1. Center for Terahertz Waves, Key Laboratory of Optoelectronics Information and Technology, Ministry of Education, Tianjin University, Tianjin 300072, China
2. School of Precision Instrument and Optoelectronics Engineering, Tianjin University, Tianjin 300072, China
3. Georgia Tech Shenzhen Institute (GTSI), Tianjin University, Shenzhen 518067, China