1. Center for Terahertz Waves, 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 OK 74078 USA