1. Hangzhou Institute for Advanced Study, University of Chinese Academy of Sciences, Hangzhou 310024, China
2. Key Laboratory of Infrared Imaging Materials and Detectors, Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai 200083, China
3. University of Chinese Academy of Sciences, Beijing 100049, China
4. National Laboratory of Infrared Physics, Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai 200083, China
5. School of Electronic and Electrical Engineering, University of Leeds, Leeds LS2 9JT, UK
6. Key Laboratory of Terahertz Solid State Technology, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai 200050, China
7. Cavendish Laboratory, University of Cambridge, Cambridge CB3 0HE, UK