Coherent Phonon Quantum Dynamics in Semiconductor Nanostructures

这项研究的主要目的是将我们利用飞秒光脉冲产生超快声学声子的知识扩展到基本的半导体材料和纳米结构。我们使用直径几微米的紧聚焦激发和探测光束，以允许以-1微米的横向空间分辨率和亚皮秒的时间分辨率来探测相干声子。为此，我们研制了一台超快Sagnac干涉仪，用于在正常入射下测量声子。主要完成了以下工作：1)设计和建造了一台用于探测皮秒尺度相干声学声子的Sagnac干涉仪。这种设计对于垂直入射测量特别方便，而且是公共路径，使其不受声学和热寄生波动的影响。2)在块状GaAs(100)中产生和检测纵向相干声子。上述带隙激发和探测光束从样品的同一侧入射。…3)在Al_&lt；0.3&gt；Ga_&lt；0.7&gt；As(100)衬底上(厚度为50 nm)的超薄(&lt；1um)GaAs(100)薄膜中产生和检测纵向相干声子。上述带隙激发和探测光束从样品的两侧入射，使我们能够在时间上将声子脉冲信号与对信号的虚假热贡献和纯电子贡献分开。4)在周期性的GaAs/AlAs超晶格中产生和检测相干声子。探索了具有原子有序重复长度的超晶格。通过将抽运光调谐到830 nm左右，实现了对GaAs光的选择性激发。尝试了一种新的激发构型，其中超晶格与法线方向成直角解理。观察到了纵向声学声子沿平面方向传播的信号。此外，平行开展的金属理论工作首次对自由电子气的非平衡驰豫进行了分析处理。要将这项技术推广到更多种类的半导体和更高频率的声学发电，还需要进一步的工作。高达-10太赫兹。较少

The main aim of this study is to extend our knowledge of ultrafast acoustic phonon generation using femtosecond light pulses to basic semiconducting materials and nanostructures. We use tightly focused excitation and probe light beams a few microns in diameter to allow the detection of coherent phonons with -1 mum lateral spatial resolution and sub-picosecond time resolution. In order to do this we have developed an ultrafast Sagnac interferometer for measurements at normal incidence.The following studies were carried out :1) Design and construction of a Sagnac interferometer for detection of coherent acoustic phonons on picosecond time scales. The design is particularly convenient for normal incidence measurements and is common path, rendering it immune to acoustic and thermal parasitic fluctuations.2) Generation and detection of longitudinal coherent acoustic phonons in bulk GaAs (100). Above band-gap excitation and probe optical beams were incident from the same side of the sample. … More The effect of coherent phonon pulses propagating into the bulk was detected.3) Generation and detection of longitudinal coherent acoustic phonons in ultrathin (<1 mum) GaAs (100) film (thickness 50 nm) on a Al_<0.3>Ga_<0.7>As (100) substrate film.(- 5 mum in thickness). Above band-gap excitation and probe optical beams were incident from opposite sides of the sample, allowing us to temporally separate phonon pulse signals from spurious thermal and purely electronic contributions to the signal.4) Generation and detection of coherent acoustic phonons in periodic GaAs/AlAs superlattices. Superlattices with atomic order repeat lengths were probed. Selective excitation of the GaAs was achieved by tuning the pump beam to around 830 nm. A new excitation configuration, in which the superlattice was cleaved at right angles to the normal direction, was attempted. Signals due to the propagation of longitudinal acoustic phonons in the in-plane direction were observed.In addition, theoretical work on metals carried out in parallel has allowed a first analytical treatment of the nonequilibrium relaxation of a free electron gas. Further work is needed to extend the technique to a wider variety of semiconductors and to higher frequency acoustic generation. up to - 10 THz. Less