1. State
Key Laboratory of Artificial Microstructure and Mesoscopic Physics,
School of Physics, Peking University, Beijing 100871, P. R. China
2. Beijing
Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences; National Center for Nanoscience and Technology (NCNST), Beijing 100083, P. R. China
3. Microsystem
and Terahertz Research Center, China Academy of Engineering Physics, Chengdu 610200, Sichuan, P. R. China