1. Institute for Quantum Electronics, Eidgenössische Technische Hochschule (ETH) Zürich, CH-8093 Zurich, Switzerland
2. Dipartimento di Fisica, Università degli Studi di Trieste, IT-34127 Trieste, Italy
3. Department of Physics and Astronomy, Interdisciplinary Nanoscience Center (iNANO), Aarhus University, 8000 Aarhus C, Denmark
4. Department of Mathematics and Physics, Università Cattolica del Sacro Cuore, IT-25133 Brescia, Italy
5. ILAMP (Interdisciplinary Laboratories for Advanced Materials Physics), Università Cattolica del Sacro Cuore, IT-25133 Brescia, Italy
6. Department of Physics and Astronomy, KU Leuven, B-3001 Leuven, Belgium
7. Department of Chemistry, Columbia University, New York, New York NY-10027, United States
8. Elettra─Sincrotrone Trieste S.C.p.A., Strada Statale 14, km 163.5, IT-34149 Trieste, Italy
9. Department of Physics, Columbia University, New York, New York 10027, United States
10. Université de Lyon, CNRS, Université Claude Bernard Lyon 1, Institut Lumière Matière, F-69622 Villeurbanne, France
11. CNR-INO (National Institute of Optics), IT-25123 Brescia, Italy
12. International Faculty, University of Cologne, Albertus-Magnus-Platz, D-50923 Cologne, Germany