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Imaging in a FLASH: Scientists Capture First-ever X-ray Laser Image Using a Single, Ultra-fast Shot

Researchers have for the first time used an extremely short and intense x-ray laser pulse to successfully obtain a high-resolution image of a nano-scale object before the laser destroyed the sample. The experiment, conducted at Deutsches Elektronen-Synchrotron (DESY) in Hamburg by a collaboration that included researchers from the Department of Energy's Stanford Linear Accelerator Center (SLAC), also set a speed record of 25 quadrillionths of a second (25 femtoseconds) of the duration of the x-ray pulse used to acquire the image. The results are published in the November 12 online edition and the December print edition of Nature Physics.

"This result is a remarkable validation of the concept of imaging using single pulses from a free electron laser," said Keith Hodgson, Director of Photon Science at SLAC and a co-author of the paper. "This is just the first glimpse of the breakthrough discoveries that will come from the Linac Coherent Light Source (LCLS) when it becomes operational in 2009."

Called "flash diffraction imaging," this experiment proves the principle behind atomic-scale imaging that will be applied when even more powerful x-ray free-electron lasers are available, such as the LCLS, now under construction at SLAC; the SPring-8 Compact SASE Source (SCSS) facility in Japan; and the European XFEL in Hamburg. According to researchers, these revolutionary lasers will give scientists unprecedented insight into fields such as materials science, chemistry, biology and medicine.

Computer models had suggested that by precisely tuning an x-ray laser, images of microscopic and even atom-sized objects could be obtained in the fraction of a second before the sample is stripped of its electrons and destroyed. However, up until now, there had been no experimental verification of this principle.

Free-electron lasers are a new class of laser that create extremely intense photon pulses in the x-ray spectrum using a beam of electrons from a particle accelerator. Once it is fully operational, the LCLS will use SLAC's existing linear accelerator to produce laser light that is 10 billion times brighter than any other x-ray source on earth.

Using the world's first soft x-ray free-electron laser at Deutsches Elektronen-Synchrotron (DESY) in Hamburg, an international collaboration led by Janos Hajdu (SLAC and Uppsala University) and Henry Chapman of (Lawrence Livermore National Laboratory [LLNL]) zapped a sample that contained nanometer-sized objects and recorded the pattern of scattered x-rays—the diffraction pattern—before the laser destroyed the sample. A special computer algorithm was then used to recreate an image of the object based on the recorded diffraction pattern.

Hajdu theorized that a single diffraction pattern could also be obtained from an atomic-scale object—such as a macromolecule, virus or cell—using an ultra-short and extremely bright x-ray free-electron laser pulse, before the sample explodes and turns into a plasma.

Successfully demonstrating the workability of this theory using soft x-rays—which have a long wavelength and are useful for imaging nano-scale objects—means that scientists can soon apply this technique using "hard x-rays," which have a much shorter wavelength suitable for studying much smaller objects. The LCLS will be the first free-electron laser capable of producing hard x-rays, and will provide an ideal platform for studying atomic- and molecule-sized objects such as proteins.

Current techniques for imaging biomolecules with x-rays requires that samples be grown into large crystals the size of a salt grain which contain many molecules arranged in a regular pattern. But many biomolecules resist crystallization. Because flash imaging can image single molecules, this new technique will finally enable scientists to closely and rapidly study all classes of proteins.

"The entire collaboration is very excited by these results," said Hajdu. "Flash imaging has implications for studying molecular structures in biology in a whole new way. A new scientific community is forming to achieve these goals by combining biology with atomic, plasma, and astrophysics for the first time."

Scientists from LLNL, Uppsala University in Sweden, SLAC, DESY, Technische Universitšt Berlin, the Center for Biophotonics Science and Technology at U.C. Davis, and the private firm Spiller X-ray Optics of Livermore, conducted the first experimental demonstration of this theory.

The work was funded in part by the U.S. Department of Energy Office of Science, by a Laboratory Directed Research and Development strategic initiative proposal for "biological imaging with fourth-generation light sources" at LLNL, and by the Swedish Research Councils.

—Brad Plummer
    SLAC Today, November 13, 2006

Above image: Researchers zapped the sample (left) with a single, 25 femtosecond pulse from the DESY soft x-ray free-electron laser to obtain a diffraction pattern (center). The figures in the image are cut into a 20-micrometer-wide square of silicon nitride film, which was destroyed by the laser pulse. A special computer algorithm then processed the diffraction pattern to reconstruct the original image (right). (Click on image for larger version.)