1. Department of Mathematics and Physics, Chongqing University of Posts and Telecommunications, 2 Chongwen Road, Chongqing 400065, China
2. Department of Physics & Astronomy, Georgia Southern University, Statesboro, Georgia 30460, U.S.A
3. State Key Laboratory of Luminescence and Applications, Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, 3888 Eastern South Lake Road, Changchun 130033, China