1. Department of Physics and Material Sciences Center, Philipps-Universität Marburg, Renthof 5, 35032 Marburg, Germany
2. NAsP GmbH, Hans-Meerwein-Straße, 35032 Marburg, Germany
3. College of Optical Sciences, University of Arizona, 1630 E. University Blvd., Tucson, Arizona 85721, USA