1. State Key Laboratory of Artificial Microstructure and Mesoscopic Physics, School of Physics, Peking University, Beijing 100871, P.R. China
2. Collaborative Innovation Center of Quantum Matter, Beijing 100871, P.R. China
3. Nano-optoelectronics Frontier Center of Ministry of Education, Peking University, Beijing 100871, P.R. China
4. Department of Physics and Key Laboratory of Artificial Mircro- and Nano-structures of Ministry of Education, Wuhan University, Wuhan 430072, P.R. China
5. State Key Laboratory for Chemo/Biosensing and Chemometrics, College of Chemistry and Chemical Engineering, and School of Physics and Electronics, Hunan University, Changsha 410082, P.R. China