1. Department of Physical Electronics and Technology, St. Petersburg Electrotechnical University, St. Petersburg 197376, Russia
2. Fachbereich Physik and Landesforschungszentrum OPTIMAS, Technische Universität Kaiserslautern, Kaiserslautern 67663, Germany
3. Department of Mathematics and Physics, Lappeenranta University of Technology, Lappeenranta 53850, Finland