1. Optics & Photonics Technology Laboratory, Ecole Polytechnique Fédérale de Lausanne (EPFL), Neuchâtel 1015, Switzerland
2. Institute
of Theoretical Solid State Physics, Karlsruhe Institute of Technology, Karlsruhe 76131, Germany
3. Institute
of Nanotechnology, Karlsruhe Institute of Technology, Karlsruhe 76021, Germany
4. Institute
of Photonics, University of Eastern Finland, P.O. Box 111, 80101 Joensuu, Finland