1. 4th Physics Institute and Stuttgart Research Center of Photonic Engineering, University of Stuttgart, 70569 Stuttgart, Germany.
2. Faculty of Physics and Center for Nanointegration Duisburg-Essen, University of Duisburg-Essen, Lotharstrasse 1-21, 47057 Duisburg, Germany.
3. Department of Electrical Engineering, Technion, Israel Institute of Technology, 32000 Haifa, Israel.
4. Institute of Applied Optics, University of Stuttgart, Pfaffenwaldring 9, 70569 Stuttgart, Germany.
5. School of Physics, University of Melbourne, Parkville, Victoria 3052, Australia.