X-ray photo uses world's fastest flash

15 Nov 2006

The first image using an extremely intense, ultrashort wavelength X-ray "flash" from a free-electron laser (FEL) provides proof-of-principle for a new lensless, atomic-scale imaging technique.

Researchers in the US and Germany have for the first time produced images using FLASH, the first X-ray FEL in the world. FLASH opened earlier this year at DESY, the particle physics and synchrotron research centre in Hamburg, Germany.

Diffraction image

Using a 32 nm-wavelength "soft" X-ray flash lasting just 25 femtoseconds, the image produced in the FLASH experiment had a resolution of 62 nm. By shortening the wavelength and focussing the beam more tightly, the researchers believe that a "hard" X-ray FEL operating at 0.15 nm wavelength could yield a resolution length of 0.3 nm.

According to the paper published in Nature Physics, the photo is "proof-of-principle" for a technique that could one-day allow atomic-scale imaging of nanoscale objects in materials science, biology and medicine (Nature Physics AOP doi: 10.1038/nphys461).

In contrast to conventional photography, where the flash is used to illuminate the object so it reflects more light into the camera, in single-pulse FEL imaging the burst of high-energy X-ray photons is scattered through the sample to create a diffraction pattern.

At the same time, the X-ray flash strips the object of all its electrons, reducing it to a plasma. However, scientists can reconstruct an image of the original sample by analysing the diffraction pattern emitted before this so-called Coulomb explosion.

Image reconstruction

Using this method, the international collaboration, lead by Henry Chapman of the Lawrence Livermore National Laboratory (LLNL) and Janos Hajdu of Uppsala University in Sweden, imaged a pattern made in a silicon nitride film using a 4x10¹³Wcm^-2 pulse containing 10¹² X-ray photons.

The pulse lasted just 25 femtoseconds, a record for flash imaging and a trillion times shorter than that used for a conventional photo. Although the flash destroyed the sample at a temperature of 60,000 K, the researchers could reconstruct an image of the object from the diffraction pattern using a computer algorithm developed at LLNL.

In the absence of any previous experimental data on such extreme photon-material interactions, the researchers could only rely on theoretical predictions, first made by Hajdu in 2000, to choose a suitable pulse length, frequency and intensity. The extremely short pulse time allowed the researchers to obtain useful diffraction data in the instant before the sample was destroyed.

According to the researchers, the lensless technique could achieve atomic-scale resolution below the optical limit. Also, unlike traditional synchrotron diffractive imaging, it is not restricted to periodic, crystalline objects -- which means that it can be used to study a wide-range of non-periodic materials like complex biological molecules.

Another, more powerful X-ray laser source, the Linac Coherent Light Source (LCLS), currently under construction at SLAC, could deliver still higher resolutions. "This is just the first glimpse of the breakthrough discoveries that will come from LCLS when it becomes operational in August 2008," said Keith Hodgson, co-author of the paper and Director of Photon Science at SLAC.

The international collaboration at DESY also included researchers from the University of California, Davis, the Technical University of Berlin, the Stanford Synchrotron Radiation Laboratory and private firm Spiller X-ray Optics of Livermore.