1. Photonics and Optoelectronics Group, Department of Physics and CeNS, Ludwig-Maximilians-Universität München, Amalienstrasse 54, 80799 Munich, Germany, Institute of Physics and Institute of Micro- and Nanotechnologies, Technical University of Ilmenau, 98684 Ilmenau, Germany, and Roche Diagnostics GmbH, Nonnenwald 2, 82372 Penzberg, Germany