Taking Pictures without a Lens

The coherent light of SwissFEL will grant new insights into nanostructured materials
In X-Ray crystallography the task of imaging periodic structures like those of crystalline substances is tackled routinely with help of well established methods. Essentially it all boils down to searching for a regular array in the diffraction pattern (this is a sort of "fingerprint" of the object being imaged). Initially, on the detectors screen only the diffraction pattern is visible, from which the real image of the object is then reconstructed, applying simple math formula to calculate the distances between the atoms in the crystal.

Much more difficult is the undertaking when it comes to non-periodic structures, in which atoms are not so neatly arranged. The need for understanding these more disordered structures is obvious. Especially materials scientists are interested in all kinds of irregularities or dislocations that can lead to failure of, say, a component of a nuclear reactor subject to radiation damage.

How pictures get unfolded

In the last years, researchers have come up with some new techniques, that allow to image such non-periodic structures with coherent X-rays. The most prominent feature of these novel methods is that they do without any lenses, which automatically removes the fundamental resolution limit inherent to every optical instrument. The coherence requirement is of fundamental significance, since only highly coherent illumination can guarantee that one samples enough data to reconstruct an image from the corresponding diffraction pattern. One of these lens less coherent imaging techniques has been recently developed at the Paul Scherrer Institute. Its name, Ptychography, is derived from the Greek word for folding. The main idea behind Ptychography consists of scanning a large sample step by step in order to enhance the resolution of the image obtained. The "folding" in the name comes from the fact that one illuminates small spots of the sample which are partially superimposed on each other. This is equivalent to folding the sample, and by doing so, one obtains enough redundant information for a successful reconstruction of the real object from its diffraction pattern. The rest of the job is mainly done by smart computer algorithms that retrieve the original image in a certain number of iteration steps. After each iteration step, the scientist only has to take care for applying sensible constraints to make sure that the image produced by the computer matches the experimental data.

Improving Computer Simulations

At present an alternative approach to study non-crystalline structures is by means of computer simulations. Scientists can use their theoretical knowledge of the forces that shape the behavior of a structure and feed this theoretical model into a computer to calculate how an ensemble of atoms evolves over time. In particular, one of these tools, called Molecular Dynamics(MD), can also be used to investigate the before mentioned defects in a material. However, Molecular Dynamics simulations only work on a number of assumptions that must be tested against experimental evidence. That's why experiments with SwissFEL, will be of great help for improving those simulations. Importantly, the experiments planned at SwissFEL will look at structures in the nanometer scale and their evolution over time scales which are similar to those accessible to MD simulations. This perfect matching will certainly be a win-win situation with the potential for strengthening both future computer experiments and X-ray science.

The World's finest detectors: a PSI Inhouse Development fuels SwissFEL success.

Besides Ptychography, the Paul Scherrer Institute has been succesful in developing state-of-the-art detectors- a key requirement for getting ever better X-Ray pictures of "messy" materials. The so-called PILATUS detectors, currently being marketed by PSI spin-off Dectris, have been especially designed to handle the high photon flux of advanced synchrotrons. They feature noise-free single photon detection capability, a high framing rate and a flexible modular build-up. At present, a new generation of PILATUS detectors is being developed to meet the specific requirements of SwissFEL experiments.