1. Institute of Electromagnetic Fields (IEF), ETH Zurich, 8092 Zurich, Switzerland.
2. Maryland NanoCenter, Institute for Research in Electronics and Applied Physics, University of Maryland, College Park, MD 20742, USA.
3. Physical Measurement Laboratory, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA.
4. Micro- and Nanosystems, ETH Zurich, 8092 Zurich, Switzerland.
5. Photonics Laboratory, Department of Microtechnology and Nanoscience, Chalmers University of Technology, Gothenburg, Sweden.