1. Department of Optics and Optical Engineering, University of Science and Technology of China, Hefei, Anhui 230026, China
2. Institute of Materials Research and Engineering, Agency for Science, Technology and Research (A*STAR), 2 Fusionopolis Way, #08-03, Innovis, Singapore 138634, Singapore
3. Singapore University of Technology and Design, 8 Somapah Road, Singapore 487372, Singapore