1. Key Laboratory of Low-Dimensional
Quantum Structures and Quantum Control of the Ministry of Education,
Department of Physics, Hunan Normal University, Changsha 410081, Hunan, P. R. China
2. State Key Laboratory of Optoelectronic Materials and Technologies, Nanotechnology Research Center, School of Physics & Engineering, Sun Yat-sen University, Guangzhou 510275, Guangdong, P. R. China