1 State Key Laboratory of Information Photonics and Optical Communications, Beijing University of Posts and Telecommunications, Beijing 100876, China
2 State Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources, North China Electric Power University, Beijing 102206, China
3 Laboratory of Nano Optoelectronics, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China
4 Engineering Research Center for Semiconductor Integrated Technology, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China
5 Department of Electronic & Electrical Engineering, University College London, London WC1E 7JE, UK