1. Photonics and Optoelectronics Group, Department of Physics and CeNS, Ludwig-Maximilians-Universität München, Amalienstraße 54, 80799 Munich, Germany
2. Department of Chemistry and Center for Integrated Protein Science, Ludwig-Maximilians-Universität München, Butenandtstraße 5-13, 81377 Munich, Germany
3. Nanosystems Initiative Munich, Schellingstraße 4, 80799 Munich, Germany
4. Department of Chemistry, New York University, Silver Center, 100 Washington Square East, Room 712, New York 10003, United States