1. Center for Terahertz Waves, College of Precision Instrument and Optoelectronics Engineering, Tianjin University, Key Laboratory of Optoelectronic Information Technology (Ministry of Education of China), 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