Femtosecond Z-scan investigation of nonlinear refraction in surface modified PbS nanoparticles

The third-order optical nonlinearities of surface modified PbS nanoparticles have been measured using the Z-scan technique with femtosecond laser pulses at 780 nm wavelength. The samples studied are the nanoparticles in micro-emulsion with the PbS concentration ranging from 0.3 × 10^-3 to 1.9 × 10^-3 M (mol/l). A Z-scan analysis method based on the Huygens-Fresnel (H-F) propagation integral is employed to extract the nonlinear refractive index from the experimental Z-scan data with a large nonlinear phase shift. The nonlinear refractive index of the PbS nanoparticles in microemulsion is found to increase linearly with the PbS concentration. The highest concentration microemulsion gives a nonlinear refractive index of -4.7 × 10^-12 cm²/W, which is approximately 3 orders of magnitude higher than those of commercially available bulk semiconductors, such as ZnS and CdS. In addition, nonlinear absorption in these samples remained unmeasurable up to 0.9 GW/cm². The mechanisms responsible for the observed large refractive nonlinearity are discussed.