1. School of Electrical and Electronic Engineering; Nanyang Technological University; Singapore 639798 Singapore
2. School of Optoelectronic Science and Engineering; University of Electronic Science and Technology; Chengdu 610051 China
3. Institute of Microelectronics; A*STAR; Singapore 117686 Singapore
4. Department of Electrical and Electronic Engineering; Auckland University of Technology; Auckland 1142 New Zealand
5. Research Center for Applied Sciences; Academia Sinica; Taipei 11529 Taiwan
6. Department of Physics; National Taiwan University; Taipei 10617 Taiwan
7. Department of Electrical and Computer Engineering; National University of Singapore; Singapore 117576 Singapore
8. NUS Suzhou Research Institute (NUSRI); Suzhou Industrial Park Suzhou 215123 China