1. Photonics
and Optoelectronics Group, Department of Physics and Center for NanoScience
(CeNS), Ludwig-Maximilians-Universität München, Amalienstrasse
54, 80799 Munich, Germany
2. Nanosystems Initiative
Munich (NIM), Schellingstrasse 4, 80799 Munich, Germany
3. Department
of Chemistry, Ludwig-Maximilians-Universität München, Butenandtstrasse
5-13 (E), 81377 Munich, Germany