Dynamics of coherent phonon wave-packet

通过在设计的半导体超晶格中产生声子波包，研究了相干声子波包在时间和空间上的动力学。我们利用不同波长的泵浦和探测脉冲分离声子的产生区和探测区，特别研究了声子动力学。利用双色泵浦探针技术测量了GaAs/AlAs超晶格中的相干折叠纵向声声子。用1色和2色泵浦探测技术得到的结果的比较表明，具有有限波矢（q≠0）的一阶FLA声子的波矢是由探测脉冲的波矢决定的。此外，利用泵浦脉冲和探测脉冲的穿透深度差，可以实时地观察声子波包的传播。在一类新的非极性同位素半导体超晶格中观察到高阶相干受限LO声子模的产生。在泵浦探测技术中，只有拉曼主动奇数阶模式。利用泵浦光束的不同偏振谱图清楚地表明，非极性半导体中相干声子的产生机制是受激拉曼散射。我们研究了在20、100和195周期的GaAs/AlAs超晶格（SLs）中产生的相干折叠纵向声学（FLA）声子的动力学特性。在20周期激光激光器的时域信号中，除了在布里渊区中心及附近具有A_1对称性的拉曼有源模式外，还发现了在100周期以上激光激光器中没有的布里渊区中心具有拉曼无活性B_2对称性的相干FLA声子模式。B_2模态的出现是由于周期总数的有限性导致模态对称性的破坏。此外，还发现在傅里叶变换光谱中观测到的相干FLA声子模式的带宽取决于周期和波矢量的有限性。少

Dynamics of coherent phonon wave-packet in time and space was investigated by generating the phonon wave-packet in designed semiconductor superlattices. We specially investigated the phonon dynamics by separating generation and detection regions using various wavelengths of the pump and probe pulses.The coherent folded longitudinal acoustic (FLA) phonons in the GaAs/AlAs superlattices were measured by using a 2-color pump-probe technique. The comparison between results obtained by using 1-color and 2-color pump-probe techniques indicates that the wave-vector of the first-order FLA phonons with finite wave vectors (q≠O) is determined by that of the probe pulses. Moreover, the propagation of the phonon wave-packet can be observed in time by using the difference of the penetration depth for the pump and probe pulses.Generation of high-order coherent confined LO phonon modes was observed in a new class of nonpolar isotope semiconductor superlattices. Only Raman active odd-number-order mode … More s are observable in the pump-probe technique. The spectra taken with the variety of polarization of pump beam show clearly that the generation mechanism of the coherent phonons in nonpolar semiconductor like Ge is stimulated Raman scattering.We have investigated dynamical properties of coherent folded longitudinal acoustic (FLA) phonons generated in GaAs/AlAs superlattices (SLs) with 20, 100 and 195 periods. In the time-domain signals of the SL with 20 periods, the coherent FLA phonon mode with the Raman-inactive B_2 symmetry at the Brillouin zone center, which is not observed in the SLs with more than 100 periods, is detected in addition to the Raman-active modes with the A_1 symmetry at and near the zone center. The appearance of the B_2 mode is caused by the break of the mode symmetry due to the finiteness of the total number of periods. Moreover, it is found that the bandwidths of the coherent FLA phonon modes observed in the Fourier transform spectra depend on the finiteness of periodicity and the wave vector. Less