1. Department of Materials Science and Engineering, City University of Hong Kong, Kowloon 999077, Hong Kong SAR, P. R. China
2. State Key Laboratory of Luminescence and Applications, Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, 3888 Dongnanhu Road, Changchun 130021, P. R. China
3. State Key Laboratory of Terahertz and Millimeter Waves, City University of Hong Kong, Kowloon 999077, Hong Kong SAR, P. R. China
4. Centre for Functional Photonics, City University of Hong Kong, Kowloon 999077, Hong Kong SAR, P. R. China
5. Key Laboratory of Advanced Materials Processing & Mold (Zhengzhou University), Ministry of Education, Zhengzhou 450002, P. R. China