1. Optoelectronics Research Centre, Physics Unit, Tampere University, Tampere 33720, Finland
2. École Polytechnique Fédérale de Lausanne, Laboratory of Physics of Nanostructures, Institute of Physics, 1015 Lausanne, Switzerland
3. Univ Rennes, CNRS, Institut FOTON - UMR 6082, F-35000 Rennes, France
4. LakeDiamond SA, Rue Galilée 7, 1400 Yverdon-les-Bains, Switzerland