Optical nonlinearity of surface-modified PbS and CdxPb1-xS nanoparticles in the femtosecond regime

摘要

Abstract: We present an investigation of third-order optical nonlinearity in surface modified PbS and Cd$-x$/Pb$- 1$MIN@x$/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 $MUL 10$+$MIN@3$/ M and Cd$-x$/Pb$-1$MIN@x$/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 $MUL 10$+$MIN@3$/ and 1.9 $MUL 10$+$MIN@3$/ M. The highest concentration microemulsion gives a nonlinear refractive index of 1.8 $MUL 10$+$MIN@11$/ cm$+2$//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$-x$/Pb$- 1$MIN@x$/S nanoparticles, Cd$-0.33$/Pb$-0.67$/S exhibits relatively larger refractive nonlinearity. For all samples, nonlinear absorption remained unmeasureable up to 0.9 GW/cm$+2$/. 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. !12