1. Center for Terahertz waves and College of Precision Instrument and Optoelectronics Engineering, Tianjin University and the Key Laboratory of Optoelectronics Information and Technology (Ministry of Education), Tianjin 300072, China
2. School of Electronic Engineering, Tianjin University of Technology and Education, Tianjin 300222, China
3. Guangxi Key Laboratory of Optoelectronic Information Processing, School of Optoelectronic Engineering, Guilin University of Electronic Technology, Guilin 541004, China
4. School of Electrical and Computer Engineering, Oklahoma State University, Stillwater, Oklahoma 74078, USA