[1] Solid State Physics Laboratory, Physics Department, Facultés Universitaires Notre-Dame de la Paix, B-5000 Namur, Belgium
[2] Optoelectronics Research Centre, University of Southampton, Southampton SO 17 1BJ, United Kingdom
[3] Optics Group, Physics Department, Martin-Luther-University Halle-Wittenberg, D-06099 Halle, Germany
[4] Photonics and New Materials Division, Institute of Optics, Information und Photonics (Max Planck Research Group), University of Erlangen-Nuremberg, D-91058 Erlangen