1. National Institute of Materials Physics, P.O.
Box MG-7, 077125, Bucharest-Magurele, Romania
2. Faculty
of Physics, University of Bucharest, 077125, Bucharest-Magurele, Romania
3. University Politehnica Bucharest, Faculty of Electronics,
Telecommunications and Information Technology, Bucharest, Romania
4. National Institute for Lasers, Plasma and Radiation
Physics, Bucharest-Magurele, Romania