1. State Key Laboratory of Artificial Microstructure and Mesoscopic Physics School of Physics Nano‐Optoelectronics Frontier Center of Ministry of Education (NFC‐MOE) Peking University Beijing 100871 P. R. China
2. Microsystem and Terahertz Research Center China Academy of Engineering Physics Chengdu 610200 P. R. China
3. Advanced Micro‐Fabrication Equipment Inc. Shanghai 201201 P. R. China
4. School of Electronic Science and Engineering University of Electronic Science and Technology of China Chengdu 610054 P. R. China