1. Department of Applied Physics and Photonics, Vrije Universiteit Brussel, Pleinlaan 2, 1050 Brussels, Belgium
2. Laboratory of Optics and Spectroscopy, Institute of Solid State Physics, 72 Tzarigradsko Chaussee Boulevard, 1784 Sofia, Bulgaria
3. Supélec, Optics & Electronics (OPTEL), LMOPS EA-4423, 2 rue Edouard Belin, F-57070 Metz, France
4. Department of Electrical and Electronic Engineering, Public University of Navarra, 31006 Pamplona, Spain