PbS nanoparticles embedded in SiO

2

thin films were prepared by the sol-gel method and the dip-coating 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 1S

e

-1S

h

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 negative component.The results indicate that the ultrafast optical nonlinearity arises from the contribution of 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 4mJ/cm

2

the magnitude of for PbS nanoparticles doped SiO

2

thin film is calculated to be 5.1×10

-10

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 CS

2

.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 sample with a narrow size distri-(bution) and high-doped concentration

the optical nonlinearity may be further increased.