1. Department of Physics, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon 999077, Hong Kong
2. Department of Physics, University of Science and Technology of China, Hefei, Anhui 230026, China
3. Department of Electronic and Computer Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon 999077, Hong Kong
4. College of Physics and Optoelectronic Engineering, Shenzhen University, 3688 Nanhai Ave, Shenzhen 518060, China