We report the first observations of large third order optical nonlinearities in AlGaAs/GaAs superlattices due to intersubband transitions. The process is triply resonant when the first intersubband transition matches the radiation from CO2 lasers. The intersubband relaxation time has also been estimated from the dispersion of χ(3) with the laser difference frequency Δω and found to be in good agreement with other published values. The magnitude and frequency dependence of the intersubband mechanism have been calculated using the diagrammatic technique.

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The theory of free-carrier-induced optical non-linearity is reviewed. Examples of such mechanisms in narrow gap semiconductors are discussed. HgTe, HgMnTe and zero-gap HgCdTe are shown to have record, picosecond speed, optical non-linearities at 10.6 pm. The largest occurs in Hg, , , Cd, , , Te, whose x(3) = 2 x lop3 ESU at 80 K. The theoretical model suggests that these non-linearities are caused by laser-induced carrier temperature modulation, which produces large carrier density variations in zero-gap materials. The thermal processes have saturation power densities in the 100 kW cm"-l MW cm" range. At such intensities, the dielectric constant of HgTe is modulated by about 10 YO.

Free-carrier induced third-order optical nonlinearities can be both large and fast in narrow-gap semiconductors. We have studied a variety of mechanisms in bulk semiconductors and heterostructures using CO2 lasers and found third-order susceptibilities as large as 2×10−3 esu with picosecond relaxation times. These mechanisms saturate at much higher intensities than do slower mechanisms and hence induce huge modulations of the dielectric function. In addition, most of these processes are nonresonant so they do not require the exact matching of material parameters and are relatively insensitive to temperature.

We report the first observation of optical nonlinearities due to subband structures in the growth direction of an n-type superlattice by four-wave mixing in an end-firing configuration. We observe a nearly eightfold increase in the third-order optical susceptibility when the electric fields of the CO2 laser are changed from a parallel to a perpendicular orientation to the plane of an Al0.08In0.92Sb/InSb superlattice. No change is observed in a control InSb epilayer under similar conditions.