1. State Key Laboratory of Information Photonics and Optical Communications, and School of Science, Beijing University of Posts and Telecommunications, Beijing 100876, P. R. China
2. Hefei National Laboratory for Physical Sciences at the Microscale, School of Chemistry and Materials Sciences, CAS Key Laboratory of Materials for Energy Conversion, CAS Center for Excellence in Nanoscience, University of Science and Technology of China, Hefei 230026, P. R. China
3. Natl Synchrotron Radiat Lab, CAS Ctr Excellence Nanosci, CAS Key Lab Strongly Coupled Quantum Matter Phys, Univ Sci & Technol China, Hefei 230029, Anhui P. R. China