We demonstrate efficient intracavity nonlinear interaction of laser modes in quantum cascade (QC) lasers with resonant nonlinear optical intersubband transitions. Sum-frequency and second-harmonic generation at 4.1 and 3.6 and 4.7 µm wavelength has been demonstrated for a two-wavelength QC laser emitting at the fundamental wavelengths 7.1 and 9.5 µm. In a structure optimized for second harmonic generation, we measure 1.5 µW of nonlinear power at 3.7 µm wavelength together with 930 mW of fundamental power at 7.5 µm wavelength. The experimental results are in very good agreement with the calculations.
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