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