1. Center for Free-Electron Laser Science, Deutsches Elektronen-Synchrotron, Notkestrasse 85, Hamburg 22607, Germany
2. Physics Department, University of Hamburg and the Hamburg Center for Ultrafast Imaging, Luruper Chaussee 149, Hamburg 22761, Germany
3. Department of Electrical Engineering and Computer Science and Research Laboratory of Electronics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA