1. Photonics and Optoelectronics Group; Department of Physics and Center for NanoScience (CeNS); Ludwig-Maximilians-Universität München; Amalienstr. 54 80799 Munich Germany
2. Nanosystems Initiative Munich (NIM); Schellingstraße 4 80799 Munich Germany
3. Department of Chemistry; Ludwig-Maximilians-Universität München; Butenandtstr. 5-13 (E) 81377 Munich Germany
4. Department of Physics and Center for NanoScience (CeNS); Ludwig-Maximilians-Universität München; Amalienstr. 54 80799 Munich Germany
5. Department of Chemistry; Division of Molecular Imaging and Photonics; KU Leuven-University of Leuven; 3001 Leuven Belgium