1. School of Electrical and Electronic Engineering, Nanyang Technological University, 639798 Singapore
2. The National Key Laboratory on High Power Semiconductor Lasers, Changchun University of Science and Technology, Changchun 130022, China
3. State Key Lab for Superlattices and Microstructures, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China
4. State Key Lab of Luminescence and Applications, Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, Changchun 130033, China