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Saclay Laser Interaction Center
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Beam shaping

Beam shaping in the spatial domain

Spatial beam shaping was studied theoretically and experimentally in our group for the generation of photoelectron bunches with minimized emittance in the framework of the EUROFEL design studies contract (FP6). The goal was to generate a 266 nm beam having a quasi top-hat profile with a FWHM of the order of one mm. Among the different approaches, we chose to use a field mapping method. We used a Keplerian telescope with two aspherical lenses to generate a collimated quasi top hat beam from a collimated quasi-gaussian beam obtained by spatial filtering of the output of the 3ω generator. Our best result is given in figure 6.

table characteristics
We also investigated theoretically the optimization of the XUV photons yield produced by High order Harmonic Generation in collaboration with the AttoPhysics group. The goal was to use a phase plate to generate out of a gaussian beam at the output of the laser, a super gaussian beam at the focal plane of a lens and to study the evolution of the field in the vicinity of the focal plane. This activity is presented in the AttoPhysics section of this report.

Beam shaping in the time (spectral) domain

Temporal or spectral beam shaping has been used for a long time in the laboratory, in particular through the use of a linear or a combination of linear systems (DAZZLERS). Most of our studies have been done in tight collaboration with T. Oksenhendler (FASTLITE). During the last years, one of our main studies was carried out within the EUROFEL program. Our goal was to generate a quasi top-hat temporal profile at 266 nm. Pulse duration of the order of 5ps and subpicosecond rise time and fall time were required. Numerical studies were first used to define the target amplitude and phase to be generated. Before the availability of a UV Dazzler, experimental studies were carried out with the IR beam and an IR Dazzler to validate our approach. The best result obtained at 266 nm with the UV Dazzler is shown on figure 7. The pulse energy was of a few µJ.

table characteristics
For these EUROFEL studies, spatial and temporal beam shaping was not done at the same time, although there are no difficulties to do so. The main problem to deal with and which was not solved experimentally, to our knowledge, is the generation of high quality (as presented above) and high energy (about 1mJ) 266 nm pulses.

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Saclay Laser-matter Interaction Center

Up date: 22-04-2009
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