Short wavelength (5.36–1.85 μm) nonlinear spectroscopy of coupled InGaAs/AlAs intersubband quantum wells

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Abstract
We report short wavelength second‐harmonic generation (SHG) spectroscopy of asymmetric coupled In0.6Ga0.4As/AlAs quantum wells (QWs). The QW is designed to show maximum second‐order nonlinear susceptibility χ(2) for SHG of 4 and 2 μm wavelengths by single and double resonance effects, respectively. SHG spectroscopy across the midinfrared is measured using both a CO2 and a free electron laser as pumps. The χ(2) of the QW is extracted from interference of the second‐harmonic fields from the QW and GaAs substrate, determined by the azimuthal dependence of the SHG power. We measure χ(2) of the QW for harmonic wavelengths between 5.36 and 1.85 μm. This is the shortest wavelength SHG to date by any QW intersubband interaction. Good agreement of experiment with theory for the dispersion of χ(2) for both singly and doubly resonant conversion is observed throughout the midinfrared.