title: Violation of the transit-time limit toward generation of ultrashort electron bunches with controlled velocity chirp.
abstract: 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.
authors.affiliation: Applied Electromagnetic Wave Research Center, Korea Electrotechnology Research Institute (KERI), Ansan 426-170, Republic of Korea.
authors.forename: Seok-Gy
authors.initials: SG
authors.lastname: Jeon
authors.affiliation: 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.
authors.forename: Dongwon
authors.initials: D
authors.lastname: Shin
authors.affiliation: Department of Physics, UNIST, Ulsan, 44919, Republic of Korea.
authors.forename: Min Sup
authors.initials: MS
authors.lastname: Hur