1 Research Laboratory of Electronics, Massachusetts Institute of Technology, 77 Massachusetts Ave., Cambridge, MA 02139, USA.
2 Department of Physics and Center for Ultrafast Imaging, University of Hamburg, Luruper Chaussee 149, 22761 Hamburg, Germany.
3 Northrop Grumman Corporation, NG Next, 1 Space Park Blvd., Redondo Beach, CA 90278, USA.
4 Centre for Research on Adaptive Nanostructures and Nanodevices (CRANN), Advanced Materials and Bio-Engineering Research Centre (AMBER), and School of Chemistry, Trinity College Dublin, Dublin 2, Ireland.
5 Center for Free-Electron Laser Science and Deutsches Elektronen-Synchrotron (DESY), Notkestraße 85, 22607 Hamburg, Germany.