Microstructural and nonlinear optical properties of silica-titania sol-gel film doped with PbS quantum dots

Thin silica-titania films doped with different concentration of PbS quantum dots (PbS/oxide molar ratios ranging from 5% to 25%) were fabricated via a sol-gel route. The structural properties were studied by x-ray diffraction, high-resolution transmission electron microscopy, and Rutherford backscattering spectrometry. The PbS crystals were found to have a mean diameter in the range 2.3-3.5 nm with narrow size distribution. The resonant nonlinear optical properties were studied by the nonlinear m-line technique and degenerate four-wave mixing. High negative nonlinear refractive indices (n(2)) were measured at 1.064 mu m. Different n(2) values were obtained for nanosecond excitation (n(2)=10(-7)-10(-8) cm(2)/kW) and for picosecond excitation (n(2)=10(-9)-10(-10) cm(2)/kW). The differences can be explained by saturation effects. Measurements at 532 nm showed n(2) values ten times higher than at 1.064 mu m. The response time of the nonlinearity for both wavelengths was below 35 ps. (C) 1999 American Institute of Physics. [S0021-8979(99)03213-2].