1. Institute of Physics, A. Gostauto 12, Vilnius 2600, Lithuania, Institute of Materials Science and Applied Research, Vilnius University, Naugarduko 24, Vilnius 2006, Lithuania, Department of General Physics and Spectroscopy, Vilnius University, Sauletekio 9, corp. 3, Vilnius 2054, Lithuania, and 4 CEA/ Saclay, DSM/DRECAM/SCM/URA331 CNRS, 91191 Gif-sur-Yvette Cedex, France