1. Electrical and Computer Engineering; National University of Singapore; 21 Lower Kent Ridge Road Singapore 117576 Singapore
2. College of Materials Science and Engineering; Shenzhen University; 3688 Nanhai Ave Shenzhen 518060 P. R. China
3. Department of Photonics and Institute of Electro-Optical Engineering; National Chiao Tung University; Hsinchu 300 Taiwan
4. Institute of Materials research and Engineering (IMRE); Singapore 138634 Singapore