1. College of Physics and Electronic Engineering, Taishan University, Taian, Shandong 271000, P. R. China
2. Institute of Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Soochow University, Suzhou, Jiangsu 215123, P. R. China
3. School of Physics, Peking University, Beijing 100871, P. R. China
4. Key Laboratory of In-fiber Integrated Optics of Ministry of Education, College of Physics and Optoelectronic Engineering, Harbin Engineering University, Harbin 150001, P. R. China