1. Key Laboratory of Terahertz Optoelectronics, Ministry of Education, Beijing Key Laboratory of Metamaterials and Devices, Advanced Innovation Center for Imaging Technology, Department of Physics, Capital Normal University, Beijing 100048, China
2. Institution of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China
3. Key Laboratory of Luminescence and Optical Information, Ministry of Education, Beijing Jiaotong University, Beijing 100044, China