1. Institute of Crystalline Materials, Shanxi University, Taiyuan, Shanxi 030006, P. R. China
2. State Key Laboratory of Quantum Optics and Quantum Optics Devices, Shanxi University, Taiyuan, Shanxi 030006, P. R. China
3. Ordered Matter Science Research Center, Nanchang University, Nanchang 330031, P. R. China