1. Division of Spectroscopy of Functional Materials, Institute of Physics, University of Zielona Góra, 4a Szafrana Str., 65-516 Zielona Góra, Poland and Spectroscopy Sector, Institute of Physical Optics, 23 Dragomanov Str., 79-005 Lviv, Ukraine
2. Institute of Physics, University of Szczecin, 15 Wielkopolska Str., 70-451 Szczecin, Poland