Degenerate four-wave mixing in semiconductor-doped glasses below the absorption edge

We report measurements of degenerate four-wave-mixing reflectivity at a fre quency below the band gap of semiconductor-doped glasses in the intensity r ange 0.5-10 GW/cm(2) Up to intensities similar to 2.5 GW/cm(2), the conjuga te reflectivity varies like the fourth power of intensity signifying a fift h-order nonlinearity due to band filling by two-photon absorption. Surprisi ngly, at a higher intensity range the conjugate signal showed a cubic depen dence on the pump intensity, which is typical of the chi((3)) process. We s how that: this cubic dependence does not necessarily indicate a third-order process as usually assumed. Instead, it is shown to arise due to a reducti on of the effective intensity by nonlinear absorption of the interacting be ams. [S0163-1829(99)01902-5].