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