通过飞秒泵浦探测技术研究PbS纳米颗粒的激发态瞬态动力学过程

Transient Dynamics of Excited States in PbS Nanoparticles by Femtosecond Pump and Probe Spectroscopy

利用飞秒泵浦探测技术研究了PbS半导体纳米颗粒复合的S iO2溶胶凝胶薄膜的瞬态动力学过程。通过改变激发探测波长和激发光强度,研究引起PbS半导体纳米颗粒的非线性吸收的两种机制。当激发探测波长选在激子吸收峰附近(620 nm)时,由于激子的饱和吸收引起的光致漂白,当激发波长选在激子能态的低能侧(753,800 nm),同时观察到激子的饱和吸收和双激子效应引起的光致吸收。研究了激子的饱和吸收和双激子效应引起的激发态吸收随激发态电子-空穴对浓度的变化关系,表明双激子效应与载流子浓度有很大关系。在高激发强度下,双激子效应引起的诱导吸收远远大于激子跃迁引起的光致漂白,双激子效应在非线性吸收中起着决定性作用。

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 （48 μJ/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 4 mJ/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.