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  • Avalanche of emissions creates first atomic x-ray laser

Photonics Spectra
Apr 2012
Ashley N. Paddock,

LIVERMORE, Calif. – The shortest, purest x-ray laser pulses ever achieved fulfill a 45-year-old prediction and could open the door to new materials, medicines and devices.

Physicists from Stanford Linear Accelerator Center (SLAC) created the pulses, which were around 40 fs, by aiming the center’s Linac Coherent Light Source (LCLS) at a capsule of neon gas, setting off an avalanche of x-ray emissions to create the first “atomic x-ray laser.”

“The atomic x-ray laser, in potential pump probe settings, will allow us to study the dynamics of chemical processes, like charge transfer, isomerization, conformational changes, etc.,” said lead investigator Nina Rohringer in an email. “Once one understands these processes and the timescales involved, one can think about using the new knowledge in designing new materials.” She collaborated with scientists from SLAC, Lawrence Livermore National Laboratory and Colorado State University.

The new laser fulfills a 1967 prediction that x-ray lasers could be made by first removing inner electrons from atoms and then inducing electrons to fall from higher to lower energy levels, releasing a single color of light in the process. But until 2009, when LCLS came online, no x-ray source was powerful enough to create this type of laser.

A powerful x-ray laser pulse from SLAC National Accelerator Laboratory’s Linac Coherent Light Source comes up from the lower-left corner (green) and hits a neon atom (center). Illustration courtesy of Gregory M. Stewart/SLAC.

To make the laser, LCLS’ powerful x-ray pulses – each a billion times brighter than any available before – knocked electrons out of the inner shells of many of the neon atoms. When other electrons fell in to fill the holes, about one in 50 atoms responded by emitting a photon in the very short hard x-ray wavelength. Those x-rays stimulated neighboring neon atoms to emit more x-rays, creating a domino effect that amplified laser light 200 million times.

Using other gases, Rohringer hopes to achieve even shorter-pulse, higher-energy atomic x-ray lasers. The team is running calculations using oxygen and nitrogen molecules and has found that nuclear dynamics play a big role in the lasing of molecules.

“We are currently considering lasing in nitrogen and oxygen,” she said. “But molecules open up new challenges. In addition to the electronic dynamics, nuclear dynamics (vibrations) play a role – this might disrupt the lasing process.

They are working on possible applications, but the scheme of x-ray pump x-ray probe experiments is virtually unexplored, “so that we have quite some work ahead of us to determine the best conceptual techniques,” Rohringer said. “One potential application is to use the two-color source created, i.e., the transmitted pump pulse (which is tunable) and the atomic x-ray laser pulse (at fixed energy).”

Next, her team will consider “Raman type” lasers, where amplification and compression of x-ray pulses via an x-ray Raman scattering process will be exploited.

For her, the most challenging and interesting question will be to transfer nonlinear optical spectroscopic techniques to x-rays. “This is going to be challenging because of the small interaction strength of x-rays, combined with the ultrashort timescales (femto- and subfemtosecond) we have to deal with in the x-ray regime,” Rohringer said. “But we would gain element specificity and atomic spatial resolution, which would be a great step toward understanding the structural dynamics of chemical and biological processes.

“I think that, with the invention of x-ray free-electron lasers, it is possible to venture [into] the field of quantum optics with x-rays,” she said. “People have been dreaming about this since the ’60s.”

The research appeared in the Jan. 26 issue of Nature (doi: 10.1038/nature10721).

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