1. Institute of Physics, University of Zielona Góra, ul. Szafrana 4a, 65-069 Zielona Góra, Poland
2. Department of Solid State Physics, University of Athens, Panepistimiopolis, 15 784 Athens, Greece
3. Institute of Physics, West Pomeranian University of Technology, Al. Piastow 17, 70-310 Szczecin, Poland
4. Faculty of Biological Sciences, University of Zielona Góra, ul. Szafrana 1, 65-516 Zielona Góra, Poland
5. Sector of Spectroscopy, Institute of Physical Optics, Dragomanov Street 23, 79-005 Lviv, Ukraine
6. Institute of Physics, Wroclaw University of Technology, Wyb. Wyspiańskiego 27, 50-370 Wroclaw, Poland