1. Yunnan Key Laboratory of Opto-electronic Information Technology, Yunnan Normal University, Kunming 650500, P. R. China
2. School of Physics and Electronics, Qian Nan Normal College for Nationalities, Duyun, Guizhou 558000, P. R. China
3. College of Engineering and Applied Sciences, Nanjing University, Nanjing 210093, P. R. China