1. Center for Terahertz waves and College of Precision Instrument and Optoelectronics Engineering Tianjin University and the Key Laboratory of Optoelectronics Information and Technology (Ministry of Education) Tianjin 300072 P. R. China
2. Optoelectronics Research Centre and Centre for Photonic Metamaterials University of Southampton Highfield Southampton SO17 1BJ UK
3. Cavendish Laboratory University of Cambridge J. J. Thomson Avenue Cambridge CB3 OHE UK
4. Guangxi Key Laboratory of Optoelectronic Information Processing Guilin University of Electronic Technology Guilin 541004 P. R. China
5. School of Electrical and Computer Engineering Oklahoma State University Stillwater OK 74078 USA