Study progress in second-order optical nonlinearitis of inorganic nanoparticles

Hyper-Rayleigh scattering (HRS) technique, an incoherent methodology, is an effective tool developed at the beginning of the nineties for the determination of the first-order hyperpolarizability beta value of molecular chromophores in solution or gas. Compared with conventional coherent second harmonic generation (SHG) or with electric-field-induced SHG technique, HRS technique is not constrained by the orientational, size, and/or charge restrictions, hence, in the past years, it has been used to measure the beta value of nanoscale particles and some important results were obtained. In this paper, its simple principle, experimental setup, and data processing method are described. According to the recent studies, reported and our work, some possible HRS mechanisms, such as surface contribution, bulk-like contribution, aggregate contribution, resonance enhancement, surface static electric field, and solvent filed, are explored. Especially, surface contribution is emphasized. The investigation of second-order nonlinear optical properties of nanoparticles with the HRS technique is an interdisciplinary field of nano-science and nonlinear optics, which is attracting more and more attentions of scientists.