1. Jiangsu Provincial Engineering Research Center of Low Dimensional Physics and New Energy, Research Center of Information Physics and College of Science, Nanjing University of Posts and Telecommunications, Nanjing 210023, P. R. China
2. College of Electronic and Optical Engineering, Nanjing University of Posts and Telecommunications, Nanjing 210023, P. R. China
3. National Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, P. R. China
4. Department of Physics, Mathematics & Science College, Shanghai Normal University, Shanghai 200234, P. R. China