0.4As/AlAs n-type quantum wells (QW s) grown on a GaAs substrate. Intersubband absorption at 4.1 and 2.1 m um wavelengths, corresponding to the 1 to 2 and 1 to 3 transitions, re spectively, are observed. SHG of 2.0 mum light is demonstrated in this doubly resonant QW. This is the shortest wavelength SHG to date in an y n-type QW system. The second order nonlinear susceptibility chi(2) i s measured using a free electron laser by interference of the second h armonic fields from the QW and substrate. At a pump wavelength of 4.0 mum, a large asymmetry in the SHG power with rotation angle of the sam ple arising from SHG from the QW is observed, and a chi(2) of magnitud e 20+/-8 nm/V, approximately 100 times that of bulk GaAs, and phase 63 -degrees+/-34-degrees relative to the GaAs substrate is measured. Comp arison of both the linear and nonlinear properties to a simple model i s discussed.