1. Accurion GmbH, Stresemannstr. 30, D-37079 Göttingen, Germany
2. Centre for Photonics Engineering, University of Southern Denmark, Alsion 2, DK-6400 Sønderborg, Denmark
3. Center for Surface- and Nanoanalytics, Johannes Kepler University, Altenbergerstr. 69, A-4040 Linz, Austria
4. Institute of Physics, University of Oldenburg, Carl-von-Ossietzky-Str. 9-11, D-26129 Oldenburg, Germany