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 of Integrated Optoelectronics, College of Electronic Sciences and Engineering, Jilin University, 2699 Qianjin Street, Changchun, 130012, P.R. China