1. Optics Laboratory, Institute of Physics, Tampere University of Technology, P.O. Box 692, FIN-33101 Tampere, Finland
2. Department of Physics, University of Joensuu, P.O. Box 111, FIN-80101 Joensuu, Finland
3. Institute of Materials Chemistry, Tampere University of Technology, P.O. Box 541, FIN-33101 Tampere, Finland