1. Department of Electronic and Computer Engineering; The Hong Kong University of Science and Technology; Clear Water Bay Kowloon Hong Kong SAR China
2. Department of Physics; The Hong Kong University of Science and Technology; Clear Water Bay Kowloon Hong Kong SAR China
3. Department of Physics and Key Laboratory of Artificial Micro- and Nano-Structures of Ministry of Education; Wuhan University; Wuhan 430072 China