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Ultra-High Intensity Laser Sparks New Behavior in Light

By focusing laser light to brightness one billion times greater than the surface of the Sun, a team of physicists from the United States and China has observed changes in a vision-enabling interaction between light and matter. Those changes yielded unique X-ray pulses with the potential to generate extremely high-resolution imagery useful for medical, engineering, scientific and security purposes.

A rendering of how changes in an electron’s motion (bottom view) alter the scattering of light (top view), as measured in a new experiment that scattered more than 500 photons of light from a single electron. Image credit: Extreme Light Laboratory, University of Nebraska-Lincoln.

A rendering of how changes in an electron’s motion (bottom view) alter the scattering of light (top view), as measured in a new experiment that scattered more than 500 photons of light from a single electron. Image credit: Extreme Light Laboratory, University of Nebraska-Lincoln.

The team, headed by Professor Donald Umstadter, director of the Extreme Light Laboratory University of Nebraska-Lincoln, fired an ultra-high-intensity laser system, DIOCLES, at helium-suspended electrons to measure how the laser’s photons scattered from a single electron after striking it.

“Under typical conditions, as when light from a bulb or the Sun strikes a surface, that scattering phenomenon makes vision possible,” Prof. Umstadter said.

“But an electron – the negatively charged particle present in matter-forming atoms – normally scatters just one photon of light at a time. And the average electron rarely enjoys even that privilege, getting struck only once every four months or so.”

Though previous laser-based experiments had scattered a few photons from the same electron, Prof. Umstadter and co-authors managed to scatter more than 500 photons at a time.

At the ultra-high intensities produced by DIOCLES, both the photons and electron behaved much differently than usual.

“When we have this unimaginably bright light, it turns out that the scattering — this fundamental thing that makes everything visible — fundamentally changes in nature,” Prof. Umstadter said.

A photon from standard light will typically scatter at the same angle and energy it featured before striking the electron, regardless of how bright its light might be.

Yet the team found that, above a certain threshold, the laser’s brightness altered the angle, shape and wavelength of that scattered light.

“So it’s as if things appear differently as you turn up the brightness of the light, which is not something you normally would experience,” Prof. Umstadter said.

“An object normally becomes brighter, but otherwise, it looks just like it did with a lower light level. But here, the light is changing the object’s appearance. The light’s coming off at different angles, with different colors, depending on how bright it is.”

That phenomenon stemmed partly from a change in the electron, which abandoned its usual up-and-down motion in favor of a figure-8 flight pattern.

As it would under normal conditions, the electron also ejected its own photon, which was jarred loose by the energy of the incoming photons.

But the researchers found that the ejected photon absorbed the collective energy of all the scattered photons, granting it the energy and wavelength of an X-ray.

“The unique properties of that X-ray might be applied in multiple ways,” Prof. Umstadter said.

“Its extreme but narrow range of energy, combined with its extraordinarily short duration, could help generate 3D images on the nanoscopic scale while reducing the dose necessary to produce them.”

Those qualities might qualify it to hunt for tumors or microfractures that elude conventional X-rays, map the molecular landscapes of nanoscopic materials now finding their way into semiconductor technology, or detect increasingly sophisticated threats at security checkpoints.

Atomic and molecular physicists could also employ the X-ray as a form of ultrafast camera to capture snapshots of electron motion or chemical reactions.

The findings were published this week in the online edition of the journal Nature Photonics.

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Wenchao Yan et al. High-order multiphoton Thomson scattering. Nature Photonics, published online June 26, 2017; doi: 10.1038/nphoton.2017.100