1. Institute of Physics, Technical University of Lodz, Wolczanska 219, 93-005 Lodz, Poland
2. Department of Applied Physics and Photonics, Vrije Universiteit Brussel, Pleinlaan 2, 1050 Brussels, Belgium
3. Institute of Solid State Physics, 72 Tzarigradsko Chaussee Boulevard, 1784 Sofia, Bulgaria