1. Centre for Micro-Photonics and CUDOS, Faculty of Engineering and Industrial Sciences, Swinburne University of Technology, Hawthorn, Victoria 3122, Australia
2. State Key Laboratory of Optoelectronic Materials and Technologies, School of Physics & Engineering, Sun Yat-Sen University, Guangzhou 510275, China
3. College of Science, South China Agriculture University, Guangzhou 510642, China