1. Department of Applied Physics, The Hong Kong Polytechnic University, Kowloon, Hong Kong SAR, China
2. Department of Physics and Materials Science, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hong Kong SAR, China
3. School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798
4. School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore 637371