1Terahertz Science Cooperative Innovation Center, University of Electronic Science and Technology of China, Chengdu 610054, China
2National Key Laboratory of Application Specific Integrated Circuit, Hebei Semiconductor Research Institute, Shijiazhuang 050051, China
3Center for Terahertz Waves, College of Precision Instrument and Optoelectronics Engineering, Tianjin University, Key Laboratory of Optoelectronic Information Technology (Ministry of Education), Tianjin 300072, China