1. Dep. of Theoretical Electrotechnics and Electrical Measurement, Technical University, Park Komenského 3, 043 89 Košice, Slovakia
2. Institue of Experimental Physics, Slovak Academy of Sciences, 043 53 Košice, Slovakia
3. Institute of Physics, A. Mickiewicz University, Umultowska 85, 61-614 Poznań, Poland
4. International Laser Centre, Ilkovičova 3, 812 19 Bratislava, Slovakia
5. Institute of Physics, P.J. Šafárik University, Park Angelinum, 041 54 Košice, Slovakia