1. Foundation for Research and Technology−Hellas, Institute of Electronic Structure and Laser, P.O. Box 1527, Heraklion Crete, Greece
2. University of Patras, Physics Department, 265 00 Rio Patras, Greece
3. Laboratoire Léon Brillouin (CEA-CNRS), C.E.N. Saclay, 91391 Gif-Sur-Yvette Cedex, France
4. Groningen University, Chemistry Department and Materials Science Center, 9747 AG Groningen, The Netherlands