1. Institute of Optics and Quantum Electronics, Friedrich-Schiller University Jena, Max-Wien-Platz 1, 07743 Jena, Germany
2. Stanford Linear Accelerator Center (SLAC), 2575 Sand Hill Road, Menlo Park, California 94025, USA
3. Helmholtz Institute Jena, Fröbelstieg 3, 07743 Jena, Germany