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

为了弄清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率调制光谱和超快时间分辨光谱研究。主要研究结果总结如下：为了研究高单重态激子态的精细结构，采用了光反射光谱（PR）技术。PR谱在各激子能级结构清晰，调制幅度大。PR信号产生的机制是电反射，这是由于光激发载流子周期性地抵消了内建的表面电场。使用Ti：蓝宝石激光器作为探测光的高分辨率PR光谱揭示了n = 6.2的单重态激子的较高成员。利用飞秒激光器进行了光谱分辨简并四波混频（FWM）光谱分析。随着两个激励脉冲之间的延迟时间增加，FWM信号明显表现出周期性的超快调制。由于两个间隔很近的跃迁的干扰，调制被指定为量子拍；自由激子和束缚激子。在ET和EL之间的不同能量下测量了激子极化子的消相时间T_2，其中ET和EL分别为横向和纵向激子能量。T_2越长，极化子能量越低，表明声子散射主要控制极化子的消相。利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。在E_T和E_L之间的能量范围内，泵浦-探针信号的光谱表现出显著的负反射率变化。反射弛豫时间在E_L附近为1.1 ps，在E_T附近为1.7 ps。这些弛豫时间与激子极化子的声子散射有关。在激子分子发光的能量处，还观察到瞬态反射率随色散型光谱形状的变化。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