1. Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and Guangdong Province College of Physics and Optoelectronic Engineering Shenzhen University Shenzhen 518060 P. R. China
2. Key Laboratory of Materials Physics Anhui Key Laboratory of Nanomaterials and Nanotechnology Institute of Solid State Physics Hefei Institutes of Physical Science Chinese Academy of Sciences Hefei 230031 P. R. China
3. University of Science and Technology of China Hefei 230026 P. R. China