1. Institute of Experimental Physics, SAS, Watsonova 47, 04001 Košice, Slovakia
2. Faculty of Electrical Engineering and Informatics, Technical University of Košice, Letná 9, 04200 Košice, Slovakia
3. Pavol Jozef Šafárik University, Faculty of Science, Park Angelinum 9, 04001 Košice, Slovakia