1. Department of Physics, State Key Laboratory of Low-Dimensional Quantum Physics and Tsinghua-Foxconn Nanotechnology Research Center, Tsinghua University, Beijing 100084, P. R. China
2. Collaborative Innovation Center of Quantum Matter, Beijing 100084, P. R. China
3. Nanophotonics and Optoelectronics Research Center, Qian Xuesen Laboratory of Space Technology, China Academy of Space Technology, Beijing 100094, P. R. China