1. Deutsches Elektronensynchrotron (DESY), Notkestrasse 85, 22607 Hamburg, Germany
2. School of Mathematics and Physics, Queen's University, University Road, Belfast BT7 1NN, United Kingdom
3. Institute of Optics and Quantum Electronics, Abbe Center of Photonics, Friedrich-Schiller-Universität Jena, Max-Wien-Platz 1, 07743 Jena, Germany
4. Helmholtzinstitut Jena, Helmholtzweg 4, 07743 Jena, Germany