1. Science and Technology on High Power Microwave Laboratory, Northwest Institute of Nuclear Technology, Xi'an, Shaanxi 710024, China
2. Key Laboratory of Physical Electronics and Devices of the Ministry of Education, Xi'an Jiaotong University, Xi'an, Shaanxi 710049, China
3. Xiangtan University, Xiangtan, Hunan 411105, China
4. Department of Engineering Physics, Tsinghua University, Beijing 100084, China
5. Department of Electrical and Computer Engineering, Michigan State University, East Lansing, Michigan 48824, USA