1. Joint Laboratory of Optics, Palacky University, 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. Technische Universität Dortmund, Experimentelle Physik 2, D-44221 Dortmund, Germany