1. Department of Physics, Shanghai University, Shanghai 200444, China
2. Laboratory of Mircostructure, Shanghai University, Shanghai 200444, China
3. Key Laboratory of Polar Materials and Devices (MOE), Department of Electronics, East China Normal University, Shanghai 200241, China
4. Institute of Laser and Opto-electronics, College of Precision Instrument and Opto-electronics Engineering, Tianjin University, Tianjin 300072, China