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Excitation mechanisms of coherent phonons unravelled by femtosecond X‐ray diffraction
Author(s) -
Bargheer M.,
Zhavoronkov N.,
Woo J. C.,
Kim D. S.,
Woerner M.,
Elsaesser T.
Publication year - 2006
Publication title -
physica status solidi (b)
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.51
H-Index - 109
eISSN - 1521-3951
pISSN - 0370-1972
DOI - 10.1002/pssb.200668073
Subject(s) - femtosecond , excitation , phonon , picosecond , superlattice , diffraction , ultrashort pulse , amplitude , materials science , condensed matter physics , optics , atomic physics , physics , laser , quantum mechanics
Coherent zone‐folded longitudinal acoustic phonons (ZFLAPs) were measured nondestructively with sub‐picometer spatial and sub‐picosecond time resolution via ultrafast X‐ray diffraction (XRD). The femtosecond excitation of a GaAs/AlGaAs superlattice with 1.5 eV photons results in spatially periodic stress, which induces a coherent ZFLAP with a 3.5 ps period. The motion is directly monitored by femtosecond X‐ray pulses at a kilohertz repetition rate. The intensity change Δ R / R = 0.03 of the −1 st order satellite peak of the (002) reflection of bulk AlGaAs is directly proportional to the induced lattice strain of Δ a / a 0 = 1.5 × 10 −4 . The phase and amplitude of the oscillatory XRD signal around a new equilibrium demonstrates the displacive excitation of coherent phonons (DECP) as the dominant mechanism for strong excitation. We discuss different mechanisms which account for impulsive stress, and argue that the electronic stress due to the deformation potential dominates. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)