How does light turn into motion within a metal? A team of researchers from European XFEL, the University of Potsdam and other participating institutions has 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. The results are published in Nature Communications.

In the study, platinum and copper layers just a few nanometers (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 nanolayers 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.”