1. Key Laboratory of Polar Materials and Devices
2. Ministry of Education, and Department of Electronic Engineering
3. East China Normal University
4. Shanghai 200241
5. P. R. China
6. Engineering Research Center for Nanophotonics & Advanced Instrument
7. Ministry of Education, and Department of Physics
8. Shanghai 200062
9. China
10. Department of Physics and Material Science
11. City University of Hong Kong
12. Kowloon