Coherent femtosecond low-energy single-electron pulses for time-resolved diffraction and imaging: A numerical study

Abstract

We numerically investigate the properties of coherent femtosecond single electron wave packets photoemitted from nanotips in view of their application in ultrafast electron diffraction and non-destructive imaging with low-energy electrons. For two different geometries, we analyze the temporal and spatial broadening during propagation from the needle emitter to an anode, identifying the experimental parameters and challenges for realizing femtosecond time resolution. The simple tip-anode geometry is most versatile and allows for electron pulses of several ten of femtosecond duration using a very compact experimental design, however, providing very limited control over the electron beam collimation. A more sophisticated geometry comprising a suppressor-extractor electrostatic unit and a lens, similar to typical field emission electron microscope optics, is also investigated, allowing full control over the beam parameters. Using such a design, we find ∼230 fs pulses feasible in a focused electron beam. The main limitation to achieve sub-hundred femtosecond time resolution is the typical size of such a device, and we suggest the implementation of more compact electron optics for optimal performance

Femtosecond electrons probing currents and atomic structure in nanomaterials

“…4c, the expected full width at half maximum (FWHM) electron pulse duration t FWHM is plotted as a function of tipsample distance for different electron energies, where the focusing condition is adjusted to provide a l t of B30 nm (described in more detail in the Methods section). The pulse duration decreases sub-linearly with shorter propagation length, t FWHM pd g , with gE0.83, which can be explained by the distance-dependent reduced inhomogeneity of the acceleration field at the apex in the diffraction mode 14 . So far, the shortest possible distances in the diffraction mode are B150 mm, restricted by vacuum breakthrough at the electron lens, limiting the electron pulse duration to B300 fs, see Fig.…”

“…With a wire radius of 15 nm, we expect a drift time of B200 fs, which agrees well with the observed 10-90 rise times of 140 fs and 230 fs of p-and n-segment, respectively. Hence, we interpret the fast initial dynamics as direct measure of radial photocurrent in the NW and conclude that the observed dynamics reflect the carrier dynamics and is not limited by the temporal resolution of our instrument, which according to simulations is expected to be o50 fs in the imaging mode 14 . These results demonstrate the feasibility of fsPPM as a novel approach for probing ultrafast currents on the nanoscale with fs temporal resolution.…”

“…Many-electron pulses can be strongly affected by space charge broadening due to Coulomb repulsion 13 . Furthermore, even single electron wave packets experience significant dispersive broadening depending on their initial energy distribution 14 . Temporal compression techniques can be used to obtain femtosecond many-electron pulses at a distant sample 15 , but have yet to be demonstrated for low electron energies.…”

Ultrafast nanoscale magnetic switching via intense picosecond electron bunches

“…Novel electron optical concepts promise a tremendous increase in temporal and spatial resolution allowing researchers to address new scientific challenges [2][3][4][5][6]. The use of relativistic MeV electron energies is an attractive avenue to deal with the issue of Coulomb repulsion in ultrashort electron pulses bringing single-shot electron microscopy on nm length-and ps time-scales within reach [7,8].…”

Development of an Ultrafast Low-Energy Electron Diffraction Setup

“…In particular, nonlinearly driven, nanometrically sharp needle emitters are suggested to minimize broadening effects [52][53][54].…”