Dynamics of coherent phonon wave-packet

点击翻译按钮获取中文摘要

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