1. Research Institute for Nuclear Problems, Belarusian State University, Minsk 220030, Belarus
2. Center for Physical Sciences and Technology, A. Gostauto 11, Vilnius LT-01108, Lithuania
3. Institute of Photonics, University of Eastern Finland, Joensuu FI-80101, Finland
4. Physics Department, Universite de Namur, 61 Rue de Bruxelles, B-5000 Namur, Belgium