1. School of Science, Beijing University of Posts and Telecommunications, Beijing 100876, China
2. State Key Laboratory of Information Photonics and Optical Communications, Beijing University of Posts and Telecommunications, Beijing 100876, China
3. Physics Department, The State University of New York at Potsdam, Potsdam, New York 13676-2294, USA
4. Center for Optoelectronics Materials and Devices, Department of Physics, Zhejiang Sci-Tech University, Hangzhou, 310018 Zhejiang, China