1. Sargent Joint Research CenterWuhan National Laboratory for Optoelectronics (WNLO) and School of Optical and Electronic InformationHuazhong University of Science and Technology Wuhan 430074 Hubei P. R. China
2. School of Materials Science and EngineeringHuazhong University of Science and Technology Wuhan 430074 Hubei P. R. China
3. School of Physics and Optoelectronic EngineeringGuangdong University of Technology Guangzhou 510006 Guangdong P. R. China