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

Abstract

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 (n2) were measured at 1.064 μm. Different n2 values were obtained for nanosecond excitation (n2=10−7–10−8 cm2/kW) and for picosecond excitation (n2=10−9–10−10 cm2/kW). The differences can be explained by saturation effects. Measurements at 532 nm showed n2 values ten times higher than at 1.064 μm. The response time of the nonlinearity for both wavelengths was below 35 ps.