中文摘要：

通过光Kerr效应研究了PbS半导体纳米颗粒的三阶光学非线性响应。在较低的激发强度下，单激子态的饱和吸收和双激子效应的激发态吸收对非线性的贡献同时存在；而在高激发强度下，双激子效应是主要的。计算了高激发强度下（4mJ／cm^2）PbS半导体纳米颗粒的三阶非线性光学极化率为5．1×10^-10esu。

英文摘要：

PbS nanoparticles embedded in SiO2 thin films were prepared by the sol-gel method and the dipcoating technique. The sample under investigation was PbS nanoparticles sol-gel film about 150 nm thick on glass substrate, its linear absorption spectrum indicate the band gap shifts to higher energy for PbS nanopar- ticles due to the quantum confinement effect. The absorption spectrum of sample exhibits a distinct band peaked at 640 nm, which is attributed to 1 Se-1 Sh transitions. An average particle diameter of 5 nm was estimated according to a simple effective mass approach. We report on the nonlinear optical properties （NLO） of PbS nanoparticles at various excitation densities. At low excitation densities, the OKE response evolution of PbS nanoparticle, including a positive component and a negative component, and a very slow decay was observed. At higher excitation densities, there is a pronounced modification of the temporal profile, the disappearance of the positive component and the existence of the ultrafast optical nonlinearity arises from the contribution of negative component. The results indicate that the the absorption saturation due to exciton transition and the photo-induced absorption due to biexciton effect. The contribution of the one-exciton state and that of biexciton to the third-order susceptibility are opposite, which agrees with Banyai＇s theoretical prediction. At excitation density of 4 mJ/cm^2, the magnitude of for PbS nanoparticles doped SiO2 thin film is calculated to be 5.1 × 10^- l0 esu, which arises from the contribution of biexciton effect at high pump intensity. The magnitude of for PbS nanoparticles is four orders of magnitude larger than that of CS2. And it is one order of magnitude larger than that of the standard optical nonlinear material CdS （5 ×10^-11 esu）. Based on the preparation method of our samples, size distribution of the nanoparticles is usually more than 10%. Wide size distribution results in a strong reduction of the optical nonlinearity. If we can obtain the