Optical nonlinearity of surface modified PbS and Cd_xPb_1-xS nanoparticles in the femtosecond regime

We present an investigation of third-order optical nonlinearity in surface modified PbS and Cd_xPb_1-xS nanoparticles using the Z-scan technique with femtosecond laser pulses at 780-nm wavelength. The samples include PbS nanoparticles in microemulsion with PbS concentration range from 0.3-2.5×10^-3 M and Cd_xPb_1-xS nanoparticles in microemulsion with x from 0 to 1. An extended Z-scan theory based on the Huygens-Fresnel principle is employed to extract the nonlinear refraction index from the experimental Z-scan data with a large nonlinear phase shift. The nonlinear refractive index in PbS nanoparticle microemulsion is found to increase linearly with PbS concentration between 0.3×10^-3 and 1.9×10^-3 M. The highest concentration microemulsion gives a nonlinear refractive index of 1.8×10¹¹ cm²/W, which is approximately 4 orders of magnitude higher than those of commercially available bulk semiconductors, such as ZnS and CdS, measured at the same conditions. In Cd_xPb_1-xS nanoparticles, Cd_0.33Pb_0.67S exhibits relatively larger refractive nonlinearity. For all samples, nonlinear absorption remained unmeasurable up to 0.9 GW/cm². The observed large refractive nonlinearity in these nanoparticles may mainly be attributed to the optical Stark effect and contribution from the surface-trapped states in the nanoparticles.