1. Institute of Physics, Polish Academy of Sciences, Aleja Lotnikow 32/46, PL-02668 Warsaw, Poland
2. Laboratoire Léon Brillouin, CEA Saclay, F-91191 Gif-sur Yvette, France
3. Faculty of Mathematics and Natural Sciences, University of Rzeszów, S. Pigonia 1, PL-35310 Rzeszów, Poland