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