1. Center for Terahertz Waves and College of Precision Instrument and Optoelectronics Engineering Tianjin University Tianjin 300072 P. R. China
2. Optoelectronics Research Centre and Centre for Photonic Metamaterials University of Southampton Highfield Southampton SO17 1BJ UK
3. School of Science Tianjin University Tianjin 300072 P. R. China
4. School of Electrical and Computer Engineering Oklahoma State University Stillwater OK 74078 USA