1. Jiangsu Provincial Key Laboratory of Advanced Photonic and Electronic MaterialsSchool of Electronic Science and EngineeringNanjing National Laboratory of MicrostructuresNanjing University Nanjing 210093 P. R. China
2. College of Engineering and Applied SciencesNanjing National Laboratory of MicrostructuresNanjing University Nanjing 210093 P. R. China
3. College of Electronic Science and EngineeringNanjing University of Posts and Telecommunications Nanjing 210023 P. R. China
4. Xiamen University Xiamen 316005 P. R. China