1. Department of Materials Science and Engineering; College of Engineering; Peking University; Beijing 100871 P. R. China
2. Key Laboratory for Photonic and Electronic Bandgap Materials; Ministry of Education; School of Physics and Electronic Engineering; Harbin Normal University; Harbin 150025 Heilongjiang P. R. China
3. BIC-ESAT; College of Engineering; Peking University; Beijing 100871 P. R. China