1. Institute of Nanotechnology, Karlsruhe Institute of Technology, P.O. Box 3640, 76021 Karlsruhe, Germany
2. Institute of Applied Physics, Abbe Center of Photonics, Friedrich-Schiller-Universitt Jena, Albert-Einstein-Str. 15, 07745 Jena, Germany
3. Institute of Theoretical Solid State Physics, Karlsruhe Institute of Technology, Wolfgang-Gaede-Str. 1, 76131 Karlsruhe, Germany
4. Fraunhofer Institute for Applied Optics and Precision Engineering IOF, Albert-Einstein-Str. 7, 07745 Jena, Germany