1. School of Microelectronics University of Science and Technology of China Hefei 230029 P. R. China
2. Department of Electrical Engineering and Computer Science University of Michigan Ann Arbor MI 48109 USA
3. Hefei National Laboratory for Physical Science at the Microscale Department of Chemical Physics University of Science and Technology of China Hefei 230027 P. R. China
4. National Synchrotron Radiation Laboratory University of Science and Technology of China Hefei 230027 P. R. China
5. Suzhou Institute of Nano‐Tech and Nano‐Bionics Chinese Academy of Sciences Chinese Academy of Sciences (CAS) Suzhou 215123 P. R. China
6. Key Laboratory of Wireless‐Optical Communications Chinese Academy of Sciences University of Science and Technology of China Hefei 230029 P. R. China