1. Palacky University, Joint Laboratory of Optics, 17. listopadu 12, 771 46 Olomouc, Czech Republic
2. Institute of Physics of the Czech Academy of Sciences, 182 21, Prague 8, Czech Republic
3. Ioffe Institute, Russian Academy of Sciences, 194021 St. Petersburg, Russia
4. Experimentelle Physik 2, Technische Universität Dortmund, D-44221 Dortmund, Germany