Multifocus off-axis zone plates for x-ray free-electron laser experiments

Abstract

X-ray free-electron lasers (XFELs) are paving the way towards new experiments in many scientific fields, such as ultrafast science, nonlinear spectroscopy, and coherent imaging. However, the strong intensity fluctuations inherent to the lasing process in these sources often lead to problems in signal normalization. In order to address this challenge, we designed, fabricated, and characterized diffractive x-ray optics that combine the focusing properties of a Fresnel zone plate with the beam-splitting capability of a grating in a single diffractive optical element. The possibility to split the incident beam into identical copies allows for direct shot-to-shot normalization of the sample signal, thereby greatly enhancing the signal-to-noise ratio in experiments with XFEL radiation. Here we propose two schemes for the design of such diffractive x-ray optical elements for splitting and focusing an incoming beam into up to three foci by merging a grating with a focusing zone plate. By varying the duty cycle of the grating or the relative shift of the Fresnel zone plate structure, we are able to tune the relative intensities of the different diffraction orders to achieve the desired splitting ratios. Experimental confirmation of the design is provided with soft x-ray light (540 eV) and shows a good agreement with calculations, confirming the suitability of this approach for XFEL experiments.

Simultaneous two-color snapshot view on ultrafast charge and spin dynamics in a Fe-Cu-Ni tri-layer

“…(c) A beam splitter grating perpendicular to the zones is integrated by periodic inversion of the zone plate patterns. 45 This splits the focus spot into twin copies with defined separation, and introduces a slight tilt to the locus of equal time points in the detector image.…”

“…2(c) , resulting in a twin spot of the off-axis zone plate in the focal plane with a defined separation. 45 In this way, one of these spots can be optically pumped while the other records the sample absorption in the unperturbed state. A consequence of this focus splitting is that the beam paths followed by the two colors are no longer exactly equal, but are slightly shorter for the color coming from the off-axis zone plate half on the same side as the focus.…”

Electron population dynamics in resonant non-linear x-ray absorption in nickel at a free-electron laser

“…(c) Setup for non-linear XAS. The split-beam-normalization scheme uses a special zone plate, 29 which generates two adjacent beam foci for transmission through the sample and a reference membrane before the beams impinge on the detector.…”

“…The zone plate combines an off-axis Fresnel structure for focusing and a line grating for beam-splitting in a single optical element. 29 It thus produces two μ m-sized, identical foci in the sample plane, 1.9 mm apart, originating from the first-order diffraction of the zone plate, as well as the positive and negative first orders of the line grating. The sample has a square support of 25 mm size, containing square Si 3 N 4 membrane windows (orange in Fig.…”

XRnanotech – Recent Developments In Nanostructured X-ray And EUV Optics

“…This includes the possibility of using zone plates for multibeam applications. [3] This can be useful to probe a sample with an FEL and at the same time obtain a beam for reference purposes. Especially with the fluctuating SASE emission of modern FELs, this proves to be a valuable tool.…”