1. Department of Physics, Key Laboratory of Atomic and Molecular Nanoscience (Ministry of Education), Tsinghua University, Beijing 100084, People’s Republic of China, Department of Physics, Hangzhou Normal University, Hangzhou, Zhejiang 310036, People’s Republic of China, and Department of Physics, Center for Optoelectronics Materials and Devices, Zhejiang Sci-Tech University, Xiasha College Park, Hangzhou, Zhejiang 310018, People’s Republic of China