1. Center for Terahertz Waves and College of Precision Instrument and Optoelectronics EngineeringTianjin University Tianjin 300072 China
2. Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and Guangdong ProvinceCollege of Optoelectronics EngineeringShenzhen University Shenzhen 518060 China
3. School of Physics and AstronomyUniversity of Birmingham Birmingham B15 2TT UK
4. Guangxi Key Laboratory of Automatic Detecting Technology and InstrumentsGuilin University of Electronic Technology Guilin 541004 China
5. Physical Science and Engineering DivisionKing Abdullah University of Science and Technology Thuwal 23955‐6900 Saudi Arabia
6. School of Electrical and Computer EngineeringOklahoma State University Stillwater OK 74078 USA