1. ICFO-The Institute of Photonic Sciences, The Barcelona Institute of Science and Technology, 08860 Castelldefels (Barcelona), Spain
2. Unidad de Materiales y Dispositivos Optoelectronics (UMDO), Instituto de Ciencias de los Materiales de la Universidad de Valencia (ICMUV), P.O. Box 22085, 46071 Valencia, Spain
3. Department of Physics and Astronomy and ARC Centre for Engineered Quantum Systems, Macquarie University, Sydney, New South Wales, 2109 Australia