1. Science Institute and Faculty of Physical Sciences, Univ. of Iceland, 107 Reykjavík, Iceland
2. St. Petersburg National Research University of Information Technologies, Mechanics and Optics, St. Petersburg, 197101, Russia
3. Dept. of Materials and Nanophysics, Electrum 229, Royal Institute of Technology (KTH), SE-16440 Kista, Sweden
4. Department of Physics, St. Petersburg State University, St. Petersburg, 198504, Russia
5. Department of Applied Physics, Aalto University, Espoo, FIN-00076, Finland