Extending the ablation imprints method of focal spot characterisation from X-ray to visible and near-infrared spectral range

Abstract

The ablation imprints method is a well-established approach to thoroughly characterising fluence distributions [J/cm2] of focused short-wavelength free-electron laser beams. For visible and near- infrared laser beams, fluence distribution of the focused beam can also be measured by other means, for example, by projecting a magnified image of the focal spot onto a camera. We studied the viability of the ablation imprints method in the visible and near-infrared spectral range and compared it to the above-mentioned conventional approach. Furthermore, we compared the effects of the X-ray, visible, and near-infrared radiation on the ablation damage. The ablation imprints method is based on creating cross-sections at varied levels of the fluence distribution by imprinting the beam into a target - PbI2 coated slab of poly(methyl methacrylate) in our case. The fluence distribution is reconstructed from the cross-sections created by varying the laser pulse energy between the PbI2 ablation threshold and a few orders of magnitude above the threshold. We characterised an X-ray astigmatic focused beam at the Small Quantum Systems instrument of the

EuXFEL. At the Prague Asterix Laser System, we successfully characterised the focal spot at 438 nm.

At 1315 nm, the ablation imprints method produced partially satisfactory results, and we compared these results with conventional methods. We conclude that the ablation imprints method can characterise focused laser beams in the visible and near-infrared spectral range.