1. Institute of Information Photonics Technology, Faculty of Science, Beijing University of Technology, Beijing 100124, P. R. China.
2. State Key Laboratory of New Ceramics and Fine Processing, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, P. R. China.
3. State Key Laboratory of Low-Dimensional Quantum Physics, Tsinghua-Foxconn Nanotechnology Research Center, Department of Physics, Collaborative Innovation Center of Quantum Matter, Tsinghua University, Beijing 100084, P. R. China.