1. Experimentelle Physik 2, Technische Universität Dortmund, 44227 Dortmund, Germany
2. Institute of Physics, Polish Academy of Sciences, 02668 Warsaw, Poland
3. Institute of Physics, Czech Academy of Sciences, 182 21 Prague 8, Czech Republic
4. Palacky University, RCPTM, Joint Laboratory of Optics, 771 46 Olomouc, Czech Republic
5. Naval Research Laboratory, Washington, DC 20375, USA
6. Ioffe Institute, Russian Academy of Sciences, 194021 St. Petersburg, Russia