1. Department of Electrical Engineering and Information Science, Ruhr-University Bochum, D-44780 Bochum, Germany
2. Electrodynamics and Physical Electronics Group, Brandenburg University of Technology Cottbus-Senftenberg, D-03046 Cottbus, Germany
3. Key Laboratory of Materials Modification by Laser, Ion and Electron Beams, School of Physics, Dalian University of Technology, Dalian 116024, China