1. Department of Materials Science and Engineering, City University of Hong Kong, Kowloon 999077, Hong Kong SAR.
2. State Key Laboratory of Terahertz and Millimeter Waves, City University of Hong Kong, Kowloon 999077, Hong Kong SAR.
3. Centre for Functional Photonics, City University of Hong Kong, Kowloon 999077, Hong Kong SAR.
4. School of Optoelectronic Science and Engineering, University of Electronic Science and Technology of China, Chengdu 610054, P. R. China.
5. Shenzhen Research Institute, City University of Hong Kong, Shenzhen 518057, P. R. China.
6. Department of Applied Chemistry, School of Engineering, University of Tokyo, Tokyo 113-8654, Japan.
7. Institute for Materials Chemistry and Engineering, Kyushu University, Fukuoka 816-8580, Japan.