Strong asymmetry of non-resonant optical four-wave mixing in PbSe and PbSnSe

Non-resonant four-wave mixing experiments were performed for a determination of the free carrier intraband energy relaxation time in semiconductors. The signal intensity was measured as a function of the pump beam detuning delta omega. For PbSe and PbSnSe this line shape showed an unexpected strong asymmetry; that is, the Stokes signal dpendence on delta omega was different from that of the anti-Stokes signal. We explain this asymmetry by an interference with resonant scattering due to a shallow impurity with binding energy of the order of a few wavenumbers