1. Department of Physics, Shanghai Normal University, 200234 Shanghai, China
2. State Key Laboratory of Infrared Physics, Shanghai Institute of Technical Physics, Chinese Academy of Sciences, 200083 Shanghai, China
3. State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, 200050 Shanghai, China
4. Key Laboratory of polar Materials and Devices, East China Normal University, 200062 Shanghai, China
5. Key Laboratory of Terahertz Solid State Technology, CAS, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, 200050 Shanghai, China
6. Photonic Laboratory, Department of Microtechnology and Nanoscience, Chalmers University of Technology, S-412 96 Göteborg, Sweden