1Center for Terahertz Waves and College of Precision Instrument and Optoelectronics Engineering, Tianjin University, and the Key Laboratory of Optoelectronics Information and Technology Tianjin, Ministry of Education of China, Tianjin 300072, China
2Physical Science and Engineering Division, King Abdullah University of Science and Technology, Thuwal 23955-6900, Saudi Arabia
3School of Electrical and Computer Engineering, Oklahoma State University, Stillwater, Oklahoma 74078, USA