1. Xinjiang Key Laboratory of Solid State Physics and Devices, College of Physics Science and Technology, Xinjiang University, Urumqi 830046, P. R. China
2. School of Physics and Technology, State Key Laboratory of Chemistry and Utilization of Carbon Based Energy Resources, Xinjiang University, Urumqi 830046, P. R. China
3. School of Physics and Electronics, Hunan Key Laboratory for Super-Microstructure and Ultrafast Process and Hunan Key Laboratory of Nanophotonics and Devices, Central South University, Changsha 410083, P. R. China