X-ray laser to enable nanometre measurements
Researchers from Osaka University, in collaboration with RIKEN and the Japan Synchrotron Radiation Research Institute (JASRI), have reduced the beam diameter of an X-ray free-electron laser to 6 nm in width. Their breakthrough, described in the Journal of Synchrotron Radiation, brings the utility of these lasers for imaging structures closer to the atomic level than was previously possible.
To ‘see’ extremely small and otherwise invisible objects, and observe ultrafast chemical processes, researchers commonly use synchrotron X-ray facilities. X-ray free-electron lasers are an alternative that can — in principle — image atomic-scale detail of, for example, a virus particle on the timescale of an electron transition, without damaging the particle. To do this, you need an incredibly bright X-ray laser that focuses extremely fast laser pulses on the nanometre scale.
“Using multilayer focusing mirrors, we narrowed the width of our laser beam down to a diameter of 6 nm,” said study lead author Takato Inoue, from Osaka University. “This is not quite the diameter of a typical atom, but we’re making good progress.”
Until now, it has been difficult to focus X-ray free-electron lasers to such small diameters due to the challenges involved in fabricating the required mirrors and confirming the focused size of the lasers. The research team addressed the focusing problem by analysing the shape of the laser’s interference patterns, known as speckle profiles.
“We generated speckle profiles by coherent X-ray scattering of randomly distributed metal nanoparticles,” said senior author Satoshi Matsuyama, also from Osaka University. “This enabled experimental measurements of the laser beam profile, which were in good agreement with theoretical calculations.”
Because the laser beam diameter can be so precisely measured, further advancements are now feasible. For example, by using atoms for the scattering analysis, X-ray free-electron laser measurements can be improved to a 1 nm focus.
The researchers anticipate that extremely high-intensity lasers, over a million trillion times brighter than the Sun, will now be useful for imaging ultrafast molecular processes — at atomic-scale detail — that are beyond the capabilities of the most advanced synchrotrons. With such technology, protein molecules and other small important biological entities can be imaged without damaging them under the strategy of ‘diffraction before destruction’, by using a single laser pulse.
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