For the Very First Time, We've Actually Filmed Sound Waves Inside Crystals

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Scientists Film Sound Waves Inside a Crystalenot-poloskun - Getty Images
  • In order to understand the macro dynamics of materials, scientists need to understand what instigates certain phenomena, which occur on infinitesimally small timescales.

  • To this end, scientists at the Technical University of Denmark and Stanford University successfully filmed sound waves in a diamond crystal structure for the first time ever using an X-ray free-electron laser (XFEL).

  • Gaining insight into these sub-microsecond processes could pave the way toward advancements in materials science and geoscience.

Understanding the crystalline structure of materials and how they react to certain phenomena is vital to advancing the field of materials physics. Being able to create new electrical circuits, photonic chips, or next-generation superconductors means understanding materials on extremely minute scales.

To achieve this, scientists use what’s known as an X-ray free-electron laser (XFEL), like the one at the Stanford Linear Accelerator Center (SLAC). At the end of this 1.8-mile-long tube is a microscope that examines materials using extremely bright X-rays at across a 1 mm sample.

Crucially, these XFEL facilities (of which there are only a handful across the globe) create “movies” capturing the atomic goings-on inside a crystalline structure. And for the first time ever, scientists from the Technical University of Denmark (DTU) and Stanford University captured sound waves as they traveled through a diamond crystal. To capture these sound waves, scientists successfully filmed the crystal sample with extremely high precision, as the wave only exists within the crystal for one-millionth of a second. The results were published in mid-September in the journal Proceedings of the National Academy of Sciences (PNAS).

“Our hair-thin X-ray and optical laser beams had to meet on the mm-sized single-crystal diamond sample with a better timing accuracy than a nanosecond before the first data could be acquired,” Henning Friis Poulsen, a physics professor at DTU and study co-author, said in a press statement. “But we did it, and I believe these results will inspire a plethora of new research.”

Using the ultrafast and ultra-bright laser pulses at SLAC’s Linac Coherent Light Source (LCLS), the team measured this acoustic phenomena with sub-picosecond accuracy—a level of detail never before captured. They were also able to capture these sound waves non-invasively, which helped preserve the structural integrity of the sample.

The video above shows different sound waves as they reflect off the surface of the crystalline structure like an impossibly tiny game of Pong. Another video shows how the sound wave diffuses and attenuates over less than a microsecond timescale.

“With this setup, we can investigate a wide range of ultrafast structural phenomena that have so far been beyond the reach of science,” DTU’s Theodor Holstad, a co-author of the study, said in a press statement. “Visualizing structural processes on a timescale of less than a microsecond is relevant for solid-state physics, materials science, and geoscience.”

Understanding these small nuances at extremely small scales can also give important insight into large scale phenomena—for example, why certain materials strengthen while others shatter when subjected to the same level of force. Stanford’s Leora Dresselhaus-Marais, who was involved with the study, says in a Stanford press statement that, though we have an advanced understanding of these more macro phenomena (such as phase transformations), scientists are still missing vital information regarding these “instigating events” that occur at incredibly short timescales.

But, she added, the gaps in our knowledge are slowly disappearing. “By showing this behavior in diamond—a crystal with the fastest sound speed—we illustrate the new opportunities now available with our microscope to study new phenomena deep inside crystals,” Dresselhaus-Marais said. “With ultrashort timescales now at our fingertips, we have the ability to hunt for these rare events at their native timescales.”

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