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Nonlinear wave-mixing in twin-waveguide GaAs/AlGaAs quantum cascade lasers

In this paper we present two-color GaAs/AlGaAs quantum cascade lasers with separate optical confinement of the two laser stacks. The laser stacks, each comprising cladding layers and an active region, are grown on top of each other by molecular beam epitaxy on GaAs substrates. The lasers operate at 10.4 µm and 9.1 µm wavelength and have built-in second-order intersubband nonlinearities. Due to coupling between the optical fields of the respective waveguides, not only the second-harmonic of each wavelength, but also the sum-frequency of the two lasers is measured. This concept can be extended by independently biasing the two laser stacks, whilst maintaining overlap between the two optical modes.