Clearly Coherence in Beamed X-rays at MAX IV

Clearly Coherence in Beamed X-rays at MAX IV

NanoMAX beamline group (from left): Maik Kahnt, Simone Sala, Alexander Björling, Sebastian Kalbfleisch, Ulf Johansson

Operational performance at MAX IV does live up to theoretical calculation. As published in the journal Optics Express, diagnostics data from NanoMAX beamline confirms the X-ray source delivers simultaneous high coherence and high intensity on sample. The beamline also achieves a diffraction-limited, and therefore extraordinary, resolution. The focused and extreme brightness of the X-rays make it possible for the imaging user community to conduct more complex and detailed experiments faster than before.

Since MAX IV began User Operations in 2017, planning and activities necessary to achieve optimal beamline and facility performance have engaged staff engineers and scientists in daily aspects of their work. To best serve instrument users, it’s important to understand the strengths and limitations of the scientific tools. In this case, the NanoMAX group sought to characterize the quality and coherence of photon flux available for user experiments.

“These results can guide users in designing their studies and proposals, since the degree of coherence and the available X-ray beam intensity decide what experiments are possible and within what timeframes,” said Alexander Björling, beamline scientist for NanoMAX.

NanoMAX is a hard X-ray beamline designed for imaging and scanning diffraction at nanometer range. The techniques are ideal to study both the structure and physical properties of samples for an array of applications in geosciences, biology, chemistry, semiconductor engineering, catalysis and other areas.

test results for X-ray source beam at MAX IV
RESULTS: (left figure) Full coherent flux of the beam, together with the flux of a “single mode” beam that can be considered fully coherent. (right figure) Beam size in the focal plane compared to the diffraction limit.

Testing, testing

The first test of the focused X-ray beam was how to characterize its coherence, a property of light where waves have the same frequency and waveform, and phase differences remain constant. Measurements were taken at NanoMAX using ptychography, a computational imaging technique that generates overlapping patterns of illumination. As a by-product, it also produces a complex wavefront of the incident radiance in the sample plane. This wavefront is useful as a tool to evaluate the beam.

In the experiment, patterned tungsten film was scanned at a range of photon energies and measured at different scanning positions. The resulting diffraction patterns or wavefronts showed that the X-ray beam sizes were significantly similar to theorized numbers for a diffraction limited beam. In simple terms, this is evidence of the precise focusing capabilities of the X-ray beam.

The degree of coherence or coherent flux—which is the number of photons per second used to create the diffraction pattern—was then calculated with a software modelling method known as probe mode decomposition. Data with full flux and flux of a single-mode showed that high beam intensity and high coherence in the beam was achievable using specific slit settings.

“All experiments that rely on coherent diffraction are made more powerful by the high coherent flux. An example might be nanoparticles, which can be imaged in situ while catalysing a reaction or performing some other function, provided that enough coherent photons can be put on them per second,” said Björling.


Ptychographic characterization of a coherent nanofocused X-ray beam
Alexander Björling, Sebastian Kalbfleisch, Maik Kahnt, Simone Sala, Karolis Parfeniukas, Ulrich Vogt, Dina Carbone, Ulf Johansson
Optics Express 28 (2020) 5069; doi: 10.1364/OE.386068

Read their related work here.


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