1. School
of Materials Science and Engineering, Beihang University, Beijing 100191, P.R. China
2. The MOE Key Laboratory
of Weak-Light Nonlinear Photonics, TEAD Applied Physics Institute
and School of Physics, Nankai University, Tianjin 300457, China
3. National Center for Nanoscience and Technology, No. 11 Beiyitiao, Zhongguancun, Beijing 100190, China
4. Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States
5. State
Key Laboratory of Superlattices and Microstructures, Institute of
Semiconductors, Chinese Academy of Sciences, Beijing 100083, China
6. School for Engineering of Matter, Transport
and Energy, Arizona State University, Tempe, Arizona 85287, United States