1. Jiangsu Provincial Key Laboratory of Advanced Photonic and Electronic Materials, School of Electronic Science and EngineeringNanjing University Nanjing 210093 P. R. China
2. School of PhysicsNanjing University Nanjing 210093 P. R. China
3. National Laboratory of Solid State Microstructures, Collaborative Innovation Center of Advanced MicrostructuresNanjing University Nanjing 210093 P. R. China