1. Center for Terahertz Waves and College of Precision Instrument and Optoelectronics Engineering, and the Key Laboratory of Optoelectronics Information and Technology (Ministry of Education), Tianjin University, Tianjin 300072, China
2. College of Physics and Materials Science, Tianjin Normal University, Tianjin 300387, China
3. School of Electrical and Computer Engineering, Oklahoma State University, Stillwater, Oklahoma 74078, United States