Recording high-resolution, X-ray holographic snapshots of dynamic processes is now possible thanks to Stefan Eisebitt of Technische Universität Berlin and colleagues there and at Helmholtz-Zentrum Berlin (HZB). Their approach relies on firmly fixed optics and computer power to extract the three-dimensional holographic information and focus the image [Eisebitt, et al., Nat. Commun. (2014), doi:10.1038/ncomms4008].
X-ray holography requires a coherent source, such as that available with the synchrotron source BESSY II. A sample is bathed with incident X-rays while a pinhole taps off a reference wave. Researchers can then create a hologram by superposing the two waves on a detector, data from which is fed to a computer for conversion into an image. Of course to be focused, the pinhole aperture must be very small, which means lower contrast in the resulting hologram.
Eisebitt and his colleagues have sidestepped this problem by swapping the pinhole for another optical element, a Fresnel zone plate. This is placed in the plane of the object and significantly increases the brightness of the reference wave. However, because the zone plate's focal point is itself not in the plane of the object (contrast the pinhole aperture), the image obtained is out of focus. Luckily, the holographic information allows any focal plane to be chosen and the image “sharpened” photographically speaking. The improved efficiency of the method means short exposure times and low contrast samples can be used as well fast dynamic processes studied.
The team demonstrated proof of principle with a sliver of gold foil in the shape of a lizard just a few micrometers from nose to tail and a scallop-shell shaped Siemens star. The highest resolution obtained within these structures was a width of no more than 46 nanometers. Grad student Jan Geilhufe, who developed the technique also devised a way to avid jitter pointing out that a car driving past a kilometer away is sufficient to vibrate the sample and ruin an experiment.
“In our process, we have firmly coupled the object to the reference optics so that the lens fluctuates exactly synchronized with the object. We have built an X-ray camera with an image stabilizer, so to speak,” he explains.
The technique will be implemented in the BESSY II RICXS instrument used to dynamic nanoscale processes in various materials, such as magnetic switching, with much-improved spatial and temporal resolution than currently available.
“In addition to experiments at large-scale X-ray facilities, another interesting application is in imaging with novel laser-based X-ray lab sources,” Eisebitt told Materials Today. “The so-called ‘high harmonic generation’ sources deliver a much lower flux of coherent X-rays but can be operated in a standard university lab. Our high efficiency holography approach will enable us to see very fast processes on the nanoscale with such bench-top instruments.”
