Hyper-Rayleigh scattering and multiphoton fluorescence of nanoscale CdS colloids in water

摘要

Optical nonlinearities of semiconductor nanoparticles are of great interest recently. Here the hyper-Rayleigh scattering (HRS) technique is used to measure the second-order nonlinear optical (NLO) response of nanoscale CdS Colloids with different surfaces in water. It shows that the HRS signals for CdS Colloids are very strong. The 'per particle' first-order hyperpolarizability values $beta are estimated to fall in the range of 10$+$MIN@26$/ esu, and 'per CdS unit' are in the range of 10$+$MIN@28$/ esu which is very large. And the $beta values are different for CdS nanoparticles with different surfaces. Further studies show that the CdS/SC(NH$-2$/)$-2$/ nanoparticle has strong multi- photon fluorescence (MPF) emission under the radiation of 1064 nm so the large signal at 532 nm is mainly from the MPF. But for Cd$+2$PLU$/-rich and S$+2$/-rich CdS nanoparticles the MPF are relatively weaker at 532 nm. These experiments reveal that the HRS or MPF signals are very sensitive to the changes of the nanoparticle surface and can provide useful information about the nanoparticle/solution interface. They also give the evidences proving that surface termination of the crystalline lattice that creates a conditions of non-centrosymmetry is contributing to the large $beta value for CdS nanoparticles.