1. Department of Microtechnology and Nanoscience, Chalmers University of Technology, 41296 Gothenburg, Sweden
2. Institute of Solid State Physics, Technische Universität Berlin, 10623 Berlin, Germany
3. Ferdinand-Braun-Institut, Leibniz-Institut für Höchstfrequenztechnik, 12489 Berlin, Germany
4. Institute of Optics and Atomic Physics, Technische Universität Berlin, 10623 Berlin, Germany