1. College of Materials Science and Opto-Electronic Technology & Key Laboratory of Vacuum Physics, University of Chinese Academy of Sciences, Beijing 100049, P. R. China
2. State Key Laboratory for Superlattices and Microstructures, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China
3. Division of Nanophotonics, CAS Key Laboratory
of Standardization and Measurement for Nanotechnology, CAS Center
for Excellence in Nanoscience, National Center for Nanoscience and Technology, Beijing 100190, China