Condensed Matter Highlight A film of picosecond vibrations of a film

Time-domain experiments on solid-state dynamics at the fastest time-scales are typically performed using all optical techniques. These experiments have been made possible by, and have driven significant advances in, pulsed laser technology. Since optical techniques only indirectly probe structure, there is a significant scientific motivation for using higher spatial resolution probes such as X-ray and electron diffraction or microscopy. Unlike lasers, ultrafast X-ray and electron sources are much less developed. Nonetheless, there has been significant advances in ultrafast science using short wavelength probes, showing a similar synergy between source and science as was seen early on in the laser-based community. For their part, electrons are much more strongly interacting with matter than X-rays (the Coulomb is much larger than the Thompson cross-section) and are particularly well suited to studies in the gas phase [1]. Despite the large cross-section for elastic scattering from nuclei, many electrons are required to make a diffraction image, and space charge forces make it difficult to deliver these pulses in a short burst. In the solid phase, most of the (short pulse short wavelength) experiments to date has centered around X-ray diffraction. Here synchrotron based sources provide very bright and very flexible beams with roughly 100 ps duration [2,3], while table top laser-plasma based sources provide subpicosecond duration pulses albeit with very low brightness (but enough photons for single crystal diffraction [4,5]). Nonetheless, time-resolved electron diffraction is a complementary technique that is showing tremendous promise for ultrafast solid-state dynamics. One such experiment is reported in this issue in the article by Park et al. [6]. They report on their success in imaging the coherent atomic motion in a thin metallic film that has been set vibrating by a femtosecond laser pulse. In