1. Center for Terahertz Waves and College of Precision Instrument and Optoelectronics Engineering Key Laboratory of the Ministry of Education on Optoelectronics Information Technology Tianjin University Tianjin 300072 China
2. Guangxi Key Laboratory of Optoelectronic Information Processing School of Optoelectronic Engineering Guilin University of Electronic Technology Guilin 541004 China
3. School of Electrical and Computer Engineering Oklahoma State University Stillwater OK 74078 USA