1. National Laboratory of Solid State Microstructures; School of Physics; Collaborative Innovation Center of Advanced Microstructures; Nanjing University; Nanjing 210093 P.R. China
2. College of Science; National University of Defense Technology; Changsha 410073 P.R. China
3. National Laboratory for Infrared Physics; Shanghai Institute of Technical Physics; Chinese Academy of Sciences; Shanghai 200083 P.R. China
4. Department of Physics; Southeast University; Nanjing 211189 P.R. China
5. Department of Chemistry; Southeast University; Nanjing 211189 P.R. China
6. School of Electronic Science and Engineering; Nanjing University; Nanjing 210093 P.R. China
7. National Institute for Materials Science; 1-1 Namiki Tsukuba 305-0044 Japan
8. Geballe Laboratory for Advanced Materials; Stanford University; Stanford CA 94305 USA
9. Stanford Institute for Materials and Energy Sciences; SLAC National Accelerator Laboratory; Menlo Park CA 94025 USA