Ultrafast nonlinear optical properties of semiconductors in low dimension

利用飞秒泵浦和探测光谱的广泛研究阐明了CdSe薄膜中光生载流子的超快动力学。在带间激励下，激发后立即观察到非热化的热载流子分布。这种分布是在200 MeV的宽能量范围内观察到的，从泵浦能量延伸到它的低能侧。根据非热化载流子与总载流子的比率，估计热化时间为20-40秒。在A激子的共振激发下，观察到B激子结构的42 meV能量展宽。这种展宽是由A激子和B激子之间的碰撞引起的。这是不同类型激子之间的散射过程的一个例子。分析表明，A-激子-B-激子的散射时间为31ps。基于刚性球散射模型的简单理论很好地解释了这一时间，在B激子的共振激发下，观察到了漂白在B激子处的快速恢复。0.9ps的恢复时间被解释为LO声子发射使B激子转变为A激子的时间。得到的时间常数与基于Frohlich相互作用计算的散射时间在一个数量级内是一致的。通过飞秒抽运和探测测量，在BiI3层状晶体的间接吸收边附近观察到了共振相干晶格振动。被飞秒泵浦脉冲脉冲激发的相干光学声子调制探测脉冲的相位。探测光谱的相应位移随着声子振荡周期的变化而变化，声子振荡周期持续100多个周期。我们推测，相干声子辅助的间接激子跃迁也对数据有贡献。

An extensive study by means of femtosecond pump-and-probe spectroscopy has clarified the ultrafast dynamics of photogenerated carriers in a CdSe thin film. Under band-to-band excitation, a nonthermalized hot carrier distribution was observed immediately upon excitation. This distribution was observed in a 200 meV broad energy range extending from the pump energy to its low energy side. From the ratio of nonthermalized to the total carriers, the thermalization time was estimated to be 20-40 fs.Under resonant excitation of the A-exciton, a 42 meV energy broadening of the B-exciton structure was observed. This broadening is caused by collisions between the A-exciton and the B-exciton. This is an example of the scattering process between different kinds of excitons. Analysis indicates that the A-exciton-B-exciton scattering time is 31 fs. This time is well explained by a simple theory based on the rigid-sphere scattering model.Under resonant excitation of the B-exciton, a fast recovery of the bleaching was observed at the B-exciton. The recovery time of 0.9 ps is explained as the transformation time of B-excitons into A-excitons by LO phonon emission. The obtained time constant agrees with the calculated scattering time based on the Frohlich interaction within an order of magnitude.Resonant coherent lattice vibrations in a BiI_3 layered crystal are observed around the indirect absorption edge by femtosecond pump and probe measurements. Coherent optical phonons that are impulsively excited by femtosecond pump pulses modulate the phase of probe pulses. The corresponding shift of the probe spectrum swings with the period of phonon oscillation, which continues for more than 100 periods. We speculate that the coherent phonon-assisted indirect exciton transitions have also contributed to the data.