**Acknowledgements**

I thank my colleagues and coworkers for help and support during the beamtimes, their preparation and analysis. These include in particular B. Pfau, M. Schneider, C. M. Günther, J. Geilhufe, P. Hessing, I. Lemesh, L. Caretta, S. Eisebitt, and the team of beamline P04 at PETRA III for the latest imaging experiments, A. Bisig and M. Kläui for valuable advice regarding the time-resolved measurements, and M. Mondol, J. Daley, B. Lägel, V. Guzenko and the LMN group at PSI, Switzerland, for help with the sample fabrication. Finally, I acknowledge financial support by the German Science Foundation.
