High-coherence picosecond electron bunches from cold atoms

Abstract

Ultrafast electron diffraction enables the study of molecular structural dynamics with atomic resolution at subpicosecond timescales, with applications in solid-state physics and rational drug design. Progress with ultrafast electron diffraction has been constrained by the limited transverse coherence of high-current electron sources. Photoionization of laser-cooled atoms can produce electrons of intrinsically high coherence, but has been too slow for ultrafast electron diffraction. Ionization with femtosecond lasers should in principle reduce the electron pulse duration, but the high bandwidth inherent to short laser pulses is expected to destroy the transverse coherence. Here we demonstrate that a two-colour process with femtosecond excitation followed by nanosecond photoionization can produce picosecond electron bunches with high transverse coherence. Ultimately, the unique combination of ultrafast ionization, high coherence and three-dimensional bunch shaping capabilities of cold atom electron sources have the potential for realising the brightness and coherence requirements for single-shot electron diffraction from crystalline biological samples.

Time-dependent resonant scattering: An analytical approach

“…[20][21][22] At the moment, these sources are primarily intended for the use in electron diffraction experiments on molecules. 37 As the electron energies are progressively lowered, effects of resonant scattering such as discussed here should become observable.…”

“…[17][18][19] Particularly relevant to the present treatment is the rapid development of highly coherent sources of cold electrons. [20][21][22] Cold-atom collisions can also be studied in the time domain. Boesten et al 23 have observed the time dependence of a cold-atom collision in a pulsed photoassociation experiment.…”

High-Coherence Electron and Ion Bunches From Laser-Cooled Atoms

“…In the same region, the emittance from the CW excitation laser was 141 ± 7 nm. Though the femtosecond emittance is larger, the corresponding coherence length is still relatively large for an electron source, at L c = 4.0 ± 0.2 nm [16].…”

“…With femtosecond excitation, the initial electron pulse duration is limited by the spatial and temporal extent of the overlap between the new femtosecond pulses and the 5 ns pulses of 480 nm light. The overlap produces a shaped pulse of electrons or ions with a minimum duration of 150 ps [16]. The high bandwidth inherent to short laser pulses might be expected to increase the excess energy spread of the electrons and thus destroy their transverse coherence.…”

“…3(a)(i) and measured the spatial distribution of the electron bunches at the MCP detector. By knowing the initial and final electron beamlet distributions, the emittance ε r can be calculated (for more details see Reference [16]). The pepperpot measurements were performed for a series of different blue laser wavelengths, similar to the temperature measurements discussed in Section 2, and compared to results with CW excitation.…”