1. School of Optical and Electronic InformationHuazhong University of Science and Technology Wuhan 430074 P. R. China
2. Key Lab of Materials PhysicsAnhui Key Lab of Nanomaterials and NanotechnologyInstitute of Solid State PhysicsChinese Academy of Science Hefei 230031 P. R. China
3. Science Island BranchUniversity of Science and Technology of China Hefei 230026 P. R. China