Table-top electron camera catches ultrafast dynamics of matter
Terahertz enhanced electron diffractometer

Date:
August 16, 2021
Source:
Deutsches Elektronen-Synchrotron DESY
Summary:
Scientists have built a compact electron camera that can capture
the inner, ultrafast dynamics of matter. The system shoots short
bunches of electrons at a sample to take snapshots of its current
inner structure and is the first such electron diffractometer that
uses Terahertz radiation for pulse compression. The developers
validated their Terahertz-enhanced ultrafast electron diffractometer
with the investigation of a silicon sample.



FULL STORY ========================================================================== Scientists at DESY have built a compact electron camera that can
capture the inner, ultrafast dynamics of matter. The system shoots
short bunches of electrons at a sample to take snapshots of its current
inner structure and is the first such electron diffractometer that uses Terahertz radiation for pulse compression. The developer team around
DESY scientists Dongfang Zhang and Franz Ka"rtner from the Center for Free-Electron Laser Science CFEL validated their Terahertz-enhanced
ultrafast electron diffractometer with the investigation of a silicon
sample and present their work in the first issue of the journal Ultrafast Science, a new title in the Science group of scientific journals.


========================================================================== Electron diffraction is one way to investigate the inner structure
of matter.

However, it does not image the structure directly. Instead, when
the electrons hit or traverse a solid sample, they are deflected in
a systematic way by the electrons in the solid's inner lattice. From
the pattern of this diffraction, recorded on a detector, the internal
lattice structure of the solid can be calculated. To detect dynamic
changes in this inner structure, short bunches of sufficiently bright
electrons have to be used. "The shorter the bunch, the faster the
exposure time," says Zhang, who is now a professor at Shanghai Jiao
Tong University. "Typically, ultrafast electron diffraction (UED)
uses bunch lengths, or exposure times, of some 100 femtoseconds, which
is 0.1 trillionths of a second." Such short electron bunches can
be routinely produced with high quality by state-of-the-art particle accelerators. However, these machines are often large and bulky, partly
due to the radio frequency radiation used to power them, which operates
in the Gigahertz band. The wavelength of the radiation sets the size for
the whole device. The DESY team is now using Terahertz radiation instead
with roughly a hundred times shorter wavelengths. "This basically means,
the accelerator components, here a bunch compressor, can be a hundred
times smaller, too," explains Ka"rtner, who is also a professor and a
member of the cluster of excellence "CUI: Advanced Imaging of Matter"
at the University of Hamburg.

For their proof-of-principle study, the scientists fired bunches with
roughly 10,000 electrons each at a silicon crystal that was heated by a
short laser pulse. The bunches were about 180 femtoseconds long and show clearly how the crystal lattice of the silicon sample quickly expands
within a picosecond (trillionths of a second) after the laser hits the
crystal. "The behaviour of silicon under these circumstances is very well known, and our measurements fit the expectation perfectly, validating our Terahertz device," says Zhang. He estimates that in an optimised set-up,
the electron bunches can be compressed to significantly less than 100 femtoseconds, allowing even faster snapshots.

On top of its reduced size, the Terahertz electron diffractometer has
another advantage that might be even more important to researchers:
"Our system is perfectly synchronised, since we are using just one laser
for all steps: generating, manipulating, measuring and compressing the
electron bunches, producing the Terahertz radiation and even heating
the sample," Ka"rtner explains. Synchronisation is key in this kind of ultrafast experiments. To monitor the swift structural changes within a
sample of matter like silicon, researchers usually repeat the experiment
many times while delaying the measuring pulse a little more each time. The
more accurate this delay can be adjusted, the better the result. Usually,
there needs to be some kind of synchronisation between the exciting
laser pulse that starts the experiment and the measuring pulse, in this
case the electron bunch. If both, the start of the experiment and the
electron bunch and its manipulation are triggered by the same laser,
the synchronisation is intrinsically given.

In a next step, the scientists plan to increase the energy of the
electrons.

Higher energy means the electrons can penetrate thicker samples. The
prototype set-up used rather low-energy electrons and the silicon
sample had to be sliced down to a thickness of just 35 nanometres
(millionths of a millimetre). Adding another acceleration stage could
give the electrons enough energy to penetrate 30 times thicker samples
with a thickness of up to 1 micrometre (thousandth of a millimetre), as
the researchers explain. For even thicker samples, X-rays are normally
used. While X-ray diffraction is a well established and hugely successful technique, electrons usually do not damage the sample as quickly as X-rays
do. "The energy deposited is much lower when using electrons," explains
Zhang. This could prove useful when investigating delicate materials.

This work has been supported by the European Research Council under
the European Union's Seventh Framework Program (FP7/2007-2013) through
the Synergy Grant AXSIS (609920), Project KA908-12/1 of the Deutsche Forschungsgemeinschaft, and the accelerator on a chip program (ACHIP)
funded by the Gordon and Betty Moore foundation (GBMF4744).

DESY is one of the world's leading particle accelerator centres and investigates the structure and function of matter -- from the interaction
of tiny elementary particles and the behaviour of novel nanomaterials and
vital biomolecules to the great mysteries of the universe. The particle accelerators and detectors that DESY develops and builds at its locations
in Hamburg and Zeuthen are unique research tools. They generate the most intense X-ray radiation in the world, accelerate particles to record
energies and open up new windows onto the universe. DESY is a member of
the Helmholtz Association, Germany's largest scientific association, and receives its funding from the German Federal Ministry of Education and
Research (BMBF) (90 per cent) and the German federal states of Hamburg
and Brandenburg (10 per cent).

========================================================================== Story Source: Materials provided by
Deutsches_Elektronen-Synchrotron_DESY. Note: Content may be edited for
style and length.


========================================================================== Journal Reference:
1. Dongfang Zhang, Tobias Kroh, Felix Ritzkowsky, Timm Rohwer, Moein
Fakhari, Huseyin Cankaya, Anne-Laure Calendron, Nicholas
H. Matlis, Franz X. Ka"rtner. THz-Enhanced DC Ultrafast
Electron Diffractometer. Ultrafast Science, 2021; 2021: 1 DOI:
10.34133/2021/9848526 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/08/210816125712.htm

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