Ultrafast Dynamics of Folded Acoustic Phonons from Semiconductor Superlattices.
[摘要] There is great interest in the coherent control of short-wavelength acoustic phonons using ultrafast laser pulses for applications such as imaging and energy transport. Metallic layers have been used extensivelyas optical transducers for generation and measurement of strain pulses. This approach is usually limited to a few hundred GHz. The folding of the acoustic branches in periodic superlattices provides an alternative way to couple light pulses with high-frequency (1 THz) crystal vibrations. This dissertation reports on novel studies of laser-generated folded acoustic phonons in semiconductor superlattices. Through ultrafast laser and x-ray experiments, important aspects of the generation, propagation and detection of these modes are obtained. Also, detailed simulations and new theoretical considerations provide insights into the fundamental physics of folded acoustic phonons.Coherent folded acoustic phonons are studied using pump-probe experiments with ultrashort laser and X-ray pulses. A scheme to study the propagation of folded acoustic phonons is introduced that uses two superlattices as acoustic transducers. The folded phonons are generated in one superlattice and detected on a second superlattice 1.2um apart. A solution of the wave equation in terms of eigenmodes accounts for the observed features in the reflectivity spectrum. The model predicts that eigenmodes with frequencies near the acoustic gaps avoid the boundaries and exhibit confinement in the superlattice. The experimental observations confirm the presence of a long lived acoustic mode in the superlattice which lies near the first zone-center folded acoustic gap. This surface-avoiding behavior is shown to ocurr in general for any wave under a periodic perturbation.Short X-ray pulses from the Advanced Photon Source are used to probe the folded acoustic phonons as they propagate from the superlattice into the substrate. Experiments show that a bulk excitation at a wavevector 2pi/D, where D is the superlattice period, is launched from the superlattice into the substrate and propagates at the speed of sound. Results are supported by calculations of the time-resolved diffraction under the presence of strain. This observation demonstrates the capabilities of ultrafast X-ray diffraction for measuring high-wavevector excitations and holds promise for studying dynamical processes in crystals throughout the full Brillouin zone.
[发布日期] [发布机构] University of Michigan
[效力级别] Ultrafast Phenomena [学科分类]
[关键词] Coherent Acoustic Phonons;Ultrafast Phenomena;Physics;Science;Applied Physics [时效性]