1. Cavendish Laboratory, University of Cambridge, JJ Thomson Avenue, Cambridge CB3 0HE, UK.
2. Department of Photonics Engineering, Technical University of Denmark, 2800 Kgs. Lyngby, Denmark.
3. EPSRC National Epitaxy Facility, University of Sheffield, Broad Lane, Sheffield S3 7HQ, UK.
4. Université Côte d’Azur, CNRS, CRHEA, rue Bernard Gregory, 06560 Valbonne, France.