Fluence scan: an unexplored property of a laser beam


This work deals with characterization of focused laser beams for the purposes of laser-matter interaction research. Detailed knowledge of transverse energy distribution within the beam profile turns out to be essential for interpretation of the quite nontrivial experimental results. Non-Gaussian beams, which are typical of X-ray lasers, require a rigorous study as well as the interactions they do induce. We present an extended theoretical background of so-called fluence scan (f-scan or F-scan) method, which is frequently being used for offline characterization of focused short-wavelength (EUV, soft X-ray, and hard X-ray) laser beams. The method exploits ablative imprints in various solids to visualize iso-fluence beam contours at different fluence and/or clip levels. An f-scan curve (clip level as a function of the corresponding iso-fluence contour area) can be generated for a general non-Gaussian beam. As shown in this paper, fluence scan encompasses important information about energy distribution within the beam profile, which may play an essential role in laser-matter interaction research employing intense non-ideal beams. Here we for the first time discuss fundamental properties of the f-scan function and its inverse counterpart (if-scan). Furthermore, we extensively elucidate how it is related to the effective beam area, energy distribution, and to the so called Liu’s dependence. A new method of the effective area evaluation based on weighted inverse f-scan fit is introduced and applied to real data obtained at the SCSS (SPring-8 Compact SASE Source) facility.

Ablative imprints in PMMA (a) and Cu/Nb multilayer (b) in dependence on increasing pulse energy. Outer ablation contours depict iso-fluence beam contours at different levels of the peak fluence and thus constitute a contour map of the beam profile. Ablative imprints were created by a focused SCSS (SPring-8 Compact SASE Source, RIKEN/Japan) beam at wavelength of 60 nm. The images were obtained by means of Nomarski (DIC – differential interference contrast) microscopy.

Previous articles on this topic:
[1] Creation and diagnosis of a solid-density plasma with an X-ray free-electron laser
Nature 482 (2012) 59-63, doi: 10.1038/nature10746
[2] Spot size characterization of focused non-Gaussian X-ray laser beams
Opt. Express 18 (2010) 27836-27845, doi: 10.1364/OE.18.027836