1. Chair for Photonics and Optoelectronics, Nano-Institute Munich, Physics Department, Ludwig-Maximilians-Universität (LMU), Königinstr. 10, 80539 Munich, Germany
2. Merck KGaA, Frankfurter Str. 250, 64293 Darmstadt, Germany
3. Nanospectroscopy Group, Nano-Institute Munich, Physics Department, Ludwig-Maximilians-Universität (LMU), Königinstr. 10, 80539 Munich, Germany