1. Research
Laboratory of Electronics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States
2. Department
of Physics and Center for Ultrafast Imaging, University of Hamburg, Hamburg, Germany
3. Northrop
Grumman Corporation, NG Next, Redondo Beach, California 90254, United States
4. Center
for
Free-Electron Laser Science, Deutsches Elektronen-Synchrotron, 22607 Hamburg, Germany