1. School of Science and Engineering, The Chinese University of Hong Kong, Shenzhen 518172, People's Republic of China
2. Hefei National Laboratory for Physical Sciences at the Microscale, Department of Physics, University of Science and Technology of China, Hefei 230026, People's Republic of China
3. College of Physics and Optoelectronics, Taiyuan University of Technology, Taiyuan 030024, People's Republic of China