1. Center for Terahertz Waves and College of Precision Instrument and Optoelectronics Engineering; and the Key Laboratory of Optoelectronics Information and Technology; Tianjin University; Tianjin 300072 P. R. China
2. Physical Science and Engineering Division; King Abdullah University of Science and Technology; Thuwal 23955-6900 Saudi Arabia
3. Guangxi Key Laboratory of Automatic Detecting Technology and Instruments; Guilin University of Electronic Technology; Guilin 541004 P. R. China
4. School of Electrical and Computer Engineering; Oklahoma State University; Stillwater OK 74078 USA