Table-top femtosecond soft X-ray laser by collisional ionization gating

Journal name:
Nature Photonics
Year published:
Published online

The advent of X-ray free-electron lasers has granted researchers an unprecedented access to the ultrafast dynamics of matter on the nanometre scale1, 2, 3. Aside from being compact, seeded plasma-based soft X-ray lasers (SXRLs) turn out to be enticing as photon-rich4 sources (up to 1015 per pulse) that display high-quality optical properties5, 6. Hitherto, the duration of these sources was limited to the picosecond range7, which consequently restricts the field of applications. This bottleneck was overcome by gating the gain through ultrafast collisional ionization in a high-density plasma generated by an ultraintense infrared pulse (a few 1018 W cm−2) guided in an optically pre-formed plasma waveguide. For electron densities that ranged from 3 × 1018 cm−3 to 1.2 × 1020 cm−3, the gain duration was measured to drop from 7 ps to an unprecedented value of about 450 fs, which paves the way to compact and ultrafast SXRL beams with performances previously only accessible in large-scale facilities.

At a glance


  1. Computed temporal dependence of the average charge state and gain coefficient of a krypton-plasma amplifier for ne = 6 × 1018 cm−3 (green), ne = 1.2 × 1020 cm−3 (blue) and ne = 4 × 1020 cm−3 (red).
    Figure 1: Computed temporal dependence of the average charge state and gain coefficient of a krypton-plasma amplifier for ne = 6 × 1018 cm−3 (green), ne = 1.2 × 1020 cm−3 (blue) and ne = 4 × 1020 cm−3 (red).

    The grey-tinted part corresponds to the OFI regime that precedes the subsequent collisional ionization. The dotted lines show the increase in ionization rate with density. Therefore, at high densities, the steep depletion in the number of Kr8+ ions leads to an ultrafast gating of the gain lifetime (CIG process). The yellow area illustrates the region in which Kr8+ ions are predominant.

  2. Schematic of the experimental arrangement.
    Figure 2: Schematic of the experimental arrangement.

    The waveguiding beam is composed of a sequence of short (130 mJ, 30 fs) and long (690 mJ, 600 ps) pulses. It is focused over the whole jet length by an axicon lens and creates, after collisional ionization and hydrodynamic expansion, a plasma channel. Then, the pump beam (1.36 J, 30 fs) is focused at the entrance of the channel with a spherical mirror and guided thereafter. Hence, an amplifier with Kr8+ lasing-ion species over the whole gas-jet length is implemented. A third infrared beam (16 mJ, 350 fs) is used to generate a high-harmonics seed in an argon-filled cell. The latter is image relayed onto the entrance of the plasma and synchronized with the gain lifetime.

  3. Far-field beam profiles.
    Figure 3: Far-field beam profiles.

    a, HH. b, Seeded SXRL for ne = 1.2 × 1020 cm−3. The SXRL energy is assessed at 2 μJ, which corresponds to more than 3 × 1011 coherent photons. The 1 ± 0.2 mrad divergence and nearly Gaussian beam profile of HH is maintained over the plasma amplification. A factor of about 75 is reported between total HH and seeded SXRL signals.

  4. Temporal dependence of the amplification factor with respect to the seeding delay.
    Figure 4: Temporal dependence of the amplification factor with respect to the seeding delay.

    Experimental (blue circles) and Maxwell–Bloch-modelling results (red squares) for a scan in the plasma densities ne = 3 × 1018 cm−3, ne = 7.9 × 1018 cm−3, ne = 3.2 × 1019 cm−3 and ne = 1.2 × 1020 cm−3. Owing to the inhomogeneity of the plasma channel, lower-density regions, for which the gain duration is longer, are accountable for the tail at the end of the experimental curve.

  5. Simulated temporal profiles of the amplified SXRL for the set of studied densities and a prospective one for ne = 4 × 1020 cm−3 (red).
    Figure 5: Simulated temporal profiles of the amplified SXRL for the set of studied densities and a prospective one for ne = 4 × 1020 cm−3 (red).

    The duration strongly depends on the density, and ranges from 6.4 ± 0.3 ps to 50 ± 18 fs r.m.s. The highest experimental density (blue) results in an amplified beam with a duration of 64 ± 21 fs FWHM (123 ± 40 fs r.m.s.). The curves are normalized to one and their r.m.s. duration is specified.


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Author information


  1. LOA, ENSTA ParisTech, CNRS, Ecole Polytechnique, Université Paris-Saclay, 828 bd des Maréchaux, Palaiseau Cedex 91762, France

    • A. Depresseux,
    • J. Gautier,
    • F. Tissandier,
    • J. P. Goddet,
    • A. Tafzi,
    • A. Lifschitz,
    • V. Malka,
    • K. Ta Phuoc,
    • C. Thaury,
    • P. Rousseau,
    • G. Iaquaniello,
    • T. Lefrou,
    • A. Flacco,
    • B. Vodungbo,
    • G. Lambert,
    • A. Rousse,
    • P. Zeitoun &
    • S. Sebban
  2. Laboratoire de Physique des Gaz et des Plasmas, CNRS-Université Paris Sud 11, Orsay 91405, France

    • E. Oliva &
    • G. Maynard
  3. ELI Beamlines Project, Institute of Physics of the ASCR, Na Slovance 2, 182 21 Prague 8, Czech Republic

    • J. Nejdl &
    • M. Kozlova
  4. Advanced Photonics Research Institute, GIST, Gwangju 500-712, Korea

    • H. T. Kim
  5. Center for Relativistic Laser Science, Institute for Basic Science (IBS), Gwangju 500-712, Korea

    • H. T. Kim
  6. LULI – CEA, CNRS, École Polytechnique, Université Paris-Saclay, UPMC Univ Paris 06, Sorbonne Universités, F-91128 Palaiseau cedex, France

    • S. Jacquemot
  7. CEA, DAM, DIF, F-91297 Arpajon, France

    • S. Jacquemot


S.S. proposed the experiment. S.S., J.G, F.T. and A.D. designed and built the set-up. G.M. developed the code OFI-0D. E.O., G.M. and A.L. performed the numerical simulations. J.P.G. and A.T. designed, built and operated the upgraded laser system of ‘Salle Jaune’. K.T.P., C.T. and P.R. provided support for the operation of the facilities. A.F. developed a phase reconstruction and Abel inversion software for the electron-density diagnostic. All the authors contributed to the data analysis and the writing of the paper. P.Z., S.J., V.M. and A.R. supported the project.

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