1. Kunming Institute of Physics 650223 Kunming Yunnan China
2. College of EngineeringSwansea University Bay Campus, Fabian Way Swansea SA1 8EN UK
3. School of Physics and AstronomyYunnan University 650091 Kunming Yunnan China
4. Department of Applied PhysicsThe Hong Kong Polytechnic University Hung Hom, Kowloon Hong Kong China