1. Center for Terahertz Waves and College of Precision Instrument and Optoelectronics Engineering, Tianjin University and the Key Laboratory of Optoelectronics Information and Technology (Ministry of Education), Tianjin 300072, China
2. School of Physics and Astronomy, University of Birmingham, Birmingham B15 2TT, UK
3. College of Advanced Interdisciplinary Studies, National University of Defense Technology, Changsha 410073, China
4. School of Electrical and Computer Engineering, Oklahoma State University, Stillwater, OK 74078, USA