1 Photonics Research Institute, Department of Electrical Engineering, The Hong Kong Polytechnic University, Kowloon, Hong Kong, China
2 Department of Electrical Engineering, Centre for Biosystems, Neuroscience, and Nanotechnology, City University of Hong Kong, Kowloon, Hong Kong, China
3 State Key Laboratory of Terahertz and Millimeter Waves, City University of Hong Kong, Kowloon, Hong Kong, China
4 Photonics Research Institute, Department of Electronic and Information Engineering, The Hong Kong Polytechnic University, Kowloon, Hong Kong, China
5 e-mail: yaoliang@m.scnu.edu.cn
6 e-mail: dptsai@cityu.edu.hk
7 e-mail: changyuan.yu@polyu.edu.hk