1. Hefei National Research Center for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei, Anhui 230026, P. R. China
2. State Key Laboratory of Information Photonic and Optical Communications, School of Science Beijing University of Posts and Telecommunications (BUPT), Beijing 100876, P. R. China
3. School of Biomedical Engineering, Research and Engineering Center of Biomedical Materials, Anhui Medical University, Hefei, Anhui 230032, P. R. China
4. National Laboratory of Solid State Microstructures, School of Physics, Collaborative Innovation Center for 5Advanced Microstructures, Nanjing University, Nanjing 210093, P. R. China
5. Department of Physics, University of Miami, Coral Gables, Florida 33146, USA