The second order optical nonlinearity of surface Cd-rich CdS nanoparticles has been studied by newly developed Hyper-Rayleigh Scattering technique(an incoherent methodology). The results inrndicated that the first hyperpolarizability β per particles was 1.3×10-26esu for CdS nanoparticles in 5nm diameter. This value is in the largest values reported for solution species. The HRS mechanism rnof nanoparticles was discussed. It is believed that the surface layer is the main origin of second order optical nonlinearity in nanoparticles. Especially the Cd-S bonding on the surface of the CdS/Cd nanoparticles act as molecule-like scatters, while surface defects such as dislocations, vacancies,dangling orbitals, surface chemical species etc make surface layer highly polarizable. Further more,the HRS signal of CdS nanoparticles was also enhanced by two-photo absorption as viewed from fluorescence spectrum.%用新发展的超瑞利散射(HRS)技术(一种非相干的方法)研究了表面富镉的CdS纳米粒子的二阶非线性光学性质。结果表明每个纳米粒子的二次非线性极化率β值在10-26esu量级，这在目前报导的具有最大β值的溶液物种之列。探讨了CdS纳米粒子产生二阶非线性的机制。认为纳米粒子大的比表面及表面缺陷结构对rnCdS纳米粒子的二阶光学非线性有很大影响。另外，双光子吸收诱导的共振增强作用亦可能贡献CdS纳米粒子的HRS信号，这种双光子共振吸收被其双光子荧光谱所证实。
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