1. Institute of Applied Physics and Computational Mathematics, Beijing 100088, China
2. Key Laboratory of Artificial Structures and Quantum Control, School of Physics and Astronomy, Shanghai Jiao Tong University, Shanghai 200240, China
3. Key Laboratory of Terahertz Solid-State Technology, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai 200050, China
4. School of Electronic and Electrical Engineering, University of Leeds, Leeds LS2 9JT, United Kingdom
5. Department of Engineering Physics, Tsinghua University, Beijing 100084, China