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.
Highlights
Here for the first time as far as we know, is very useful in understanding the conventional upper barrier of AC frequency, and in finding a condition to break the frequency barrier by suppressing the appearance of excluded transit-phase (ETP)
With a biased DC electric field determined by our theory, extension of the gap distance (Fig. 1c) significantly violating the conventional transit-time limit can lead to the prevention of bunch-collapse in the entire frequency range
The electric force exerting on each electron is, from a z-directional AC electric field in parallel with a DC-bias field, written by: Ez(t) = −Ea sin(ωt + φ) + Ed, 0 ≤ φ < 2π, (1)
Summary
Analysis of the electron bunch-collapse in a diode. Our analysis is performed on one-dimensional (1-D) motion of electrons along z-direction across the cathode and the anode (Fig. 1). The transit-phase θ is numerically calculated for thousands of sample electrons, varying the initial phase φ of the AC electric field and the modulation frequency f normalized by ωo = 1.05 × 1010 (rad/s) for different normalized DC-bias field Ed. The horizontal width of strips corresponds to the bunch-length and the normalized current density is scaled by color. In 3-D PIC simulations, a spatiotemporally well-localized ultrashort micro-bunch train of electrons with 1.41 picosecond periodicity (0.707 THz), 46.4 femtosecond bunch-length and 0.014 c mean velocity could be extracted from a 50 μm-wide two-plate diode demanding a transit-phase of 17 × 2π, which is apparently prohibited by the conventional transit-time limit. The simulation result verifies our theoretical prediction that the bunch-length of electron bunches from vacuum diodes can be made ultrashort in the THz frequency regimes by violating the transit-time limit
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