1. National Laboratory of Solid State Micro-Structures and Department of Physics, Collaborative Innovation Center of Advanced Micro-Structures, Nanjing University, Nanjing 210093, P.R.China
2. School of Information and Engineering, Zhejiang Ocean University, Zhoushan 316000, P.R.China
3. Analysis and Testing Center, Soochow University, 199 Renai Road, Suzhou 215123, P.R.China