1. Center for Terahertz waves and College of Precision Instrument and Optoelectronics Engineering, Key Laboratory of Optoelectronics Information and Technical Science, Ministry of Education, Tianjin University, Tianjin 300072, People's Republic of China
2. Division of Physical Science and Engineering, King Abdullah University of Science and Technology, ThuWal 23955-6900, Saudi Arabia
3. School of Electrical and Computer Engineering, Oklahoma State University, Stillwater, Oklahoma 74078, USA