1. National Laboratory for Infrared Physics, Shanghai Institute of Technical Physics, Chinese Academy of Sciences; 200083 Shanghai P.R. China
2. Research Center for Advanced Materials and Devices, Shanghai Institute of Technical Physics, Chinese Academy of Sciences; 200083 Shanghai P.R. China
3. Key Laboratory of Polar materials and Devices, Ministry of Education, East China Normal University; 200062 Shanghai P.R. China
4. Physics Department, Shanghai University; 200444 Shanghai P.R. China