Scientists devise a way to see atomic-scale acoustic waves in nanostructures

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Washington, July 5 : Scientists at Lawrence Livermore National Laboratory claim to have found a physical phenomenon that can help see atomic-scale acoustic waves in nanostructures.

The researchers say that the highest frequency acoustic waves in materials, with nearly atomic-scale wavelengths, promise to be useful probes of nanostructures like LED lights.

They say that the detection of high frequency waves, which has so far been considered a difficult task, can be achieved by combining molecular dynamics simulations of shock waves with an experimental diagnostic, terahertz (THz) radiation.

During the study, the researchers performed computer simulations of the highest frequency acoustic waves forming spontaneously at the front of shock waves or generated by sub-picosecond pulse-length lasers.

They observed that, under some circumstances, when such a wave crossed an interface between two materials, tiny electric currents were generated at the interface.

According to them, such currents produce electromagnetic radiation of THz frequencies, which can be detected a few millimetres away from the interface.

The researchers said that part of the wave was effectively converted to electromagnetic radiation, which propagated out of the material where it could be measured.

Most molecular dynamics simulations of shock waves connect to experiments through electronic properties, such as optical reflectivity.

"But this new approach connects to the much lower frequency THz radiation produced by the individual atoms moving around in the shock wave. This kind of diagnostic promises to provide new information about shocked materials like the dynamics of crystals pushed to ultra-high strain rates," said Evan Reed, lead author of a paper that appears in the journal Physical Review Letters.

The researchers used molecular dynamics simulations, and showed that the time-history of the wave could be determined with potentially sub-picosecond, nearly atomic time, and space resolution by measuring the electromagnetic field.

The group studied the effect for an interface between two thin films, which are used in LED (light-emitting diode) nanostructures, and are piezoelectric (electric currents that are generated when they are squeezed).

The researchers are of the opinion that the new THz radiation technique may help improve the time resolution of such approaches.

They say that the technique has other applications as well, for it can be applied to determine the structure of many kinds of electronic devices that are constructed using thin film layered structures, such as field-effect transistors.

"The detection of high frequency acoustic waves also has been proposed for use in imaging of quantum dot nanostructures used in myriad optical devices, possibly including solar cells in the future. The technology is not there yet for that application, but our work represents a step closer," Reed said.


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