1. BioMolekulare Optik, Department für Physik, Ludwig-Maximilians-Universität, Oettingenstr. 67, D-80538 München, Germany, Center for Nanoscience (CENS), Department für Physik, Ludwig-Maximilians-Universität, Amalienstr. 54, D-80799 München, Germany, and National Nanotechnology Laboratory of CNR−INFM c/o Istituto Superiore Universitario di Formazione Interdisciplinare, Università di Lecce, Via per Arnesano, 73100 Lecce, Italy