1. Center for Terahertz Waves and College of Precision Instrument and Optoelectronics Engineering Key Laboratory of Optoelectronic Information Technology (Ministry of Education of China) Tianjin University Tianjin 300072 China
2. Cavendish Laboratory University of Cambridge J. J. Thomson Avenue Cambridge CB3 OHE UK
3. School of Electronic Engineering Tianjin University of Technology and Education Tianjin 300222 China
4. Guangxi Key Laboratory of Optoelectronic Information Processing School of Optoelectronic Engineering Guilin University of Electronic Technology Guilin 541004 China
5. School of Electrical and Computer Engineering Oklahoma State University Stillwater OK 74078 USA