1. Key Laboratory of Materials Modification by Laser, Ion and Electron Beams, Ministry of Education, School of Physics and Optoelectronic Technology, Dalian University of Technology, Dalian 116024, China
2. General Atomics, P.O. Box 85608, San Diego, California 92186-5608, USA
3. Southwestern Institute of Physics, P.O. Box 432, Chengdu 610041, China