1. Department of Electronic Science and Applied Physics, Kunming University of Science and Technology, Kunming, Yunnan 650500, P. R. China
2. Shizuishan No. 1 Middle School, Shizuishan, Ningxia 753200, P. R. China
3. Department of Applied Mathematics, Kunming University of Science and Technology, Kunming, Yunnan 650500, P. R. China