1. Microsystem & Terahertz Research Center, CAEP, Chengdu 610200, P. R. China
2. Terahertz Technology Innovation Research Institute, Terahertz Spectrum and Imaging Technology Cooperative Innovation Center, Shanghai Key Lab of Modern Optical System, University of Shanghai for Science and Technology, Shanghai 200093, P. R. China
3. Department of Physics, National Taiwan University, Taipei 10617, Taiwan
4. National Laboratory of Solid State Microstructures and Department of Physics, Nanjing University, Nanjing 210093, P. R. China
5. Department of Mechanical and Energy Engineering, Southern University of Science and Technology, Shenzhen 518055, P. R. China
6. Department of Mechanical Engineering, National University of Singapore, 117516 Singapore
7. School of Physics and Optoelectronics Engineering Science, Anhui University, Hefei 230601, P. R. China
8. Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, P. R. China
9. Department of Physics, Shanghai University, Shanghai 200444, P. R. China
10. ICQD, Hefei National Laboratory for Physical Sciences at Microscale, University of Science and Technology of China, Hefei 230026, P. R. China
11. Department of Physics, University of Science and Technology of China, Hefei 230026, P. R. China