1. National Key Laboratory of Science and Technology on Vacuum Electronics, School of Physical Electronics, University of Electronic Science and Technology of China, Chengdu, Sichuan 610054, China
2. Institute for Laser Technology, Suita, Osaka 565-0781, Japan
3. Institute of Laser Engineering, Osaka University, Suita, Osaka 565-0781, Japan
4. Beijing Vacuum Electronics Research Institute, Beijing 100016, China
5. Laboratory of Advanced Science and Technology for Industry, University of Hyogo, Ako, Hyogo 678-1205, Japan