1. Nano-Optics and Biophotonics Group, Department
of Experimental Physics 5, University of Würzburg, Am Hubland, D-97074 Würzburg, Germany
2. Department of Physics and Research Center OPTIMAS, University of Kaiserslautern, Erwin-Schrödinger-Str. 46, D-67663 Kaiserslautern, Germany
3. I. Physikalisches
Institut, Georg-August-Universität Göttingen, Friedrich-Hund-Platz
1, D-37077 Göttingen, Germany
4. Röntgen Center for Complex Material Systems (RCCM), Am Hubland, D-97074 Würzburg, Germany