Abstract
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 ≈ 1019 cm−3) and decays on a nanosecond time scale. We derive σ, the ratio between the refractive-index change and N (σ = 3 − 4 × 10−21 cm3 for CdTe nanocrystals) and compare it with the values for bulk semiconductors.
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