1. Key Laboratory of Low-Dimensional Quantum Structures and Quantum Control of Ministry of Education, and Department of Physics,Hunan Normal University, Changsha 410081, Hunan, P. R. China, and State Key Laboratory of Optoelectronic Materials and Technologies, Institute of Optoelectronic and Functional Composite Materials, Nanotechnology Research Center, School of Physics Science & Engineering, Zhongshan University, Guangzhou 510275, Guangdong, P. R. China