1. Department of Physics, South University of Science and Technology of China, Shenzhen 518055, People's Republic of China
2. City University of Hong Kong Shenzhen Research Institute, Shenzhen, People's Republic of China
3. Department of Physics and Materials Science and Center for Functional Photonics, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, People's Republic of China