1. VI Systems GmbH, Hardenbergstr. 7, Berlin D-10623, Germany
2. Optoelectronics Research Centre, Tampere University of Technology, Tampere FI-33720, Finland
3. A. F. Ioffe Physical Technical Institute of the Russian Academy of Sciences, Politekhnicheskaya 26, St. Petersburg 194021, Russia
4. Institut für Festkörperphysik, Technische Universität Berlin, Hardenbergstrasse 36, Berlin D-10623, Germany