1. School of Physics & Astronomy, University of Birmingham; Birmingham B15 2TT UK
2. Institute of Solid State Physics; Shanxi Datong University; Datong 037009 China
3. Center for Terahertz Waves and College of Precision, Instrument and Optoelectronics Engineering; Tianjin University; Tianjin 300072 China
4. School of Electrical and Computer Engineering; Oklahoma State University; Stillwater Oklahoma 74078 USA