Size dependence of second-order optical nonlinearity of CdS nanoparticles studied by hyper-Rayleigh scattering

The second-order optical nonlinearity of CdS nanoparticles with different mean diameters of 28.0, 30.0, 31.5, 50.0, and 91.0 A was studied by the incoherent hyper-Rayleigh scattering technique. Results show that the first-order hyperpolarizability beta value per CdS particle decreases with decreasing size from 91.0 to 31.5 A; however, as CdS particle size further decreases, this trend is reversed and the beta value increases. Substantially, the normalized value of the first-order hyperpolarizability per CdS formula unit, beta(0), exhibits systematic enhancement with decreasing size. This is interpreted in terms of a so-called surface contribution mechanism. The two aspects of quantum size effects, the size dependence of optical band-gap and oscillator strength, cannot be adopted to explain the size dependence of the second-order optical nonlinearity of CdS nanoparticles.