Towards attosecond 4D imaging of atomic-scale dynamics by single-electron diffraction

Many physical and chemical processes which define our daily life
take place on atomic scales in space and time. Time-resolved
electron diffraction is an excellent tool for investigation of
atomic-scale structural dynamics (4D imaging) due to the short de
Broglie wavelength of fast electrons. This requires electron pulses
with durations on the order of femtoseconds or below. Challenges
arise from Coulomb repulsion and dispersion of non-relativistic
electron wave packets in vacuum, which currently limits the
temporal resolution of diffraction experiments to some hundreds of
femtoseconds. In order to eventually advance the temporal
resolution of electron diffraction into the few-femtosecond range
or below, four new concepts are investigated and combined in this
work: First, Coulomb repulsion is avoided by using only a single
electron per pulse, which does not repel itself but interferes with
itself when being diffracted from atoms. Secondly, dispersion
control for electron pulses is implemented with time-dependent
electric fields at microwave frequencies, compressing the duration
of single-electron pulses at the expense of simultaneous energy
broadening. Thirdly, a microwave signal used for electron pulse
compression is derived from an ultrashort laser pulse train.
Optical enhancement allows a temporal synchronization between the
microwave field and the laser pulses with a precision below one
femtosecond. Fourthly, a cross-correlation between laser and
electron pulses is measured in this work with the purpose of
determining the possible temporal resolution of diffraction
experiments employing compressed single-electron pulses. This novel
characterization method uses the principles of a streak camera with
optical fields and potentially offers attosecond temporal
resolution. These four concepts show a clear path towards improving
the temporal resolution of electron diffraction into the
few-femtosecond domain or below, which opens the possibility of
observing electron densities in motion. In this work, a compressed
electron pulse's duration of 28±5 fs full width at half maximum
(12±2 fs standard deviation) at a de Broglie wavelength of 0.08 Å
is achieved. Currently, this constitutes the shortest electron
pulses suitable for diffraction, about sixfold shorter than in
previous work. Ultrafast electron diffraction now meets the
requirements for investigating the fastest primary processes in
molecules and solids with atomic resolution in space and time.