1. Institute of Physical Chemistry, University of Hamburg, Bundesstr. 45, 20146 Hamburg, Germany, Photonics and Optoelectronics Group, Physics Department and CeNS, University of Munich, Amalienstr. 54, 80799 München, Germany, and Humboldt-Universität zu Berlin, Institut für Physik, Hausvogteiplatz 5-7, 10117 Berlin, Germany