1. Department of Electrical Engineering, City University of Hong Kong 1 , Kowloon, Hong Kong 999077, People's Republic of China
2. Centre for Biosystems, Neuroscience, and Nanotechnology, City University of Hong Kong 2 , Kowloon, Hong Kong 999077, People's Republic of China
3. The State Key Laboratory of Terahertz and Millimeter Waves, City University of Hong Kong 3 , Kowloon, Hong Kong 999077, People's Republic of China