1. Photonics and Optoelectronics Group, Department
of Physics and Center for Nanoscience (CeNS), Ludwig-Maximilians-Universität (LMU), Amalienstaße 54, 80799 Munich, Germany
2. Nanosystems Initiative Munich (NIM), Schellingstraße 4, 80799 Munich, Germany
3. Instituto de Ciencia de Materiales de Sevilla (CSIC-Universidad de Sevilla), C/Américo Vespucio 49, 41092 Sevilla, Spain