1. a University “Politehnica Bucuresti”, Department of Physics , Splaiul Independentei 313, Bucharest , 77206 , Romania
2. b Institute of Physics, Academy of Sciences of the Czech Republic , Na Slovance 2, 18221, Prague 8 , Czech Republic
3. c Institute of Applied Physics, Military University of Technology , 2 Kaliskiego 00-908, Warsaw , Poland