Laser Pas de Deux Enhances Resolution in Time and Space
Synchronized optical and x-ray wavelengths permit imaging of minute dynamic processes.
Lynn M. Savage
To completely grasp the physics underlying such phenomena as cellular function, chemicals changing from one state to another, or the formation of cracks and fissures in polymer thin films, one must watch such events as they occur, as close up as possible. Optical microscopes long ago reached the boundaries of usefulness for studying dynamic forces in action — because of the diffraction limit — but not even electron microscopes or other specialized devices were able to penetrate the physical world with both high temporal and high spatial resolution.
Now, however, members of an international collaboration — led by researchers at Lawrence Livermore National Laboratory in California — have used synchronized x-ray and optical laser pulses to achieve 50-nm spatial resolution and 10-ps temporal resolution while imaging the laser ablation of a test material. The temporal resolution is limited only by the 25-fs “shutter speed” of the x-ray pulses provided by free-electron laser sources.
Synchronized beams from an x-ray and an optical laser provide high temporal and spatial resolution, enabling imaging of ultrafast dynamic processes. While a pump laser ablates the sample, the beam from a free-electron laser passes through the sample, and the diffracted x-rays reflect off the mirror onto the CCD chip. Direct x-rays pass through a hole in the mirror so as not to burn out the chip. Images courtesy of DESY, Hamburg, Germany.
Single-shot pulsed electron diffraction imaging is used to study ultrafast processes in solids, but that technique currently is capable of only nanosecond resolution for samples of this class. According to lead researchers Henry N. Chapman and Anton Barty, the group improved the resolution by more than three orders of magnitude and expects to improve on that result by another factor of 50 or so, down to the x-ray pulse length of 25 fs.
The imaging system, which was designed and built at Lawrence Livermore National Laboratory, features a unique x-ray camera and an Nd:YLF laser operating at 523 nm that was used to ablate a silicon nitride sample with nanoscale patterns etched into it. The laser generated 12.5-ps pulses at about 25 μJ each. The system also includes a back-illuminated direct-detection CCD chip for recording images. The CCD is blinded to the 523-nm light by a 100-nm-thick zirconium filter. Furthermore, the camera chip is protected from ablation by an x-ray mirror with a hole through its center; the hole allows direct x-ray radiation to pass through, while the mirror reflects diffracted x-rays onto the CCD chip.
A scanning electron microscope image depicts the sample, a 20 × 20-μm silicon nitride substrate with a pattern etched into it (top). A CCD camera acquired single-shot images before and after the sample was ablated with a 523-nm laser (right).
The duration of the x-ray pulses set the frame rate of the imaging system. To obtain the fastest possible rate, the researchers used femto-second pulses generated by the FLASH soft x-ray free-electron laser facility at Deutsches Elektronen Synchrotron (DESY) in Hamburg, Germany. An optical delay line in the pump laser path helped synchronize the optical and x-ray laser beams — the investigators tested delays of 10 to 140 ps.
“Currently, there is only one free-electron x-ray laser in the world, and demand is intense,” Chapman said. “However, many more soft x-ray free-electron lasers are being planned or are under construction in the world.”
The group — which includes members from Uppsala University in Sweden, Universität Duisburg-Essen in Germany and the University of Oxford in the UK, along with the previously mentioned institutes — intends to demonstrate better spatial and temporal resolutions using shorter wavelengths and improved synchronization, as well as to take the technique beyond the demonstration stage.
Nature Photonics, July 2008, pp. 415-419.
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