Optical nonlinearity of surface-modified PbS and Cd x Pb 1-x S nanoparticles in the femtosecond regime

Abstract

We present an investigation of third-order optical nonlinearity in surface modified PbS and Cd<SUB>x</SUB>Pb<SUB>1-x</SUB>S 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 X 10<SUP>-3</SUP> M and Cd<SUB>x</SUB>Pb<SUB>1-x</SUB>S nanoparticles in microemulsion with x from 0 to 1. An extended Z-scan theory based on the Huygens- Fresnel princeps 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 X 10<SUP>-3</SUP> and 1.9 X 10<SUP>-3</SUP> M. The highest concentration microemulsion gives a nonlinear refractive index of 1.8 X 10<SUP>-11</SUP> cm<SUP>2</SUP>/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<SUB>x</SUB>Pb<SUB>1-x</SUB>S nanoparticles, Cd<SUB>0.33</SUB>Pb<SUB>0.67</SUB>S exhibits relatively larger refractive nonlinearity. For all samples, nonlinear absorption remained unmeasureable up to 0.9 GW/cm<SUP>2</SUP>. 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.