1. School of Physics and Telecommunication Engineering; Zhoukou Normal University; Zhoukou 466001 P.R. China
2. Key Laboratory of Optoelectronic Technology of Jiangsu Province; School of Physics and Technology; Nanjing Normal University; Nanjing 210023 P.R. China
3. Faculty of Mathematics and Physics; Huaiyin Institute of Technology; Huaian 223003 P.R. China