1. Department of Physics and Research Center OPTIMAS, Technische Universität Kaiserslautern, P.O. Box 3049, 67653 Kaiserslautern, Germany
2. School of Mechanics, Civil Engineering and Architecture, Northwestern Polytechnical University, Xi’an 710072, China
3. School of Physics and Information Technology, Shaanxi Normal University, Xi’an 710119, China