1. Tianjin Key Laboratory of Low Dimensional Materials Physics and Preparing Technology, Department of Physics, School of Science, Tianjin University 1 , Tianjin 300354, China
2. Department of Basic Courses, Tianjin Sino-German University of Applied Sciences 2 , Tianjin 300350, China
3. State Key Laboratory of Luminescence and Applications, Changchun Institute of Optics, Fine Mechanics and Physics Chinese Academy of Sciences 3 , Changchun 130033, China