1. Guangxi Key Laboratory of Optoelectronic Information Processing, School of Optoelectronic Engineering , Guilin University of Electronic Technology , Guilin 541004 , China
2. Beijing National Laboratory for Condensed Matter Physics, Chinese Academy of Sciences , Institute of Physics , Beijing 100190 , China
3. College of Electronic and Information Engineering , Shandong University of Science and Technology , Qingdao 266590 , P. R. China
4. Center for Terahertz Waves and College of Precision Instrument and Optoelectronics Engineering , Tianjin University , Tianjin 300072 , China