1. Key Laboratory of Materials Physics and Anhui Key Laboratory of Nanomaterials
and Nanotechnology, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, P. R. China
2. Anhui Provincial Key Laboratory of Photonics Devices
and Materials, Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Hefei 230031, P. R. China
3. University of Science and Technology of China, Hefei 230026, P. R. China