1. Department of Materials Science and Engineering; City University of Hong Kong; Hong Kong 999077 P. R. China
2. Shenzhen Research Institute; City University of Hong Kong; Shenzhen 518057 P. R. China
3. State Key Laboratory of Terahertz and Millimeter Waves (SKLTMW); City University of Hong Kong; Hong Kong 999077 P. R. China
4. Centre for Functional Photonics (CFP); City University of Hong Kong; Hong Kong 999077 P. R. China