1. Photonics
and Optoelectronics Group, Department of Physics and Center for Nanoscience
(CeNS), Ludwig-Maximilians-Universität München, Amalienstraße
54, Munich 80799, Germany
2. Department
of Physics and Center for Nanoscience (CeNS), Ludwig-Maximilians-Universität München, Geschwister-Scholl-Platz 1, Munich 80539, Germany
3. Clarendon
Laboratory, Department of Physics, University of Oxford, Parks Road, Oxford OX1 3PU, United Kingdom
4. Nanosystems Initiative Munich (NIM), Schellingstraße 4, Munich 80539, Germany