Control of coherent phonons with excitonic quantum beats

本文研究了分子束外延生长的GaAs/AlAs多量子阱中激子量子拍打与相干纵向光学声子的动态相互作用。利用飞秒脉冲激光对时域信号进行了时间分辨反射型泵浦探测光谱检测。激子量子热能量受随层厚变化的量子尺寸效应和随外加电场变化的量子受限斯塔克效应控制。发现在量子拍能与LO声子能量共振的条件下，相干类gaas声子的强度急剧增强。层厚度依赖性和泵浦能量依赖性的系统结果清楚地表明，激子量子拍打和相干LO声子通过纵向极化相互耦合，形成耦合模式，极化耦合是相干LO声子增强的驱动力。激子量子拍和相干LO声子之间耦合的发现为我们提供了一种用激子量子拍控制相干LO声子的新范式，从而导致相干声子的操纵。此外，我们首次证明了MQW中相干类砷化镓声子产生的频率为8.8太赫兹的强单色太赫兹电磁波。此外，我们还研究了真空沉积制备的CuI薄膜中的相干LO声子。研究发现，在泵浦脉冲能量的激子共振条件下，相干LO声子的强度显著增强，这表明在CuI薄膜中，脉冲受激拉曼散射主导了强度增强。

We have investigated dynamical interactions between excitonic quantum beats and coherent longitudinal optical (LO) phonons in various GaAs/AlAs multiple quantum wells (MQWs) grown by molecular beam epitaxy. The time-domain signals were detected using time-resolved reflection-type pump-probe spectroscopy with a femto-second pulse laser. The excitonic quantum-beat energy is controlled by the quantum-size effect with a change of layer thickness and a quantum-confined Stark effect with a change of applied electric field. It is found that the intensity of the coherent GaAs-like LO phonon is drastically enhanced under the condition that the quantum-beat energy is resonant with the LO-phonon energy. The systematic results of the layer-thickness dependence and the pump-energy dependence clearly demonstrates that the excitonic quantum beat and coherent LO phonon couples with each other via longitudinal polarization, which results in the formation of a coupled mode, and that the polarization coupling acts as a driving force for the enhancement of the coherent LO phonon. The finding of the coupling between the excitonic quantum beat and coherent LO phonon provides us a novel paradigm of control of the coherent LO phonon with the excitonic quantum beat, which leads to manipulation of coherent phonons. Furthermore, we have demonstrated for the first time that intense and monochromatic THz electromagnetic wave with a frequency of 8.8 THz is generated by the coherent GaAs-like LO phonon in the MQW. In addition, we have investigated the coherent LO phonons in CuI thin films grown by vacuum deposition. It is found that the intensity of the coherent LO phonon is markedly enhanced under an exciton resonant condition of the pump-pulse energy, which suggests that impulsive stimulated Raman scattering dominates the enhancement of the intensity in the CuI thin film.