A high-intensity highly coherent soft X-ray femtosecond laser seeded by a high harmonic beam

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Synchrotrons have for decades provided invaluable sources of soft X-rays, the application of which has led to significant progress in many areas of science and technology. But future applications of soft X-rays—in structural biology, for example—anticipate the need for pulses with much shorter duration (femtoseconds) and much higher energy (millijoules) than those delivered by synchrotrons. Soft X-ray free-electron lasers1 should fulfil these requirements but will be limited in number; the pressure on beamtime is therefore likely to be considerable. Laser-driven soft X-ray sources offer a comparatively inexpensive and widely available alternative, but have encountered practical bottlenecks in the quest for high intensities. Here we establish and characterize a soft X-ray laser chain that shows how these bottlenecks can in principle be overcome. By combining the high optical quality available from high-harmonic laser sources (as a seed beam) with a highly energetic soft X-ray laser plasma amplifier, we produce a tabletop soft X-ray femtosecond laser operating at 10 Hz and exhibiting full saturation, high energy, high coherence and full polarization. This technique should be readily applicable on all existing laser-driven soft X-ray facilities.

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Figure 1
Figure 2: Experimental spectra under various conditions.
Figure 3: Calculated line shapes for the HHG (seed), the ASE SXRL, and the seeded SXRL for two levels of seeding.
Figure 4: Three-dimensional images (false colour) of the seeded SXRL cross-section as recorded by the soft X-ray CCD detector after reflection on the monochromatic soft X-ray mirror.
Figure 5: Variation of the HHG (open diamond) and seeded SXRL (black square) intensities versus the angle of polarization.


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We thank J. L. Charles for technical support. T.M. was supported by the Marie Curie Individual Fellowship of the European Commission.

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Correspondence to Ph. Zeitoun.

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The authors declare that they have no competing financial interests.

Supplementary information

Supplementary Discussion

The choice of the Optical Field Ionisation amplifier among all the x-ray lasers developed up to now has been done according to several precise criteria, discussed in this text. (DOC 26 kb)

Supplementary Figure

This figure shows the amplification law as measured with plasma lenghts ranging from 0 up to 6 mm. Comparison between the ASE and the seeded x-ray laser intensities is shown. (DOC 161 kb)

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