1. Department of Materials Science and Engineering, City University of Hong Kong 1 , Kowloon, Hong Kong 999077, China
2. Institute for Materials Chemistry and Engineering, Kyushu University 2 , Fukuoka 816-8580, Japan
3. State Key Laboratory of Terahertz and Millimeter Waves, City University of Hong Kong 3 , Kowloon, Hong Kong 999077, China