Berlin 2005 – wissenschaftliches Programm
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HL: Halbleiterphysik
HL 63: Ultrakurzzeitph
änomene
HL 63.11: Vortrag
Mittwoch, 9. März 2005, 13:15–13:30, TU P-N201
Coherent atomic motions in a semiconductor superlattice studied by femtosecond x-ray diffraction — •Matias Bargheer1, Nikolai Zhavoronkov1, Yuri Gritsai1, J. C. Woo2, Dai-Sik Kim2, Michael Woerner1, and Thomas Elsaesser1 — 1Max-Born-Institut, Berlin — 2Seoul National University, Seoul, Korea
We report on the first nondestructive femtosecond x-ray diffraction measurements of minute reversible structural changes in a nano-structured solid. As a prototype sample representative for a larger class of inorganic and organic nanostructures, we directly measured coherent lattice plane motions with sub-picosecond and sub-picometer accuracy in a GaAs/AlGaAs superlattice [1]. The spatially periodic femtosecond excitation of the lowest subband in the wells of the superlattice results in coherent lattice motions with a 3.5 ps period, directly monitored by a novel kHz femtosecond-XRD setup. Small changes Δ R / R = 0.01 of weak Bragg reflexes (R = 0.005) are detected. Phase and amplitude of the XRD signal demonstrate the displacive excitation of zone-folded longitudinal acoustic phonons as the dominant mechanism for strong excitation. This results is in contrast to all optical pump-probe experiments on the same material in the weak excitation regime which was shown to be impulsive Raman excitation [2]. [1] Bargheer et al. "Coherent Atomic Motions in a Nanostructure Studied by Femtosecond X-ray Diffraction", Science 2004 (in print). [2] A. Bartels et al. Phys. Rev. Lett. 82, 1044 (1999).