1. Department of Photonics Engineering, Technical University of Denmark, DK-2800 Kongens Lyngby, Denmark
2. Center for Nanostructured Graphene, Technical University of Denmark, DK-2800 Kongens Lyngby, Denmark
3. Nano-Optics and Biophotonics Group, Experimentelle Physik 5, Physikalisches Institut, Universität Würzburg, D-97074 Würzburg, Germany
4. Center for Nano Optics, University of Southern Denmark, DK-5230 Odense M, Denmark
5. Danish Institute for Advanced Study, University of Southern Denmark, DK-5230 Odense M, Denmark