1. Technical University of Denmark, DTU Fotonik, Ørsteds Plads 343, 2800 Kongens Lyngby, Denmark
2. Chinese Academy of Sciences, Institute of Semiconductors, A35 Qinghua East Road, Haidian, Beijing 100083, China
3. Technical University of Denmark, DTU Nanotech, Ørsteds Plads 345, 2800 Kongens Lyngby, Denmark
4. Lund University, Division of Solid State Physics and The Nanometer Structure Consortium, Box 118, SE-22100 Lund, Sweden