1. Department of Physics and Department of Chemistry, National University of Singapore, 2 Science Drive 3, Singapore, 117542
2. Centre for Micro-Photonics, Faculty of Engineering and Industrial Science, Swinburne University of Technology, Hawthorn, Victoria 3122, Australia
3. College of Material Science and Engineering & State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, Donghua University, Shanghai 201620, China