Semiconductor nanoparticles have recently attracted much attention for their potential applications in making photonic devices

which can benefit from the control of the size

spectral tunability

enhancement of the third-order nonlinear optical properties

and the ultrafast relaxation dynamics.PbS-doped glass

recently reported as a saturable absorber for mode locking of a Cr:forsterite laser

is one of the most noteworthy examples of utilization of optical nonlinearities in PbS semiconductor nanoparticles.The study of nonlinear absorption and its dynamic behavior in PbS semiconductor nanoparticles is an important research topic both for the basic research and for its applications.Silicon dioxide films incorporated with lead sulfide semiconductor nanoparticles were synthesized by using the sol-gel method and the dip-coating technique.We have investigated the transient dynamics of PbS nanoparticles by one-color femtosecond pump-probe method at different wavelengths and various excitation densities.The experimental result shows that the absorption saturation of exciton resonance results in photobleaching at 620 nm in low excitation density condition.The decay process of the bleaching includes two components

the fast one from free carriers scattering and the slow one from trapped carriers scattering.The photo-induced absorptions at 753 nm and 800 nm were caused by the biexciton effect.The experimental result indicates that when the excitation and probe wavelength were chosen on the low-energy side of the exciton resonance

it could overlap the ground-state-biexciton transition.At low excitation density(48μJ/cm

2

)

the average number of excited e-h pairs in each nanoparticle was much less than one

and photo-induced absorption of the biexciton effect and photobleaching of the exciton transition were observed simultaneously. As pump density increases

the absorption saturation gradually disappears

and only the photo-induced absorption can be observed at the pump density of 4mJ/cm

2

indicating that the biexciton effect is depends strongly on the density of excited carriers and plays a significant role in the multiple e-h pair in PbS nanoparticles.