Control of coherent phonons with excitonic quantum beats

研究了分子束外延GaAs/AlAs多量子威尔斯阱中激子量子拍频与相干纵光学声子之间的动力学相互作用。时域信号的检测使用时间分辨反射型泵浦探测光谱与飞秒脉冲激光。激子的量子拍频能量由量子尺寸效应和量子限制斯塔克效应控制。结果表明，在量子拍频能量与LO声子能量共振的条件下，相干类GaAs LO声子的强度得到显著增强。层厚依赖性和泵浦能量依赖性的系统结果清楚地表明，激子量子拍频和相干LO声子通过纵向偏振相互耦合，这导致耦合模的形成，并且偏振耦合作为增强相干LO声子的驱动力。激子量子拍频与相干LO声子之间耦合的发现为我们提供了一种用激子量子拍频控制相干LO声子的新范例，从而实现对相干声子的操控。此外，我们还首次证明了多量子阱中的类GaAs LO声子产生了频率为8.8THz的强单色THz电磁波。此外，我们还研究了真空沉积法生长的CuI薄膜中的相干LO声子。结果表明，在激子共振条件下，相干LO声子的强度显著增强，说明脉冲受激拉曼散射是CuI薄膜中相干LO声子强度增强的主要原因.

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.