Excitation mechanisms of coherent phonons unravelled by femtosecond X-ray diffraction

Abstract

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 ZFLAF with a 3.5 ps period. The motion is directly monitored by femtosecond X-ray pulses at a kilohertz repetition rate. The intensity change ΔR/R = 0.03 of the -1st order satellite peak of the (002) reflection of bulk AlGaAs is directly proportional to the induced lattice strain of Δa/a0 = 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,.