Imaging atomic motions in materials

The ultra-fast X-ray diffraction technique has now become widely used. Many experiments using this technique are, in principle, a so-called pump-probe measurement, using a Ti:sapphire laser system (wavelength 800 nm, 1-kHz repetition rate with 5-mJ pulse energy and 45-fs duration) and, for example, a moving, 20-mm-thick Cu band to generate characteristic X-ray pulses. Recently, a German group reported the successful imaging of coherent atomic motions in a GaAs/AlGaAs superlattice. The motions are of great interest and are due to the excitation of electron-hole pairs in the GaAs subband. Both expansion of the GaAs layers and contrast of the AlGaAs layers were observed, mainly because bonding in the GaAs layers was affected by the excitation. For more information, see the paper, "Coherent Atomic Motions in a Nanostructure Studied by Femtosecond X-ray Diffraction", M. Bargheer et al., Science, 306, 1771-1773 (2004).