1. Key Laboratory of Terahertz Optoelectronics, Ministry of Education 1 , Beijing 100048, China
2. Beijing Key Laboratory for Terahertz Spectroscopy and Imaging 2 , Beijing 100048, China
3. Beijing Advanced Innovation Centre for Imaging Theory and Technology 3 , Beijing 100048, China
4. Department of Physics, Capital Normal University 4 , Beijing 100048, China