1. Pico group, QTF Centre of Excellence, Department of Applied Physics, Aalto University School of Science, P.O. Box 13500, 00076 Aalto, Finland
2. Department of Microtechnology and Nanoscience, Chalmers University of Technology, 412 96 Gothenburg, Sweden
3. Moscow Institute of Physics and Technology, 141700 Dolgoprudny, Russia
4. Department of Physics, Chemistry and Biology, Linkoping University, 581 83 Linköping, Sweden