1. Department of Physics, The Hong Kong University of Science and Technology 1 , Hong Kong, China
2. Department of Electrical Engineering, City University of Hong Kong 2 , Hong Kong, China
3. State Key Laboratory of Terahertz and Millimeter Waves, City University of Hong Kong 3 , Hong Kong, China
4. 4 William Mong Institute of Nano Science and Technology, The Hong Kong University of Science and Technology, Hong Kong, China