1. School of Materials Science and Engineering, Nanyang Technological University 50, Nanyang Avenue, 637819 Singapore
2. Department of Engineering and System Science, National Tsing Hua University, 101, section 2 Kuang Fu Road, Hsinchu 300, Taiwan
3. Photonics Research Centre, Microelectronics Division, School of Electrical and Electronic Engineering, Nanyang Technological University, 639798 Singapore