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
1Institute of Physics, Academy of Sciences of the Czech Republic, Na Slovance 2, 182 21 Prague 8, Czech Republic
2Faculty of Electrical Engineering, Czech Technical University in Prague, Technická 2, 166 27 Prague 2, Czech Republic
3Univ. Bordeaux, CEA, CNRS, CELIA (Centre Lasers Intenses et Applications) UMR5107, F-33400 Talence, France
4RIKEN/SPring-8 Kouto 1-1-1, Sayo, Hyogo, 679-5148, Japan
5Institute of Physics, Polish Academy of Sciences, Al. Lotników 32/46, Warsaw, PL-02-668, Poland
6SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, CA 94025, USA
Copyright © 2008-2014, Fyzikální ústav AV ČR, v. v. i.
