The first tunable undulator source for femtosecond hard X-rays in the range 4–12 keV is now in operation at the SLS storage ring. The source combines accelerator and laser technology relevant for future seeded free electron lasers. It provides inherently synchronized femtosecond laser ‘pump’ and X-ray ‘probe’ pulses to enable time-resolved diffraction and absorption experiments. By using X-ray diffraction to probe laser-induced coherent optical phonons in bulk bismuth, we estimate an X-ray pulse duration of 140 ± 30 fs FWHM with timing drifts below 30 fs rms measured over 5 days. The excellent spatial and temporal stability of the source allows quantitative measurement of ultrafast lattice dynamics and associated phase transitions in real space with atomic resolution and instrumental time resolution of 85 fs rms. Studying semimetals such as bismuth, we have demonstrated (i) grazing incidence femtosecond X-ray diffraction on single crystals that allows us to more completely characterize the ultrafast structural dynamics of solids, and (ii) optical control of real space coherent atomic motion. Both methods will be used to study the dynamics of photo-induced phase transitions in strongly correlated systems such as manganites. The time resolution of such measurements could be improved by one order of magnitude at future XFEL facilities at much higher flux. Femtosecond linear and non-linear resonant X-ray scattering employing full polarization control both in the soft (0.3–3 keV) and hard (4–15 keV) X-ray regime will become feasible which allows direct measurement of orbital degrees of freedom. However, a laser and hard X-ray cross-correlation technique with sufficient temporal resolution and signal-to-noise ratio appropriate for single shot operation has yet to be realized. Again, laser seeding using a phase stabilized few-cycle (5–7 fs) laser may be a viable option to generate <10 fs X-ray pulses.
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