1. State Key Laboratory of Transient Optics and Photonics, Xi'an Institute of Optics and Precision Mechanics, Chinese Academy of Sciences, Xi'an 710119, P. R. China
2. University of Chinese Academy of Sciences, Beijing 100049, P. R. China
3. Department of Physics, School of Science, Xi'an Polytechnic University, Xi'an 710048, P. R. China