Gp. Banfi et al., OPTICAL NONLINEARITY OF SEMICONDUCTOR-DOPED GLASSES AT FREQUENCIES BELOW THE BAND-GAP - THE ROLE OF FREE-CARRIERS, Journal of the Optical Society of America. B, Optical physics, 12(4), 1995, pp. 621-628
We have studied, through time-resolved degenerate-four-wave mixing, th
e 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 t
he intensity range 0.4-1.6 GW/cm2. We found that the third-order nonli
nearity 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 fr
ee carriers generated by two-photon absorption, does not saturate up t
o the highest intensity (corresponding to an electron-hole density in
the crystallites N almost-equal-to 10(19) cm-3) and decays on a nanose
cond time scale. We derive sigma, the ratio between the refractive-ind
ex change and N (sigma = 3-4 x 10(-21 cm3 for CdTe nanocrystals) and c
ompare it with the values for bulk semiconductors.