1. Key Laboratory for Micro-Nano Physics and Technology of Hunan Province, College of Materials Science and Engineering, School of Physics and Electronics, and State Key Laboratory of Chemo/Biosensing and Chemometrics, Hunan University, Changsha, Hunan 410082, P.R. China
2. School of Physics and Telecommunication Engineering, Yulin Normal University, Yulin, Guangxi 537000, P.R. China