[1] Fisika Aplikatua II Saila, Zientzi Fakultatea, Euskal Herriko Unibertsitatea, PK 644, 48080 Bilbo, Spain
[2] Institute of Physics, Acad of Sci of the Czech Republic, Na Slovance 2, CZ-18040 Praha 8, Czech Republic
[3] Laboratoire Léon Brillouin, Centre d'Etudes de Saclay, 91191 Gif-sur-Yvette Cédex, France