1. QTF Centre of Excellence, Department of Electronics and Nanoengineering, Aalto University, Tietotie 3, FI-02150 Espoo, Finland.
2. Beijing National Laboratory for Condensed Matter Physics; Key Laboratory for Nanoscale Physics and Devices, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China.
3. School of Physical Sciences, University of Chinese Academy of Sciences, Beijing 100190, China.
4. Institute of Experimental Physics, Faculty of Physics, University of Warsaw, ul. Pasteura 5, 02-093 Warsaw, Poland.
5. Songshan-Lake Materials Laboratory, Dongguan, Guangdong Province 523808, China.
6. Department of Physics, University of California at Berkeley, Berkeley, CA, USA.
7. Institute of Photonics and Photon Technology, Northwest University, Xi’an 710069, China.