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Revolutionary X-ray microscope unveils sound waves deep within crystals

October 01, 2023



Researchers at the SLAC National Accelerator Laboratory, a division of the Department of Energy. A state-of-the-art X-ray microscope developed by Stanford University and Denmark Technical University can directly observe sound waves at the smallest size possible—the lattice level within a crystal. These results, which were released in the Proceedings of the National Academy of Sciences last week, have the potential to alter how researchers investigate extremely quick changes in materials and their subsequent characteristics.

"The atomic structure of crystalline materials gives rise to their properties and associated 'use-case' for an application," stated Leora Dresselhaus-Marais, an assistant professor at SLAC and Stanford University, who was one of the researchers.

The reason why certain materials undergo strengthening while others fracture when subjected to the same stress can be explained by crystalline flaws and atomic size displacements. Our capacity to regulate certain faults has been refined by blacksmiths and semiconductor manufacturers, but there are currently few methods that can photograph these dynamics in real-time at the right scales to determine how the distortions relate to the bulk characteristics."

In this new effort, the scientists imaged the small distortions inside the crystalline lattice directly by creating soundwaves inside a diamond crystal using the newly built X-ray microscope. They accomplished this by utilizing the ultrafast and ultrabright pulses available at SLAC's Linac Coherent Light Source (LCLS) to achieve these atomic-scale vibrations at the timeframes at which they naturally occur.

In order to eliminate the "perfectly packed" part of the crystal and focus on structural faults and sound wave distortions, the researchers positioned a specialized X-ray lens along the beam diffracted by the crystalline lattice.

We utilized this to depict the process by which an ultrafast laser converts light energy into heat by reflecting the out-of-equilibrium sound wave off the crystal's front and rear surfaces successively, according to Dresselhaus-Marais. "We demonstrate this in diamond, the crystal with the fastest sound speed, to highlight the new opportunities our microscope has created for studying new phenomena deep within crystals."

The results point to a non-destructive method for observing ultrafast material changes. The instruments employed by researchers to observe these changes were far too sluggish prior to this discovery. This is significant because a lot of things rely on these quick changes, such as the propagation of sound waves and the movement of heat.

This discovery has broad ramifications for a number of academic subjects, including physics, materials science, geology, and manufacturing. Scientists can get a better grasp of transformations, melting processes, and chemical reactions in materials by comprehending the atomic-level changes that result in bigger observable events in materials—thereby opening up new 13 orders of magnitude of timeframes.

Dresselhaus-Marais stated, "This new tool offers us a unique opportunity to study how rare events leading to macroscopic changes in materials are caused by defects, atomic-distortions, or other localized stimuli inside a lattice."

"Although we have a considerable grasp of the macroscopic changes in materials, we frequently lack the information on the precise 'instigating processes' that lead to the phase transformations, melting, or chemistry we see at larger scales. We can now search for these uncommon occurrences at their natural timings because we have access to ultrashort timelines."