1. Nanofabrication Laboratory, CAS Key Laboratory for Nanophotonic Materials and Devices, National Center for Nanoscience and Technology, Beijing 100190, P. R. China
2. Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing 100049, P. R. China
3. China University of Geosciences, Beijing 100083, P. R. China