1. Hubei Key Laboratory of Optical Information and Pattern Recognition, Wuhan Institute of Technology, Wuhan, Hubei 430205, P. R. China
2. School of Electronics and Information Engineering, Hubei University of Science and Technology, Xianning, Hubei 437100, P. R. China
3. College of Science, Huazhong Agricultural University, Wuhan, Hubei 430070, P. R. China
4. School of Physics and Technology, Key Laboratory of Artificial Micro- and Nano-structures of the Ministry of Education and School of Physics and Technology, Wuhan University, Wuhan, Hubei 430072, P. R. China