1. Department of Physics, Harbin Institute of Technology, Harbin 150001, 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. School of Materials Science and Engineering, UNSW Australia, Sydney, NSW 2052, Australia.
4. Key Laboratory of In-Fiber Integrated Optics of Ministry of Education, College of Science, Harbin Engineering University, Harbin 150001, P. R. China.
5. College of Precision Instruments and Optoelectronics Engineering, Tianjin University, Tianjin 300072, P. R. China.
6. Condensed Matter Science and Technology Institute, Harbin Institute of Technology, Harbin 150001, P. R. China.
7. Materials Research Institute, The Pennsylvania State University, University Park, PA 16802, USA.