Soundwaves propagating in a diamond crystal, captured with the new X-ray technique. Image: Theodor S. Holstad et al./DTU.
Soundwaves propagating in a diamond crystal, captured with the new X-ray technique. Image: Theodor S. Holstad et al./DTU.

Solid crystalline materials – such as metals, ceramics, rock and bone – are notoriously difficult to model. They are made up of grains, domains and defects and are exposed to many competing forces that exert their influence on several levels. To model such materials, scientists have utilized highly specialized X-rays that can characterize materials at a resolution as low as 100nm.

Resolution, however, is not the only important criteria. There is also the fact that what is happening in these materials occurs over time, but up to now scientists have only ben able to follow these processes over time periods of milliseconds to seconds. Inside crystals, though, some things happen very fast, like when they change shape or transfer heat. These processes are related to how the atoms in the crystals are arranged and move, and often happen in microseconds – sometimes even in nano- or picoseconds (one billionth or one trillionth of a second, respectively).

This is the case with sound waves travelling through a crystalline material, which happens in less than a millisecond. Now, in a paper in the Proceedings of the National Academy of Sciences, researchers from Denmark and the US report capturing images of soundwaves traveling through a 1mm diamond sample.

“We want to see these changes in 3D, but until now, it couldn’t be done fast enough or without damaging the crystals,” explains Henning Friis Poulsen, a professor in the Department of Physics at the Technical University of Denmark (DTU). “Our new technology can do it faster and non-invasively and will work for many crystals.

“To do imaging at the speed of sound, it was necessary to build an entirely new microscope at the end of a 3km-long X-ray free-electron laser (XFEL). It is not a given that you’ll succeed when aiming a 3km-long X-ray source through multiple lenses and onto a sample that is 1mm across, while you hope to see a soundwave that only exists for a millionth of a second. Our hair-thin X-ray and optical laser beams had to meet on the millimeter-sized single-crystal diamond sample with a better timing accuracy than a nanosecond before the first data could be acquired. But we did it, and I believe these results will inspire a plethora of new research.”

Co-author Theodor Holstad from DTU says that their approach could apply to all types of crystalline materials. “With this setup, we can investigate a wide range of ultrafast structural phenomena that have so far been beyond the reach of science. Visualizing structural processes on a timescale of less than a microsecond is relevant for solid-state physics, materials science and geoscience. For example, when you want to understand processes in meta-materials, photonic crystals, thermoelectric materials or even soft materials such as perylene and hybrid perovskites. Finally, it may be useful in geoscience to test seismological models of how sound travels in planetary materials.”

The researchers took snapshots of the soundwaves with timesteps as small as just a few picoseconds and then edited those snapshots into small films. This allowed them to capture movies of different types of sound waves travelling and reflecting off the surface of a crystal and of dispersion (the wave spreading out over time) and attenuation (weakening of the wave) over microsecond timescales.

Finally, they demonstrated that just a single X-ray pulse less than a thousandth of a nanosecond long was sufficient for imaging, opening the door to visualizing stochastic and irreversible processes in real-time on timescales of microseconds or less.

This story is adapted from material from the Technical University of Denmark, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.