MIR Laser Creates High-Flux X-Ray Pulses

Studying time-dependent incidents on short timescales requires femtosecond, high-flux x-ray pulses. A new high-intensity mid-infrared laser system accomplishes this challenging feat.

A team led by the Vienna University of Technology developed the new hard x-ray plasma source, which is driven by intense, sub-100-fs pulses at 3.9-µm wavelengths. The longer the wavelength, the researchers said, the higher x-ray flux, which could ultimately provide more accurate measurements and imaging in medicine and materials science.

“Previously, this experiment was done with a common 0.8 µm laser,” said postdoctoral researcher Skirmantas Alisauskas. “The wavelength of our laser pulses is five times as long, which translates into a 25 times higher x-ray flux.”

Postdoctoral researcher Skirmantas Alisauskas adjusts an optical table at the Photonics Institute at the Vienna University of Technology.

He noted that each laser pulse leads to 1 billion x-ray photons, emitted in all directions.

The laser works by emitting MIR light, which hits a copper plate. The intensity of that light is so high that electrons are ripped out of the atoms. The laser field turns the electrons around and accelerates them, prompting them to hit the copper again at very high energies. During this impact, x-ray radiation is emitted.

“The flux of the x-ray radiation depends on the wavelength of the laser,” Alisauskas said. “If the laser wavelength is long, then every electron spends a lot of time in the laser field before it returns to the copper atoms. It has more time to gain energy and hits the copper plate even harder.”

Creation of the new laser proved technologically challenging, according to the researchers, given the push for longer wavelengths and intensity.

“Building lasers with such long wavelengths is hard, but the main problem was making the laser light intense enough,” Alisauskas said.

Further development of the laser system is ongoing, as the researchers work to create even longer wavelengths and potentially increase the rate at which the laser pulses.

“At the moment, we have a repetition rate of 20 laser pulses per second,” said professor Dr. Andrius Baltuska. “That is perfectly fine for a proof of principle, but for technological applications we need to increase that.”

“Theoretical simulations account for the experimental results in a wide range of driving fields and predict a further enhancement of x-ray flux,” the researchers wrote in the study.

The research was published in Nature Photonics (doi: 10.1038/nphoton.2014.256).

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