1. Center for Terahertz Waves and College of Precision Instruments and Optoelectronics Engineering, and Key Laboratory of Optoelectronic Information Technology (Ministry of Education) , Tianjin University , Tianjin 300072 , China
2. School of Physics and Electronics , Central South University , Changsha 410083 , China
3. School of Electrical and Computer Engineering , Oklahoma State University , Stillwater , Oklahoma 74078 , USA