1Laboratoire de Physique de la Matiére Condensée, Ecole Polytechnique, Palaiseau, France
2Optoelectronics Research Group, University of Glasgow, Glasgow, UK
3School of Physics and Astronomy, University of Saint Andrews, Fife, UK
4Institut de Micro et Opto-éectronique, Ecole Polytechnique Fédérale de Lausanne, Lausanne, Switzerland