Laser transformation of crystal structures into super slow motion

Artist impression of the charge density wave in the ultra-fast transmission electron microscope. Credit: Dr. Florian Sterl (Sterltech Optics)

Physicists from Göttingen first managed to film a phase transition with extremely high spatial and temporal resolution.

Laser beams can be used to change the properties of materials in an extremely accurate way. This principle has already been widely used in technologies such as rewritten DVDs. However, underlying processes generally take place at such unimaginable speeds and on such a small scale that direct observation has so far been avoided. Researchers at the University of Göttingen and the Max Planck Institute (MPI) for Biophysical Chemistry in Göttingen have now managed to film, for the first time, the laser transformation of a nanometer-resolution, slow-moving crystal structure into an electron microscope. The results have been published in the journal science.

The team, which includes Thomas Danz and Professor Claus Ropers, took advantage of an unusual property of a material consisting of thin atomic layers of sulfur and tantalum atoms. At room temperature, its crystalline structure is distorted into smaller structures like those of waves – a “charge density wave” is formed. At higher temperatures, a phase transition occurs in which the original microscopic waves suddenly disappear. Electrical conductivity also changes drastically, an interesting effect for nano-electronics.

In their experiments, the researchers induced this phase transition with short laser pulses and recorded a charge density wave reaction film. “What we notice is the formation and rapid growth of small regions where the material was passed to the next stage,” explains the first author Thomas Danz from the University of Göttingen. “The ultra Fast transmission electron microscope developed in Göttingen offers the highest time resolution for such an image in the world today.” The special feature of the experiment lies in a newly developed imaging technique, which is particularly sensitive to the specific changes observed in this phase transition. Göttingen physicists use it to take images that consist exclusively of electrons that are scattered by the oscillation of the crystal.

Their forward approach allows researchers to gain basic knowledge about light-induced structural changes. “We are now able to transfer our imaging technique to other crystal structures,” says Professor Claus Ropers, head of Ultrafast Nano-Optics and Dynamics at the University of Göttingen and Director at the MPI for Biophysical Chemistry. “In this way, we not only answer fundamental questions in solid state physics, but also open up new perspectives for future optically interchangeable materials, intelligent nano-electronics.”

Reference: “Ultra fast nanoimaging of the order parameter in a structural phase transition” by Thomas Danz, Till Domröse and Claus Ropers, January 22, 2021, science.
DOI: 10.1126 / science.abd2774

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