Femtosecond relaxation dynamics of excitons localized by disorder in semiconductors

1)我们研制了一套被动锁模染料激光器系统。染料激光器以100 MHz的速率产生一串100 fs的脉冲。波长调谐范围为580-620 nm。2）用变温光子回波法测量了GaSe中激子的相位弛豫时间。发现在30 K以上，光学退相是由声子散射引起的。3）发现GaSe中自由激子的辐射寿命与相位关联时间之间存在密切的相关性。这一结果为最近的理论预测提供了直接的实验证明。4）我们对GaSe中的自由激子线进行了两种不同的烧孔实验：脉冲传输延迟和泵浦探测实验。这两个结果支持部分激子局域化的论点堆叠无序。5）我们提出了两种新的方法来研究飞秒过程：泵浦-探测和上转换方法使用时间非相干光。6）使用一种新的泵浦-探测技术研究了亚皮秒精度的热载流子在CdSe中的时间行为。其中利用光纤中的自相位调制效应来扩展探测脉冲的光谱宽度。通过电子-空穴等离子体的形成和等离子体随后的热弛豫解释了泵浦引起的变化。7）研究了20 K下短周期GaAs/AlAs超晶格的光致发光谱与流体静压和单轴压力的关系。发现最低激发态是Xz处的间接激发态。

1) We made a passively mode-locked dye laser system. The dye laser produces a train of 100 fs pulses at the rate of 100 MHz. The wavelength tuning range is 580-620 nm.2) The phase relaxation time of the exciton in GaSe was measured by using temperature-dependent photon echo method. It is found that the optical dephasing is caused by phonon scattering above 30 K.3) A close correlation was found between the radiative lifetime and the phase correlation time of the free excitons in GaSe. This result gives a direct experimental proof of the recent theoretical prediction.4) We made two different hole burning experiments in the free exciton line in GaSe: pulse propagation delay and pump-probe experiments. Both results support arguments for partial exciton localization by stacking disorder.5) We proposed two new methods to investigate femtosecond processes: pump-probe and upconversion methods using temporally incoherent lights.6) Temporal behavior of hot carriers in CdSe was investigated with subpicosecond accuracy, by using a novel pump-probe technique. in which the self-phase modulation effect in an optical fiber is utilized to extend the spectral width of the probe pulse. Pump-induced changes is explained by the formation of an electron-hole plasma and a succeeding thermal relaxation of the plasma.7) Photoluminescence spectra of short-period GaAs/AlAs superlattices were studied as a function of hydrostatic and uniaxial pressure at 20 K. It is found that lowest excited state is an indirect exciotns at Xz.