OPTICAL NONLINEARITY OF SEMICONDUCTOR-DOPED GLASSES AT FREQUENCIES BELOW THE BAND-GAP - THE ROLE OF FREE-CARRIERS

We have studied, through time-resolved degenerate-four-wave mixing, the nonlinear response below the band gap of glasses doped with CdTe and CdS1-xSex nanocrystals by using 30-ps pulses from a Nd:YAG laser in the intensity range 0.4-1.6 GW/cm2. We found that the third-order nonlinearity is overshadowed by a fifth-order one when the band gap of the semiconductor crystallites became smaller than twice the photon energy. The fifth-order response, which is due to the refraction from the free carriers generated by two-photon absorption, does not saturate up to the highest intensity (corresponding to an electron-hole density in the crystallites N almost-equal-to 10(19) cm-3) and decays on a nanosecond time scale. We derive sigma, the ratio between the refractive-index change and N (sigma = 3-4 x 10(-21 cm3 for CdTe nanocrystals) and compare it with the values for bulk semiconductors.