Focused on coherence at NanoMAX

Focused on coherence at NanoMAX

photo: B. Libberton

We may have state-of-the-art detectors, the worlds most modern synchrotron facility and an intense X-ray beam shining at our sample through a carefully put together beamline – but if we can’t measure the properties of that beam, the data we from the sample will not be possible to interpret correctly.

In a paper recently published in Journal of Synchrotron Radiation, researchers from Institute for X-ray physics at the University of Göttingen and MAX IV Laboratory have characterized the focal spot size and coherence properties of the X-ray beam at one of the two experiment setups at NanoMAX using a 35 nm wide, square X-ray waveguide as a slit. In a waveguide, the light is guided along a material with a higher refractive index surrounded by a material with a lower refractive index. An example for visible light is an optical fiber. The focus was determined by observing the X-ray intensity transmitted through the waveguide as it was scanned through the beam. The beam size could be focused down to 56 nm, measured at full width half maximum in the horizontal direction.

Coherence is a property of the wavefronts of the electromagnetic light wave, and a highly coherent X-ray beam is one of the major advantages of the MAX IV accelerator. The wave expands from its source like the ripples on a pond when you throw a rock in the water. If the wavefronts don’t cross each other at any point, the light source is said to be fully coherent. The degree of coherence was determined while selecting a smaller or larger portion of the beam, using the variable opening called secondary source aperture that is placed about halfway down the beamline. The coherence of a light source can always be made greater if you look at a smaller portion of the emitted light. Closing the aperture means a higher degree of coherence but a less intense beam and vice versa.

The degree of coherence was concluded by observing the fringes that form close to the focal point and comparing them to a simulation of what they should look like for a certain degree of coherence. It was found to be larger than 50% for secondary source aperture sizes below 11 micrometers at an X-ray energy of 14 keV and 87% for a secondary source aperture size below 5 micrometers.

With a beam having a high degree of coherence, the researchers can do experiments that would otherwise not have been possible. One example is lens-less imaging, where the X-ray beam is scattered off a nanometer sized object. The scattered light captured on the detector can then be directly transformed into a high-resolution image of the object under study. It’s a complement to methods like transmission electron microscopy for both biological and materials science samples.

 

Read the paper here (Open Access)

Focus characterization of the NanoMAX Kirkpatrick–Baez mirror system
M. Osterhoff et al.
J. Synch. Rad. 26(4), 1173–1180

https://doi.org/10.1107/S1600577519003886