1. Key Laboratory of Luminescence and Optical Information, Ministry of Education, School of Physical Science and Engineering, Beijing Jiaotong University, Beijing 100044, China
2. Institute of Optoelectronics Technology, Beijing Jiaotong University, Beijing 100044, China
3. Sivananthan Laboratories, Bolingbrook, Illinois 60440, United States
4. Department of Physics and Astronomy, Washington State University, Pullman, Washington, 99164, United States