1. Center for Terahertz Waves and College of Precision Instrument and Optoelectronics Engineering; Tianjin University and the Key Laboratory of Optoelectronics Information and Technology; Tianjin (Ministry of Education); Tianjin 300072 P.R. China
2. Advanced Nanofabrication and Imaging Core Lab; King Abdullah University of Science and Technology (KAUST); Thuwal 23955-6900 Saudi Arabia
3. School of Physics and Astronomy; University of Birmingham; Birmingham B15 2TT UK
4. School of Electrical and Computer Engineering; Oklahoma State University; Stillwater OK 74078 USA