1. Department of Applied Physics, The Hong Kong Polytechnic University, Kowloon, Hong Kong
2. Department of Physics, The University of Hong Kong, Pokfulam Road, Hong Kong, Hong Kong
3. State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China
4. College of Science, China University of Petroleum (East China), Qingdao 255680, China
5. School of Mathematics and Physics, Suzhou University of Science and Technology, Suzhou 215009, China