Physicists from Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU) and Deutsches Elektronen-Synchrotron (DESY, Hamburg) have developed a method to improve the quality of X-ray images over conventional methods. The technique, incoherent diffractive imaging (IDI), could image individual atoms in nanocrystals or molecules faster and with a much higher resolution.
For more than 100 years, X-rays have been used in crystallography to determine the structure of molecules. At the heart of the method are the principles of diffraction and superposition, to which all waves are subject: Light waves consisting of photons are deflected by the atoms in the crystal and overlap like water waves generated by obstacles in a slowly flowing stream. If a sufficient number of these photons can be measured with a detector, a characteristic diffraction pattern or wave pattern is obtained, from which the atomic structure of the crystal can be derived. This requires that photons are scattered coherently, meaning that there is a clear phase relationship between incident and reflected photons. To stay with the water analogy, this corresponds to water waves that are deflected from the obstacles without vortexes or turbulences. If photon scattering is incoherent, the fixed phase relationship between the scattered photons disperses, which makes it impossible to determine the arrangement of the atoms, just as in turbulent waters.
But coherent diffractive imaging also has a problem: "With X-ray light, in most cases incoherent scattering dominates, for example, in the form of fluorescence resulting from photon absorption and subsequent emission," says Anton Classen, member of the FAU working group Quantum Optics and Quantum Information. "This creates a diffuse background that cannot be used for coherent imaging and reduces the reproduction fidelity of coherent methods."
Making use of incoherent radiation
It is exactly this seemingly undesirable incoherent radiation that is key to the FAU researchers' novel imaging technique. "In our method, the incoherently scattered X-ray photons are not recorded over a longer period of time, but in time-resolved short snapshots," says Professor Joachim von Zanthier. "When analysing the snapshots individually, the information about the arrangement of the atoms can be obtained."
The trick is that the light diffraction is still coherent within short sequences. However, this is only possible with extremely short X-ray flashes with durations of no more than a few femtoseconds—that is, a few quadrillionths of a second—which has only been achieved recently using free-electron lasers like the European XFEL in Hamburg or the Linac Coherent Light Source (LCLS) in California.
Visualising single molecules is possible
Since the new method uses fluorescence light, a much stronger signal can be obtained, which is also scattered to significantly larger angles, gaining more detailed spatial information. In addition, filters can be used to measure the light of specific atomic species only. This makes it possible to determine the position of individual atoms in molecules and proteins with a significantly higher resolution compared to coherent imaging using X-ray light of the same wavelength. This method could improve the study of proteins in structural biology and medicine.
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Anton Classen et al, Incoherent Diffractive Imaging via Intensity Correlations of Hard X Rays, Physical Review Letters (2017). DOI: 10.1103/PhysRevLett.119.053401