1. Department of Mechanical and Automation Engineering, The Chinese University of Hong Kong, Shatin, Hong Kong
2. Key Lab of Semiconductor Materials Science, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, P. R. China
3. Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing 100049, P. R. China
4. Key Laboratory of In-Fiber Integrated Optics of Ministry of Education, Harbin Engineering University, Harbin 150001, China
5. Joint Lab of Digital Optical Chip, Wuyi University, Jiangmen, 529020, P. R. China