1. Center for Terahertz Waves and College of Precision Instrument and Optoelectronics Engineering Tianjin University Tianjin 300072 China
2. College of Electronic and Information Engineering Shandong University of Science and Technology Qingdao 266590 P. R. China
3. Centre for Photonic Metamaterials and Optoelectronics Research Centre Zepler Institute University of Southampton Southampton SO17 1BJ UK
4. Research Institute of Superconductor Electronics (RISE) School of Electronic Science and Engineering Nanjing University Nanjing 210093 P. R. China
5. National Synchrotron Radiation Laboratory University of Science and Technology of China Hefei 230029 P. R. China
6. School of Electrical and Computer Engineering Oklahoma State University Stillwater OK 74078 USA