1. University of Vienna; Faculty of Physics; VCQ, QuNaBioS, Boltzmanngasse 5, A-1090 Vienna Austria
2. University of Rostock; Institut für Physik, Albert-Einstein-Straße 23, D-18055 Rostock; Germany
3. Stanford University; Physics Department; 382 Via Pueblo Mall Stanford, CA 94305-4060 USA
4. ICFO Institut de Ciènces Fotòniques, 08860 Castelldefels (Barcelona); Spain
5. The Center for Nanosciences and Nanotechnology & School of Chemistry; The Raymond and Beverly Faculty of Exact Sciences, Tel Aviv University, Tel Aviv 69978; Israel