1. Department of Physics and Astronomy, University of Turku, FI-20014 Turku, Finland
2. Department of Electronics and Nanoengineering, Aalto University, FI-02150 Espoo, Finland
3. Department of Future Technologies, University of Turku, FI-20014 Turku, Finland
4. Ioffe Physical-Technical Institute, Russian Academy of Sciences, St. Petersburg 194021, Russian Federation