1. Ketek GmbH, 81737 Munich, Germany
2. Department of Physics, Universität der Bundeswehr München, Werner-Heisenberg-Weg 39, 85579 Neubiberg, Germany
3. Faculty of General Sciences and Microsystems Technology, OTH Regensburg, 93053 Regensburg, Germany
4. School of Mathematics and Natural Sciences, University of Wuppertal, 42219 Wuppertal, Germany