1. ICFO—Institut de Ciències Fotòniques, The Barcelona Institute of Science and Technology, Castelldefels, 08860 Barcelona, Spain
2. Institut für Optik und Atomare Physik, Technische Universität Berlin, 10623 Berlin, Germany
3. Max-Born-Institut, 12489 Berlin, Germany
4. ALBA Synchrotron Light Source, Cerdanyola del Vallès, E-08290 Barcelona, Spain
5. Department of Physics and Astronomy, Vanderbilt University, Nashville, Tennessee 37235-1807, United States