1. Department of Experimental Physics, Institute of Physics, Faculty of Science and Technology, University of Debrecen, 4032 Debrecen, Hungary
2. Department of Optics and Quantum Electronics, University of Szeged, 6720 Szeged, Hungary
3. Department of Solid State Physics, Institute of Physics, Faculty of Science and Technology, University of Debrecen, 4032 Debrecen, Hungary