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Hannover Physicists discover a new method to image ultrafast electron motions in atoms

Hannover Physicists discover a new method to image ultrafast electron motions in atoms

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© Credit: Sonja Smalian/PhoenixD
An international team led by researchers from the Cluster of Excellence PhoenixD at Leibniz University Hannover – (from left) Ihar Babushkin, Uwe Morgner and Ayhan Demircan – discovered a novel access to the smallest temporal and spatial scales in the atomic world.
© Credit: Ihar Babushkin/PhoenixD
The diagram shows the ionization of a molecule in a strongly elliptically polarized laser field and the resulting radiation.

Researchers at Leibniz University Hannover observe electron dynamics with light at longest wavelengths

An international team led by researchers from the Cluster of Excellence PhoenixD at Leibniz University Hannover (Germany) imaged the fastest and tiniest details of the electron dynamics in atoms using light with wavelengths which were until now considered far too long for this task. "The discovery will allow novel, much easier access to the smallest temporal and spatial scales in the atomic world," says Dr. Ihar Babushkin, Theoretical Physicist and member of the Cluster of Excellence PhoenixD at Leibniz University Hannover (LUH).

How can I measure the flight path of a butterfly when the smallest scale on my ruler is as big as the Empire State Building? This question may sound grotesque because, normally, no one would probably want to measure such a small animal with a scale many times larger. This requires a tape measure whose unit of measurement is smaller than the butterfly. Such differences in size can be found also in the smallest particles: For example, the size of atoms is measured with the unit of Ångström. One Ångström corresponds to the ten-millionth part of a millimeter (10-10 meters). If atoms are now measured with the aid of light, the wavelength of the light serves as the unit of measurement, the "division of the ruler". Consequently, wavelengths in the Ångström range should be the most suitable for this task. These would be X-rays, and an observer would not be expected to see much or anything at all when observing the atom in visible light with a wavelength 3000 times longer.

These ratio rules apply not only to the observation of space, but also of time: For instance, in atomic physics, one of the fastest processes is the tunnelling of an electron away from the atom when the latter is placed in a very strong electric field. Ionization takes place at attosecond time scales (10-18 second), whereas the period of a single oscillation of visible light is around one femtosecond (10-15 second). "To study processes like this, researchers use up to now much shorter light wavelengths or the electrons escaping the atoms. Both types of measurements have significant disadvantages - they are difficult to produce and handle. But we found a solution," says Babushkin. His research was funded by the DFG (German Research Foundation) Priority Program 1840 (QUTIF), which was initiated and is coordinated by LUH.

A group of 21 scientists headed by members of the Cluster of Excellence PhoenixD has now discovered a new way to access the smallest atomic scales. With their research, they showed that clear signatures of electron dynamics are preserved in visible light; both on the time and space scales. Moreover, much longer wavelengths - down to the millimeter (terahertz) range - can be used. This means that it is possible to scale up the dynamics at the atomic level to the size of the known macroscopic world. The journal Nature Physics published the discovery of the researchers in its latest issue https://www.nature.com/articles/s41567-022-01505-2.

In the course of ionization in strong fields, the electron leaves the atom and is accelerated. As any accelerated charged particle, electron radiates light. Since the ionization process is very short in time, the spectrum of this radiation is very broad and includes components in ultraviolet, visible and even terahertz ranges. The key is to look at the polarization of this emitted light. The polarization is very sensitive to the smallest details of the electron dynamics. "Measuring light polarization allows reconstructing many aspects of electron dynamics with excellent precision," says Babushkin.

This new type of imaging opens broad perspectives: It promises experimental setups that are tens or even hundreds of times less expensive than before and thus affordable to many more researchers worldwide. "Besides, this allows us to observe the electron dynamics in situations when neither light at short wavelengths nor electrons are available for detection, for instance, in the bulk of solids," says Ayhan Demircan, theoretical Physicist and member of the Cluster of Excellence PhoenixD. Finally, optical polarization measurements can be very precise, allowing thus scientists to measure the electron dynamics as accurate as never before. "In the future," says Babushkin, "these findings will contribute to our understanding of the light-matter interaction at the edge of possible resolution both in time and space.

The Cluster of Excellence PhoenixD: Photonics, Optics, and Engineering - Innovation across Disciplines at Leibniz University Hannover comprises around 120 scientists working on novel integrated optics. Between 2019 and 2025, PhoenixD will receive approximately 52 million euros of funding from the German Research Foundation (DFG). Further information: www.phoenixd.uni-hannover.de

Original article
I. Babushkin, A. J. Galan, J. R. C. Andrade, A. Husakou, F. Morales, M. Kretschmar, T. Nagy, V. Vaicaitis, L. Shi, D. Zuber, L. Bergé, S. Skupin, I.A.Nikolaeva, N.A.Panov, D.E.Shipilo, O. G. Kosareva, A. N. Pfeiffer, A. Demircan, M. J. J. Vrakking, U. Morgner, and M. Ivanov
All-optical attoclock for imaging tunnelling wavepackets
Nature Physics, (2022)

Note to editors:
For further information, please contact the scientists and visit www.phoenixd.uni-hannover.de.
Priv.-Doz. Dr. Ihar Babushkin: Phone +49 511 762 3381, Email babushkin@iqo.uni-hannover.de
Apl Prof. Dr. Ayhan Demircan: Phone +49  511 762 17219, Email demircan@iqo.uni-hannover.de
Prof. Dr. Uwe Morgner: Phone +49 511 762 2452/2589, Email morgner@iqo.uni-hannover.de