1. Center for Terahertz Waves and College of Precision Instrument and Optoelectronics Engineering, and the Key Laboratory of Optoelectronics Information and TechnologyTianjin University Tianjin 300072 P. R. China
2. Research Institute of Superconductor Electronics (RISE)School of Electronic Science and EngineeringNanjing University Nanjing 210093 P. R. China
3. National Synchrotron Radiation LaboratoryUniversity of Science & Technology of China Hefei 230029 P. R. China
4. School of Electrical and Computer EngineeringOklahoma State University Stillwater OK 74078 USA