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Sci Rep. 2016 Sep 22;6:32567. doi: 10.1038/srep32567.

Violation of the transit-time limit toward generation of ultrashort electron bunches with controlled velocity chirp.

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Applied Electromagnetic Wave Research Center, Korea Electrotechnology Research Institute (KERI), Ansan 426-170, Republic of Korea.
Department of Physics, UNIST, Ulsan, 44919, Republic of Korea.


Various methods to generate ultrashort electron bunches for the ultrafast science evolved from the simple configuration of two-plate vacuum diodes to advanced technologies such as nanotips or photocathodes excited by femtosecond lasers. In a diode either in vacuum or of solid-state, the transit-time limit originating from finite electron mobility has caused spatiotemporal bunch-collapse in ultrafast regime. Here, we show for the first time that abrupt exclusion of transit-phase is a more fundamental origin of the bunch-collapse than the transit-time limit. We found that by significantly extending the cathode-anode gap distance, thereby violating the transit-time limit, the conventional transit-time-related upper frequency barrier in diodes can be removed. Furthermore, we reveal how to control the velocity chirp of bunches leading to ballistic bunch-compression. Demonstration of 0.707 THz-, 46.4 femtosecond-bunches from a 50 μm-wide diode in three-dimensional particle-in-cell simulations shows a way toward simple and compact sources of ultrafast electron bunches for diverse ultrafast sciences.

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