1. National Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, China
2. Jiangsu Key Laboratory of Artificial Functional Materials, Department of Materials Science and Engineering, Nanjing University, Nanjing 210093, China
3. Department of Mathematics and Physics, Nanjing Institute of Technology, Nanjing 211167, China
4. College of Electronic and Optical Engineering, Nanjing University of Posts and Telecommunications, Nanjing 210046, China