1. Department of Physics, Capital Normal University, Beijing 100048, China; Key Laboratory of Terahertz Optoelectronics, Ministry of Education, Beijing 100048, China; Beijing Key Laboratory for Terahertz Spectroscopy and Imaging, Beijing 100048, China; and Beijing Advanced Innovation Centre for Imaging Technology, Beijing 100048, China
2. School of Materials Science and Engineering, Beijing Institute of Technology, Beijing 100081, China