1. Department of Solid State Electronics Faculty of Physics Uzhhorod National University Voloshin Str. 54 Uzhhorod 88000 Ukraine
2. Department of Applied and Nonlinear Optics Institute for Solid State Physics and Optics Wigner Research Centre for Physics Konkoly-Thege Miklós Str. 29-33 Budapest 1121 Hungary
3. Institute of Physics P. J. Šafárik University Park Angelinum 9 Košice 040 00 Slovakia