1. School of Materials Science & Engineering; Beijing Institute of Technology; 5 Zhongguancun South Street, Haidian District Beijing 100081 China
2. Wuhan National High Magnetic Field Center and School of Physics; University of Science and Technology; Wuhan 430074 China
3. Department of Physics and Materials Science & Centre for Functional Photonics (CFP); City University of Hong Kong; 83 Tat Chee Avenue Kowloon Hong Kong
4. Department of Electronic Engineering; East China Normal University; Shanghai 200241 China