1. Key Laboratory for Micro-Nano Physics and Technology of Hunan Province, School of Physics and Electronics, Hunan University 1 , Changsha, Hunan 410082, People's Republic of China
2. Hunan Institute of Optoelectronic Integration, Hunan University 2 , Changsha 410082, China
3. Key Laboratory for Micro-Nano Physics and Technology of Hunan Province, College of Materials Science and Engineering, Hunan University 3 , Changsha, Hunan 410082, People's Republic of China
4. School of Physics and Electronics, Hunan Normal University 4 , Changsha, Hunan 410081, People's Republic of China