1. Laboratory of Optoelectronics Materials and Devices, 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. Department of Applied Physics, The Hong Kong Polytechnic University, Hong Kong, China
4. The Hong Kong Polytechnic University Shenzhen Research Institute, Shenzhen 518057, China
5. Department of Physics, The State University of New York at Potsdam, Potsdam, New York 13676-2294, USA
6. Center for Optoelectronics Materials and Devices, Department of Physics, Zhejiang Sci-Tech University, Hangzhou, Zhejiang 310018, China