1. Solid State Physics and NanoLund, Lund University, Box 118, SE-221 00 Lund, Sweden
2. School of Information Technology, Halmstad University, Box 823, SE-301 18 Halmstad, Sweden
3. Institute for Optoelectronics, Friedrich-Alexander Universität Erlangen-Nürnberg, Konrad-Zuse Str. 3/5, D-91052 Erlangen, Germany