Abstract
High resolution metrology of beam profiles is presently a major challenge at X-ray free electron lasers. We demonstrate a characterization method based on beam imprints in poly (methyl methacrylate). By immersing the imprints formed at 47.8 eV into organic solvents, the regions exposed to the beam are removed similar to resist development in grayscale lithography. This allows for extending the sensitivity of the method by more than an order of magnitude compared to the established analysis of imprints created solely by ablation. Applying the Beer-Lambert law for absorption, the intensity distribution in a micron-sized focus can be reconstructed from one single shot with a high dynamic range, exceeding 103. The procedure described here allows for beam characterization at free electron lasers revealing even faint beam tails, which are not accessible when using ablation imprint methods. We demonstrate the greatly extended dynamic range on developed imprints taken in focus of conventional Fresnel zone plates and spiral zone plates producing beams with a topological charge.
Highlights
With the ongoing construction of X-ray free electron lasers (XFELs), a set of extremely brilliant light sources has become available to science
We successfully demonstrated the adaption of grayscale lithography to characterize the intensity distribution of a focused extreme ultraviolet (EUV) beam from a seeded Free-electron lasers (FELs) over a dynamic range exceeding 1000 in a single shot
By developing PMMA films which have been exposed to an EUV beam, the ablation imprint method can be extended by more than an order of magnitude in sensitivity and dynamic range
Summary
With the ongoing construction of X-ray free electron lasers (XFELs), a set of extremely brilliant light sources has become available to science. An elegant method to characterize the shape of intense laser beams by investigating ablated craters was introduced by Liu already in 1982 [1] This method was applied to study the ablation behavior of several polymers in the EUV regime [2] and has been introduced to XFELs some years ago by Chalupský, Juha and others [2]. Assuming that absorption occurs ideally according to the Beer-Lambert law [8] and that the ablation depth is directly related to absorbed photon dose, surface analysis of the imprint craters provides quantitative information about the penetration depth at which the remaining photon intensity drops below the ablation threshold [3] This is generally the case for non-thermal ablation processes, which are referred to as photochemical models where electronic excitation leads to bond breaking, directly resulting in material loss of the molecular fragments. Irradiating polymer films with a focused XFEL beam and their subsequent development proves as a highly sensitive method for determining the intensity distribution in focus over a high dynamic range
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