1. National Key Laboratory of Science and Technology on Vacuum Electronics in Chengdu, School of Electronic Science and Engineering, University of Electronic Science and Technology of China, Chengdu, Sichuan 611731, China
2. Nanjing Sanle Microwave Technology Development Co., Ltd., Nanjing, Jiangsu 211899, China
3. Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA