1. Fujian Key Laboratory of Semiconductor
Materials and Applications, Department of Physics, Xiamen University, Xiamen 361005, P. R. China
2. Key Laboratory of Optoelectronic
Technology, Fujian Province University, Xiamen University of Technology, Xiamen 361024, P. R. China
3. State Key Laboratory of Artificial
Microstructure and Mesoscopic Physics, School of Physics, Peking University, Beijing 100871, P. R. China