1. Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
2. Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing 100049, China
3. School of Physical Sciences, University of Chinese Academy of Sciences, Beijing 100190, China
4. Optical and Microwave Technology Research Department, Huawei Technologies Co., Ltd., Shenzhen 518129, China
5. Department of Electronic and Electrical Engineering, University College London, London WC1E 7JE, UK