Doubly resonant second harmonic generation of 2.0 μm 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 (QWs) grown on a GaAs substrate. Intersubband absorption at 4.1 and 2.1 μm wavelengths, corresponding to the 1 to 2 and 1 to 3 transitions, respectively, are observed. SHG of 2.0 μm light is demonstrated in this doubly resonant QW. This is the shortest wavelength SHG to date in any n-type QW system. The second order nonlinear susceptibility χ(2) is measured using a free electron laser by interference of the second harmonic fields from the QW and substrate. At a pump wavelength of 4.0 μm, a large asymmetry in the SHG power with rotation angle of the sample arising from SHG from the QW is observed, and a χ(2) of magnitude 20±8 nm/V, approximately 100 times that of bulk GaAs, and phase 63°±34° relative to the GaAs substrate is measured. Comparison of both the linear and nonlinear properties to a simple model is discussed.