The world is constantly looking for faster and more efficient computers. Since the technologies of today have nearly maximised the use of electric currents for this purpose, scientists are looking beyond. One critical area that may fit our needs is called strain engineering: it uses mechanical pressure at the atomic level to alter a material’s properties.

That is, by physically squeezing or stretching the grid of atoms — called the lattice — scientists can change how a material conducts electricity or stores magnetic information.

To reach the speeds required for next-generation technology, however, these mechanical changes must happen trillions of times per second, or at terahertz (THz) frequencies.

The main challenge to high-frequency strain engineering in metals is the behaviour of free electrons. When a metal is struck by a fast laser pulse, the electrons absorb the energy. In most metals, these electrons move so quickly that they spread the energy across the material before the pulse can exert a localised mechanical force. This delocalisation prevents the formation of high-frequency strain waves.

In particular, THz-frequency vibrations are needed for advanced applications like spintronics — a field that uses the magnetic spin of electrons rather than their charge to store and move data.

‘That surprised us’

A new study by researchers from France, Germany, and Sweden, published in Nature Communications, has reported a way to use laser pulses to induce THz waves. The researchers used a superlattice: atoms of platinum and copper atoms arranged in alternating layers, each just a few nanometres thick. They chose these metals because of their contrasting electronic properties: platinum can hold a large amount of electronic energy while copper allows electrons to move quickly.

Using ultrafast X-ray diffraction, the team observed the atoms moving in real-time following a laser pulse. And they found that they could set the atoms vibrating in a coherent way at a frequency of 1 THz with a strain amplitude of 1%. That is, the atoms were displaced by 1% on average from their original positions. This is considered large.

Crucially, the team found that electron pressure, rather than heat, to be the driving force. When the laser hit the platinum layers, the electrons gained energy and immediately exerted a physical pressure against the atomic lattice.

“That surprised us. The oscillation is not caused by the pressure of the heated lattice, but by electron pressure, particularly in the platinum layers,” Jan-Etienne Pudell of European XFEL, the facility that provided the ultrafast X-ray diffraction, said in a press note.

The study showed that electron pressure is a dominant and engineerable mechanism that can be used to rapidly ‘kick’ atoms into motion.

Nanostructured materials

The discovery is an advance in the broader field of thermoacoustic metamaterials. These are nanostructured materials designed to manipulate sound and heat in ways that do not occur in nature.

The study also demonstrated that these effects can be achieved in materials created by sputtering. Sputtering is a standard industrial process used to coat everything from microchips to glass. That thermoacoustic metamaterials can be produced using cost-effective, scalable manufacturing suggests the technology could move from the laboratory to the factory floor relatively quickly.