The diffraction of coherent X-rays is routinely used to determine the structure of crystals and molecules, and underpinned the discovery of the double-helix structure of DNA in 1953. As the method relies on diffraction and interference it requires the X-rays to be scattered coherently. When this is not the case, the incident and diffracted waves are not in phase, and X-ray imaging methods cannot generate the diffractive patterns needed to reconstruct the arrangement of the atoms in a crystal. This poses a big limit to coherent X-ray diffractive imaging since incoherent scattering predominates in the X-ray domain and much effort is needed to ensure coherence.
Now, this prerequisite no longer stands. Joachim von Zanthier and colleagues have demonstrated that incoherently scattered photons can be used to image tiny, complex structures (Nat. Phys. https://dx.doi.org/10.1038/nphys4301; 2017). Specifically, they have shown that incoherently scattered X-rays from a free-electron laser (FEL) can image 2D objects with a spatial resolution close to or even below the Abbe limit. The imaging capability in two dimensions was surprising to the researchers considering the much enlarged parameter space for the possible phase combinations used to determine the higher-order correlation functions. It allows, for example, imaging of arbitrary 2D objects on a substrate, and potentially transfers the ideas of quantum imaging from visible wavelengths to shorter wavelengths.
The team performed the experiment at the PG2 beamline of the Free-Electron Laser Hamburg (FLASH) at Deutsches Elektronen-Synchrotron (DESY), Hamburg. The FEL beam runs in a 10 Hz pulsed mode at 13.2 nm. It passes a monochromator and impinges on a moving diffusor. The pseudo-thermal light scattered by the diffusor is used to illuminate an object and the light passing through the object is measured by a charge-coupled device (CCD) image sensor. In the experiment, a 2D object mask, consisting of six square-cut holes in a hexagonal arrangement to mimic the carbon atoms in a benzene molecule, on the micrometre scale, was used to generate six quasi-monochromatic independently radiating incoherent sources. The researchers showed that they were able to determine the entire benzene structure based on the 10,800 single-shot speckle patterns (see image) obtained by the CCD detector.
“The requirements for the implementation — high brilliance, ultrashort excitations and high repetition rates — are well met by the FEL facility at DESY. Our next step is to apply the scheme in the hard X-ray regime to reveal structures of crystals, nanoparticles, or even single molecules at the atomic scale,” said von Zanthier, who also added that the approach will likely improve structural analyses in biology and medicine.
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Won, R. Incoherent success. Nature Photon 12, 6 (2018). https://doi.org/10.1038/s41566-017-0080-5