1Center for Terahertz Waves and College of Precision Instrument and Optoelectronics Engineering, and Key Laboratory of Optoelectronics Information and Technology, Tianjin University, Tianjin 300072, China
2School of Materials Science and Physics, China University of Mining and Technology, Xuzhou 221116, China
3Optoelectronics Research Centre, University of Southampton, Southampton SO17 1BJ, UK
4Guangxi Key Laboratory of Optoelectronic Information Processing, School of Optoelectronic Engineering, Guilin University of Electronic Technology, Guilin 541004, China
5School of Electrical and Computer Engineering, Oklahoma State University, Stillwater, Oklahoma 74078, USA