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

We report measurements of degenerate four-wave-mixing reflectivity at a frequency below the band gap of semiconductor-doped glasses in the intensity range 0.5-10 GW/cm(2) Up to intensities similar to 2.5 GW/cm(2), the conjugate reflectivity varies like the fourth power of intensity signifying a fifth-order nonlinearity due to band filling by two-photon absorption. Surprisingly, at a higher intensity range the conjugate signal showed a cubic dependence on the pump intensity, which is typical of the chi((3)) process. We show that: this cubic dependence does not necessarily indicate a third-order process as usually assumed. Instead, it is shown to arise due to a reduction of the effective intensity by nonlinear absorption of the interacting beams. [S0163-1829(99)01902-5].