1. School of Physics and Optoelectronic Engineering, Dalian Univ. of Technology, Dalian 116024, CN.
2. School of Physics and Optoelectronic Engineering, Dalian Univ. of Technology, Dalian 116024, CN
3. State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, 865 Changning Rd., Shanghai 200050, CN (Corresponding author), e-mail: hwliang@dlut.edu.cn
4. State Key Laboratory on Integrated Optoelectronics, School of Electronic Science and Engineering, Jilin Univ., Changchun 130012, CN.