1. Department of Physics and Materials Science; City University of Hong Kong; 83 Tat Chee Avenue Kowloon Hong Kong
2. Shenzhen Research Institute; City University of Hong Kong; Shenzhen 518057 P. R. China
3. Key Laboratory of Design and Assembly of Functional Nanostructures; Fujian Institute of Research on the Structure of Matter; Chinese Academy of Sciences; Fuzhou 350108 P. R. China
4. State Key Laboratory of Millimeter Waves; City University of Hong Kong; 83 Tat Chee Avenue Kowloon Hong Kong