1. Jiangsu Key Laboratory of Advanced Laser Materials and Devices, School of Physics and Electronic Engineering, Jiangsu Normal University, Xuzhou 221116, China
2. Department of Physics and Astronomy, and Nanoscale Science and Engineering Center, University of Georgia, Athens, Georgia 30602
3. Department of Physics, Virginia Commonwealth University, Richmond, Virginia 23284
4. Department of Physics, Hong Kong Baptist University, Hong Kong 999077, China