Resonant generation of difference harmonics in heterostructures with closely space levels

The second order susceptibility for difference-frequency mixing in a heterostructures with closely spaced levels are considered. We have described both the excitations of electrons from the couple of levels to the single level and the transition from the ground level to the couple of closely spaced levels. Parameters of the structures under consideration correspond to excitation by CO2 laser and the photon energy for difference harmonic is around 10-20 meV.