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. It
is the first such electron diffractometer that uses Terahertz radiation for
pulse compression. The developer team around DESY scientists Dongfang Zhang
and Franz Kärtner 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 Kärtner, 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 behavior 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 optimized 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 synchronized, 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,"
Kärtner explains. Synchronization 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 synchronization 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 synchronization 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 nanometers (millionths of a
millimeter). Adding another acceleration stage could give the electrons
enough energy to penetrate 30 times thicker samples with a thickness of up
to 1 micrometer (thousandth of a millimeter), 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).
Reference:
Dongfang Zhang et al, THz-Enhanced DC Ultrafast Electron Diffractometer,
Ultrafast Science (2021).
DOI: 10.34133/2021/9848526
Tags:
Physics