1. Institute of Materials Research and Engineering, Agency for Science, Technology and Research (A*STAR), 2 Fusionopolis Way, #08-03, Innovis, Singapore 138634, Singapore
2. Centre for Disruptive Photonic Technologies, The Photonic Institute, SPMS, Nanyang Technological University, Singapore 637371, Singapore
3. Department of Physics and Astronomy, University College London, London WC1E 6BT, United Kingdom
4. State Key Laboratory of integrated optoelectronics, College of Electronic Science and Engineering, Jilin University, Changchun, Jilin 130012, China
5. Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore 637371, Singapore
6. Department of Optics and Optical Engineering, University of Science and Technology of China, Hefei, Anhui 230026, China