Tuesday, May 17, 2022

Seeing ultrafast dynamics of matter via a compact electron camera

DESY team demonstrates the first Terahertz enhanced electron diffractometer.

Using a newly developed compact electron camera, scientists at DESY could capture the inner, ultrafast dynamics of matter. The camera captures short bunches of electrons at a sample to take snapshots of its current inner structure.

This compact electron camera is an electron diffractometer that uses Terahertz radiation for pulse compression. The THz-compressed electron beams produce high-quality diffraction patterns and observe the ultrafast structural dynamics with improved time resolution.

Electron diffraction– a technique to investigate the inner structure of matter- image the structure when the electrons hit or traverse a solid sample, they are deflected systematically by the electrons in the solid’s inner lattice.

The detector detects this pattern of diffraction, the internal lattice structure of the solid can be calculated. Short bunches of adequately splendid electrons need to be used to recognize dynamic changes in this inner structure.

DESY scientists Dongfang Zhang said, “The shorter the bunch, the faster the exposure time. Typically, ultrafast electron diffraction (UED) uses bunch lengths, or exposure times, of some 100 femtoseconds, which is 0.1 trillionths of a second.”

Advance particle accelerators can produce high-quality short electron bunches. Although, such machines are large and bulky. Also, they use radiofrequency radiation to power themselves.

Scientists, in this study, using Terahertz radiation instead with roughly a hundred times shorter wavelengths.

Franz Kärtner from the Center for Free-Electron Laser Science CFEL said, “This means, the accelerator components, here a bunch compressor, can be a hundred times smaller, too.”

Scientists fired bunches with roughly 10,000 electrons each at a silicon crystal heated by a short laser pulse. The bunches were about 180 femtoseconds long and clearly show how the silicon sample’s crystal lattice quickly expands within a picosecond (trillionths of a second) after the laser hits the crystal.

Zhang said, “The behavior of silicon under these circumstances is very well known, and our measurements fit the expectation perfectly, validating our Terahertz device.”

“In an optimized set-up, the electron bunches can be compressed to significantly less than 100 femtoseconds, allowing even faster snapshots.”

Kärtner said, “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.”

The secret of this kind of ultrafast experiment lies in Synchronization. Scientists usually repeat the experiment while delaying the measuring pulse a little more each time. Through this, they can monitor the swift structural changes within a sample of matter like silicon.

The more accurate this delay can be adjusted, the better the result. Usually, there needs to be some 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 is triggered by the same laser, the Synchronization is intrinsically given.

Scientists are further planning to increase the energy of the electrons. The higher the energy, the thicker samples are penetrated by the electrons. 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.

Scientists noted“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).”

“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. This could prove useful when investigating delicate materials.”

Journal Reference:
  1. Dongfang Zhang et al., THz-Enhanced DC Ultrafast Electron Diffractometer, Ultrafast Science (2021). DOI: 10.34133/2021/9848526


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