Beijing Institute of Techonology, Beijing, China,Tsinghua National Laboratory for Information Science and Technology,State Key Laboratory on Integrated Optoelectronics,Department of Electronic Engineering,Tsinghua University,Beijing Key Laboratory for Terahertz Spectroscopy and Imaging,Key Laboratory of Terahertz Optoelectronics,Ministry of Education,Department of Physics,Capital Normal University,Beijing, China,Beijing Key Laboratory for Terahertz Spectroscopy and Imaging,Key Laboratory of Terahertz Optoelectronics,Ministry of Education,Department of Physics,Capital Normal University,Beijing, China,Tsinghua National Laboratory for Information Science and Technology,State Key Laboratory on Integrated Optoelectronics,Department of Electronic Engineering,Tsinghua University,Beijing Key Laboratory for Terahertz Spectroscopy and Imaging,Key Laboratory of Terahertz Optoelectronics,Ministry of Education,Department of Physics,Capital Normal University,Beijing, China