1. Key Laboratory of Excited State Physics, Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, and Graduate school of Chinese Academy of Sciences, 16 Eastern Nan-Hu Road, Changchun 130033, P.R. China, and State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, Changchun 130012, P.R. China