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Published on October 31st, 2014 | By: April Gocha, PhD

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New technique captures ultrafast processes like a flip-book

Published on October 31st, 2014 | By: April Gocha, PhD

 

 

Don’t turn your back. Don’t look away. And don’t blink.

 

Otherwise you might miss it.

 

While the above imperatives could be sound advice to reduce your trick:treat ratio and avoid any sleepless nights induced by all those nightmarish ghouls, ghosts, goblins, demons, devils, monsters, and zombies roaming the Halloween streets, that’s not what I’m referring to.

 

The real reason not to blink is that you might miss the latest experiment from McGill University (Canada) scientists, an experiment that is peering into materials on a femtosecond time scale—that’s one quadrillionth of a second, if you were already beginning to count those -15 decimal places.

 

The scientists investigated vanadium dioxide, a material with some interesting temperature-dependent properties. Room temperature finds VO2 a semiconductor, but a slight bump up to just 68°C finds the material with a rearranged atomic structure that makes it a very conductive metal. The material also has temperature-dependent reflective properties, altering transparency to infrared light with VO2’s temperature.

 

And because they happen so fast, these mood ring-esque properties make VO2 an interesting material for a variety of electronic applications. But beyond knowing that it happens, scientists didn’t know if the switch was caused by changes in VO2’s crystal structure or altered electron interactions—an important distinction for those applications.

 

To get an atomic glimpse at the ultrafast process, McGill researchers had to build an elaborate setup of a maze of lasers, amplifiers, and lenses along with a custom electron microscope, all housed on a vibration-free table in a basement laboratory.

 

While spending nearly four years to prepare the equipment for a single experiment is enough to give any grad student nightmares, senior researcher Bradley Siwick knows the work has paid off—the results are published in a recent issue of Science.

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Bradley Siwick tweaking up the laser in his McGill University lab. Credit: Allen McInnis; McGill U.

 

“We’ve developed instruments and approaches that allow us to actually look into the microscopic structure of matter, on femtosecond time scales (one millionth of a billionth of a second) that are fundamental to processes in chemistry, materials science, condensed-matter physics, and biology,” Siwick says in a McGill press release. “We’re able to both watch where nuclei go, and separate that from what’s happening with the electrons. And, on top of that, we are able to say what impact those structural changes have on the property of the material. That’s what’s really important technologically.”

 

The team captured snapshots of the transition in thin films of VO2 (just 70 nm-thick) and showed that they could induce a metastable state that, although it retained the semiconductor lattice distortion, had metal-like optical properties.

 

Beyond VO2, however, the experiment is an important proof-of-concept of the technique and its ability to capture ultrafast—and microscopic—processes in action, technology that could be useful in a wide range of fields that explore microscopic processes such as chemistry, biology, physics, and materials science.

 

To get all the study’s details, check out the paper: “A photoinduced metal-like phase of monoclinic VO2 revealed by ultrafast electron diffraction” (DOI: 10.1126/science.1253779).

 

 

 


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