1. Division of Nanophotonics CAS Center for Excellence in Nanoscience National Center for Nanoscience and Technology Beijing 100190 China
2. University of Chinese Academy of Sciences Beijing 100049 P. R. China
3. Department of Electronics and Nanoengineering Aalto University Tietotie 3 FI‐02150 Espoo Finland
4. QTF Centre of Excellence Department of Applied Physics Aalto University FI‐00076 Aalto Finland
5. Department of Electrical and Computer Engineering University of Minnesota Keller Hall 200 Union St S.E. Minneapolis MN 55455 USA
6. ICFO‐The Institute of Photonic Sciences The Barcelona Institute of Science and Technology 08860 Barcelona Spain
7. ICREA‐Institució Catalana de Recerca I Estudis Avancąts Passeig Lluís Companys 23 08010 Barcelona Spain
8. IBM T. J. Watson Research Center Yorktown Heights NY 10598 USA