Ultrafast, Intense Laser Captures Nanoscale Images

Using a single, extremely short and intense x-ray laser pulse, an international team of scientists have, for the first time, taken a high-resolution diffraction image of an object such as a protein before the intensity of the radiation destroyed the sample. The experiment was the first successful application of "flash diffractive imaging" and begins a new era in structural research.

The new method will be applicable to atomic-resolution imaging of complex biomolecules when even more powerful x-ray lasers, currently under construction, are available. The technique will allow scientists to gain insight into the fields of materials science, plasma physics, biology and medicine.

The single-shot image of a sample made at FLASH. At left is the diffractive pattern of the micro structure, right is the image of the probe damaged by the laser pulse. (Image: H. N. Chapman et al.)

The scientists, part of an international collaboration led by Lawrence Livermore National Laboratory's (LLNL) Henry Chapman and Janos Hajdu of Uppsala University in Sweden, achieved the feat using the world's first soft x-ray free-electron laser, located at the FLASH facility at Deutsches Elektronen-Synchrotron (DESY) in Hamburg. Their work will appear on the cover of the December issue of the journal Nature Physics.

"The entire collaboration is very excited by these results," said Hadju, who is also affiliated with the Stanford Linear Accelerator Center (SLAC). "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."

FLASH generates high-power soft x-ray pulses by the principle of self-amplification of spontaneous emission. The pulses are 10 million times brighter than today's brightest x-ray sources, synchrotrons. In addition, this experiment showed that it only takes a 25-femtosecond pulse duration to capture the image. Patterned into a silicon nitride film, the image was taken at around a trillion times faster than a conventional flash photograph -- just moments before the film evaporated at a temperature of 60,000 °C.

Single-molecule diffractive imaging with an x-ray free-electron laser. Individual biological molecules will be made to fall through the x-ray beam, one at a time, and their structural information recorded in the form of a diffraction pattern. The pulse will ultimately destroy each molecule, but not before the pulse has diffracted from the undamaged structure. The patterns are combined to form an atomic-resolution image of the molecule. (Graphic: LLNL)

A computer algorithm developed at LLNL in Livermore, Calif., was used to recreate an image of the object based on the recorded diffraction pattern. This 'lensless' imaging technique could be applied to atomic-resolution imaging because it is not limited by the need to build a high-resolution lens. The flash images could resolve features 50 nm in size, which is about 10 times smaller than what is achievable with an optical microscope.

The experiment suggests that in the near future, images from nanoparticles and even large individual macromolecules -- viruses or cells -- may be obtained using a single, ultrashort high-intensity laser pulse before the sample explodes and turns into a plasma. This means that scientists could better understand the structure of macromolecular proteins without crystallizing them, which is required in conventional x-ray structure analysis, and have the ability to rapidly study all classes of proteins.

"These results could become a standardized method," Chapman said. "This imaging could be applied at the cellular, subcellular and down on to single-molecule scale."

Atom trajectories computed by a hydrodynamic model show a 2-nm protein exploding after it is hit by a 20-femtosecond, 12-kiloelectronvolt x-ray pulse that is 0.1 µm wide. Models indicate that atomic-resolution imaging can be achieved with pulses shorter than 20 femtoseconds. They also show that a water tamper on the protein slows its destruction so that longer pulses could be used. (Graphic: LLNL)

Free-electron lasers represent an exciting development in fields ranging from structural biology to nanotechnology. Although atomic-scale resolution is not demonstrated in the present work, this could soon be possible when the first of a new generation of hard x-ray free-electron lasers, such as the Linac Coherent Light Source at SLAC, are complete.

FLASH was commissioned in 2004 and has been used for research with shortwave ultraviolet and soft x-ray radiation since 2005. The 260-meter-long facility was first called VUV-FEL (Vacuum Ultraviolet Free-Electron Laser), and renamed FLASH (Free-Electron Laser in Hamburg) in April 2006.

In the first measuring period at FLASH, during which the Chapman and Hajdu team carried out its flash diffraction experiment, the facility already held the world record of the shortest wavelengths ever achieved with a free-electron laser, with pulses at 32 nm. In 2006, it reached a new record with a wavelength of only 13.1 nm, and intense so-called third harmonic radiation at 4.4 nm.

The research team included scientists from LLNL, Uppsala, DESY, Technische Universität Berlin, the Stanford Synchrotron Radiation Laboratory, SLAC, the Center for Biophotonics Science and Technology at the University of California, Davis, and private firm Spiller X-ray Optics of Livermore.

For more information, visit: www.llnl.gov