1. Department of Electronic Information and PhysicsChangzhi UniversityChangzhi Shanxi 046011 China
2. School of Physics and Optoelectronic EngineeringXidian UniversityXi'an 710071 China
3. School of Physics and Telecommunication EngineeringSouth China Normal UniversityGuangzhou 510006 China
4. Department of PhysicsShaanxi University of Science & TechnologyXi'an 710021 China