1. Laboratory of Semiconductor Materials, Institute of Materials, Faculty of Engineering, École Polytechnique Fédérale de Lausanne, 1015 Lausanne, Switzerland
2. Photon Science Institute, Department of Electrical and Electronic Engineering, University of Manchester, Alan Turing Building, Oxford Road, Manchester M13 9PL, UK
3. Institute of Physics, Faculty of Basic Sciences, École Polytechnique Fédérale de Lausanne, 1015 Lausanne, Switzerland