1. State Key Laboratory of Information Photonic and Optical Communications, and School of Science, Beijing University of Posts and Telecommunications (BUPT), Beijing 100876, P. R. China
2. College of Chemistry, Beijing Normal University, Beijing 100875, P. R. China
3. Department of Materials Science and Engineering, Southern University of Science and Technology, Shenzhen, Guangdong Province 518055, P. R. China