1. Photonics and Optoelectronics Group, Physics Department and CeNS, Ludwig-Maximilians-Universität München, Amalienstrasse 54, 80799 Munich, Germany, Roche Diagnostics GmbH, Nonnenwald 2, 82372 Penzberg, Germany, and Cavendish Laboratory, University of Cambridge, J J Thomson Avenue, Cambridge CB3 0HE, United Kingdom