1. Department of Electrical and Computer Engineering, National University of Singapore, 4 Engineering Drive 3, Singapore 117583
2. Center for Sensors and MEMS, National University of Singapore, 4 Engineering Drive 3, Singapore 117583
3. Institute of Microelectronics (IME), Agency for Science, Technology and Research (A*STAR), 11 Science Park Road, Singapore 117685
4. Key Laboratory of Optoelectronic Technology & Systems, Ministry of Education, Chongqing University, Chongqing 400044, People's Republic of China