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 1 , Tianjin 300072, China
2. School of Electrical and Computer Engineering, Oklahoma State University 2 , Stillwater, Oklahoma 74078, USA
3. Guangxi Key Laboratory of Optoelectronic Information Processing, School of Optoelectronic Engineering, Guilin University of Electronic Technology 3 , Guilin 541004, China