1. Key Lab of Artificial Micro- and Nano-Structures of Ministry of Education of China
2. School of Physics and Technology
3. Wuhan University
4. Wuhan 430072
5. P. R. China
6. Department of Physics
7. College of Science
8. Huazhong Agricultural University
9. Wuhan 430070
10. School of Physics and Electronic-Information Engineering
11. Hubei Engineering University
12. Xiaogan