1. a School of Physics and Electronic Information Yunnan Normal University Kunming 650092, P.R. China
2. b School of Physics and Electronic Information Yunnan Normal University Kunming 650092, P.R. China
3. c School of Physics and Electronic Information Yunnan Normal University Kunming 650092, P.R. China
4. d School of Physics and Electronic Information Yunnan Normal University Kunming 650092, P.R. China