1. Hunan Institute of Optoelectronic Integration and Key Laboratory for MicroNano Physics and Technology of Hunan Province, State Key Laboratory of Chemo/Biosensing and Chemometrics, College of Materials Science and Engineering, Hunan University, Changsha 410082, P. R. China
2. School of Physics and Astronomy, Faculty of Science, Monash University, Melbourne, Victoria 3800, Australia
3. Department of Physics, Imperial College London, London SW7 2AZ, U.K.
4. School of Physics and Electronics, Hunan Normal University, Changsha 410081, P. R. China