Hyper-Rayleigh scattering of nanoscale CdS colloid and its formation process

Nanoscale CdS colloid has exhibited great second-order optical nonlinearities measured by hyper-Rayleigh scattering (HRS) technique. Here a further study on electrolyte concentration dependence of HRS signal of nanoscale CdS colloid is carried out in order to exclude the possible contributions of some third-order nonlinear optical processes based on colloidal particle surface electric field and solvent field. Results show that the HRS signal remains unchanged with increasing electrolyte concentration, implying that the HRS signal arises indeed from second-order polarization (including bulk-like polarization and surface polarization) of CdS nanoparticles rather than the third-order processes. Furthermore, HRS signal variation during the formation of CdS colloid is studied for the first time. An obvious enhancement of HRS signal intensity is observed immediately once Cd2+ and S2− ions are mixed. After that, HRS signal decreases toward a constant value with time. This is explained in terms of surface contribution due to large surface-to-volume atomic ratio for nanoparticles.