1. State Key Laboratory of Information Photonics and Optical Communications School of Science Beijing University of Posts and Telecommunications Beijing 100876 P. R. China
2. State Key Laboratory of New Ceramics and Fine Processing School of Materials Science and Engineering Tsinghua University Beijing 100084 P. R. China
3. Department of Orthodontics Peking University School and Hospital of Stomatology Beijing 100081 P. R. China
4. High Magnetic Field Laboratory Hefei National Laboratory for Physical Sciences at the Microscale Chinese Academy of Sciences Hefei 230031 P. R. China