1. Department of Applied Physics The Hong Kong Polytechnic University Hung Hom Kowloon Hong Kong P. R. China
2. The Hong Kong Polytechnic University Shenzhen Research Institute Shenzhen 518057 P. R. China
3. School of Electronic Science and Engineering Nanjing University Nanjing 210093 P. R. China
4. Department of Electronic and Computer Engineering Hong Kong University of Science and Technology Clear Water Bay Kowloon Hong Kong P. R. China