1. School of Physics and Electronic Information Engineering; Hubei Engineering University; Xiaogan 432000 P. R. China
2. 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 430072 P. R. China
3. College of Physics and Electronic Science; Hubei Normal University; Huangshi 435003 P. R. China