Formation and relaxation dynamics of excitons in a weakly electron-lattice coupling system

为了阐明激子的形成和随后的弛豫过程，我们对β-ZnP_2中的激子系统进行了高分辨调制光谱和超快时间分辨光谱研究。主要研究结果如下：1.光反射（PR）光谱已被用来研究更高的单重态激子的精细结构。PR谱在每个激子能级上显示出清晰的结构，具有较大的调制幅度。产生PR信号的机制是由于光激发载流子周期性地抵消内置表面电场而引起的电反射。高分辨率的PR光谱使用钛：蓝宝石激光器作为探测光揭示了更高的成员的单重态激子高达n = 6.2。利用飞秒激光实现了光谱分辨的简并四波混频（FWM）光谱。FWM信号清楚地表现出作为延迟的函数的周期性超快调制 关于我们 两个激励脉冲之间的时间。调制被分配为由于两个紧密间隔的跃迁的干涉的量子拍频;这是自由激子和束缚激子。在不同能量下测量了激子极化激元的退相时间T_2，其中ET和EL分别为横向和纵向激子能量。极化激元能量越低，T_2越长，表明声子散射主要控制极化激元的退相.用飞秒激光泵浦探测光谱研究了β-ZnP_2中激子带的超快反射率变化。泵浦-探测信号的光谱在E_T和E_L之间的能量范围内表现出显著的负反射率变化。反射弛豫时间在E_L附近为1.1ps，在E_T附近为1.7ps。这些弛豫时间与激子极化激元的声学声子散射有关。在激子分子发光出现的能量处也观察到具有色散型光谱形状的瞬态反射率变化。少

In order to clarify the formation and subsequent relaxation processes of excitons, we have performed high- resolution modulation spectroscopy as well as ultrafast time-resolved spectroscopy for the exciton system in β-ZnP_2. The main results are summarized as follows.1. Photoreflectance (PR) spectroscopy has been performed in order to investigate fine structure of the higher singlet exciton states. The PR spectrum shows clear structure at each exciton level with large modulation amplihtude. The mechanism by which the PR signal is produced is electroreflectance due to the periodic cancellation of the built- in surface electric field by photoexcited carriers. A high-resolution PR spectrum using a Ti: sapphire laser as a probe light reveals the higher members of singlet excitons up to n = 6.2. Spectrally resolved degenerate four-wave mixing (FWM) spectroscopy has been done using a femtosecond laser. The FWM signal clearly exhibits a periodic ultrafast modulation as a function of the delay … More time between two excitation pulses. The modulation is assigned as the quantum beats due to the interference of two closely spaced transitions; the is free exciton and the bound exciton. Dephasing times T_2 of the is exciton polariton are measured at various energies between ET and EL, where ET and EL are transverse and longitudinal exciton energies, respectively. The T_2 is longer as the polariton energy is lower, suggesting that the phonon scatterings mainly govern the polariton dephasing.3. Ultrafast reflectivity changes of exciton bands in (β-ZnP_2 have been investigated by pump-probe spectroscopy using a femtosecond laser. Spectrum of the pump-probe signal shows a significant negative reflectivity change in the energy range between E_T and E_L. Reflection relaxation time is 1.1 ps near E_L and 1.7 ps near E_T. These relaxation times are related to the acoustic phonon scattering of exciton polaritons. Transient reflectivity changes with a spectral shape of dispersion type are also observed at the energy where the excitonic molecule luminescence appears. Less