1. a Vacuum Electronics National Lab, School of Physical Electronics, University of Electronic Science and Technology of China, Chengdu 610054, P. R. China, Research Laboratory of Electronics, Massachusetts Institute of Technology, Cambridge, MA 02139-4307, USA
2. b Vacuum Electronics National Lab, School of Physical Electronics, University of Electronic Science and Technology of China, Chengdu 610054, P. R. China
3. c Vacuum Electronics National Lab, School of Physical Electronics, University of Electronic Science and Technology of China, Chengdu 610054, P. R. China
4. d Vacuum Electronics National Lab, School of Physical Electronics, University of Electronic Science and Technology of China, Chengdu 610054, P. R. China
5. e Research Laboratory of Electronics, Massachusetts Institute of Technology, Cambridge, MA 02139-4307, USA
6. f Research Laboratory of Electronics, Massachusetts Institute of Technology, Cambridge, MA 02139-4307, USA