1. Department of Experimental Physics, Faculty of Fundamental Problems of Technology, Wrocław University of Science and Technology, Wybrzeże Wyspiańskiego 27, 50-370 Wrocław, Poland
2. Nanores, Bierutowska 57-59, 51-317 Wrocław, Poland
3. Department of Optical Spectroscopy, W. Trzebiatowski Institute of Low Temperature and Structural Research of the Polish Academy of Sciences, Okólna 2, Wroclaw 50-422, Poland