1. Department of Applied Physics, School of Mathematics and Statistics, Hunan University of Technology and Business, Changsha 410205, P. R. China
2. School of Information and Electronic Engineering, Hunan City University, Yiyang 413000, P. R. China
3. School of Physics and Electronics, Central South University, Changsha 410083, P. R. China
4. School of Mechanical Engineering, Hunan Institute of Engineering, Xiangtan, 411104, P. R. China