1. National Laboratory for Infrared Physics, Shanghai Institute of Technical Physics, Chinese Academy of Sciences, 200083 Shanghai, China
2. State Key Laboratory for Superlattices and Microstructures, Institute of Semiconductors, Chinese Academy of Sciences, 100083 Beijing, China
3. Key Laboratory of Polar Materials and Devices, Ministry of Education, East China Normal University, 200062 Shanghai, China
4. Physics Department, Shanghai University, 200444 Shanghai, China