1. Faculté des Sciences, Département de Physique, Université Libre de Bruxelles (U.L.B.), CP. 231, Campus Plaine, 1050 Bruxelles, Belgium
2. Departamento de Física, Facultad de Ciencias Físicas y Matemáticas, Universidad de Chile, Casilla 487-3, Santiago, Chile
3. Department of Applied Physics and Photonics, Brussels Photonics (B-PHOT), Vrije Universiteit Brussel, Pleinlaan 2, 1050 Brussels, Belgium
4. Institute of Solid State Physics, 72 Tzarigradsko Chaussee Boulevard, 1784 Sofia, Bulgaria