| Jun 18, 2026 |
How does light turn into motion within a metal?Scientists have shown that ultrashort optical laser pulses can trigger extremely rapid lattice vibrations in periodically layered metal structures - not primarily by heating the atomic lattice, but through the pressure exerted by hot electrons.(Nanowerk News) In the study (Nature Communications, "Electron pressure drives THz phonons in metal–metal superlattices"), platinum and copper layers just a few nanometres (millionths of a millimeter) thick were stacked to form an artificial metal lattice. After being excited by a laser pulse, the artificial crystal lattice began to oscillate at around one terahertz: at a rate of roughly one trillion times per second, the platinum nano layers expand and squeeze the copper layers. The oscillation, which begins immediately, is too rapid to be explained by conventional lattice heating via heat transfer from the electrons. |
| “That surprised us,” says Jan-Etienne Pudell of European XFEL. “The oscillation is not caused by the pressure of the heated lattice, but by electron pressure, particularly in the platinum layers.” |
| “We’re not simply seeing a metal heating up and expanding here,” says Matias Bargheer, spokesperson for the Collaborative Research Centre (SFB) 1636 “Elementary Processes of Light-Driven Reactions at Nanoscale Metals” at the University of Potsdam. “We see that the electrons themselves exert pressure within less than one trillionth of a second, and in a way pound from within on the surface of the metal. This is highly exciting for the chemistry of nanometres thin metal layers, because it sheds new light on the question of hot electrons, heat, atomic movements all the way to chemical reactions.” |
| The results also show that such processes can be tailored by the choice of material and layer thickness. |
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| Schematic illustration of the experiment. (Image: University of Potsdam) |
| For its measurements, the international team used the MID (Materials Imaging and Dynamics) instrument at European XFEL. The platinum-copper lattice was excited using very short laser pulses on the order of 10-15 seconds (trillionths of a second) and examined using equally short high-energy X-ray pulses. The X-ray pulses can directly resolve the deep structural changes within the material. The experiment thus provide information that is sensitive to both the material and depth, revealing how the different metal layers shift following laser excitation. |
| “The MID instrument was built precisely to address questions such as: How do atoms and electrons move in complex materials when they are brought out of equilibrium by light?” says Jan-Etienne Pudell. “Here, we were not only able to observe the emergence of a terahertz oscillation, but also to determine the physics driving it.” |
| The results are particularly relevant to the CRC/SFB 1636 because the pressure of the hot electrons in the platinum arises from reflection at the surface and at the interfaces between metals. The pressure is a measure of the electrons’ pounding on the platinum layer’s surface, through which energy can be transferred to molecules bound to the surface. |
| “This creates a new experimental and conceptual link between plasmonic chemistry, the dynamics of high-energy electrons, heat flow and ultrafast structural changes,” explains Bargheer. |
| Source: European XFEL (Note: Content may be edited for style and length) |
