Measuring the temporal structure of few-femtosecond free-electron laser X-ray pulses directly in the time domain

Journal name:
Nature Photonics
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Short-wavelength free-electron lasers are now well established as essential and unrivalled sources of ultrabright coherent X-ray radiation. One of the key characteristics of these intense X-ray pulses is their expected few-femtosecond duration. No measurement has succeeded so far in directly determining the temporal structure or even the duration of these ultrashort pulses in the few-femtosecond range. Here, by deploying the so-called streaking spectroscopy technique at the Linac Coherent Light Source, we demonstrate a non-invasive scheme for temporal characterization of X-ray pulses with sub-femtosecond resolution. This method is independent of photon energy, decoupled from machine parameters, and provides an upper bound on the X-ray pulse duration. We measured the duration of the shortest X-ray pulses currently available to be on average no longer than 4.4 fs. Analysing the pulse substructure indicates a small percentage of the free-electron laser pulses consisting of individual high-intensity spikes to be on the order of hundreds of attoseconds.

At a glance


  1. Experimental set-up and measurement principle at the LCLS.
    Figure 1: Experimental set-up and measurement principle at the LCLS.

    Experimental set-up in the AMO hutch at the LCLS. The X-ray laser and NIR streaking laser are coupled into the vacuum chamber and are co-linearly focused onto a Ne gas target. The generated photoelectrons are then energy-resolved with a magnetic bottle electron spectrometer. The inset on the left depicts two distinctive cases of temporal overlap of the FEL with respect to the streaking field, one at the zero-crossing and one at a maximum of the NIR vector potential. The respective photoelectron spectra are also shown. More details are given in the text.

  2. Dressed single-shot X-ray photoelectron spectra and correlation plots.
    Figure 2: Dressed single-shot X-ray photoelectron spectra and correlation plots.

    ai, Simulations (blue, ac, df) for the sideband regime with 16 fs FEL pulses (tperiod/tX-ray = 1/2, ac) and for the streaking regime with 4 fs FEL pulses (tperiod/tX-ray = 2, df) are compared with experimental findings (gi, red). On the left (a,d,g), colour-coded intensity images of 100 consecutive dressed photoelectron (PE) spectra are plotted. In the middle (b,e,h), 15 single-shot spectra are extracted for the three cases. For each, the spectral width (orange, dashed lines) and centre position (black crosses) are marked, with black lines connecting the centres of consecutive spectra. On the right, the centre positions and widths of the spectra for 1,000 simulated shots (c,f) and for the experimental data (i) are mapped out in correlation scatter plots.

  3. Streaking of few-femtosecond X-ray pulses.
    Figure 3: Streaking of few-femtosecond X-ray pulses.

    a, Principle of X-ray pulse duration evaluation. be, Four different streaked photoelectron spectra as measured in our experiment (blue solid lines) at different values of the NIR vector potential, and the respective linear streaking ramps (orange dashed lines) derived from the maximum shifted peak of each spectrum. In b, the streaked spectrum spans from −32 eV to 27 eV; that is, the NIR laser quarter-cycle of 2 fs is mapped onto the maximum shift of |−32| eV and the complete spectral width of 59 eV therefore corresponds to a pulse duration upper limit of 3.7 fs (all pulse durations are FWHM). In a similar manner we derive pulse duration upper limits of 3.4 fs (c), 2.8 fs (d) and 2.6 fs (e). Considering the ambiguity of the energy-to-time mapping, all those shots are in principle also compatible with the doubled pulse lengths.

  4. Average FEL pulse duration upper limit.
    Figure 4: Average FEL pulse duration upper limit.

    Histogram of all possible pulse duration upper limits in agreement with our measurement for each shot (blue), for the parameter range used at LCLS (20 pC low-charge mode, 1,791 eV X-ray photon energy, electron bunch near full compression at 10 kA, slotted foil inserted into the electron beam path). The red solid line shows a Gaussian fit to the distribution, indicating an average X-ray pulse duration upper limit of ∼4.4 fs FWHM.

  5. Measurement of an attosecond FEL X-ray pulse.
    Figure 5: Measurement of an attosecond FEL X-ray pulse.

    a, Measured spectrum (right) of a single FEL intensity spike (solid dark blue line) and the corresponding Gaussian fit (dashed red line). The shift of the peak from the unstreaked central energy at ∼921 eV (green dotted line) defines the energy-to-time ramp (left), with the help of which the width of the deconvoluted spectrum (yellow dashed-dotted line) can be converted into a measure for the pulse duration upper limit, depicted as the light blue curve (bottom left). b, Derived upper limits for the pulse durations of all single-spike shots as a function of the energy shift of the peak. The pulse described in a is marked as the bigger light blue dot. Most of the shots with an energy shift between 10 eV and 20 eV are in the region of 1.6 fs FWHM and below (indicated by the green oval in b).


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Author information

  1. These authors contributed equally to this work

    • W. Helml &
    • A. R. Maier


  1. Technische Universität München, Physik-Department E11, James-Franck-Straße, 85748 Garching, Germany

    • W. Helml &
    • R. Kienberger
  2. Max-Planck-Institut für Quantenoptik, Hans-Kopfermann-Straße 1, 85748 Garching, Germany

    • W. Helml,
    • W. Schweinberger,
    • J. Gagnon &
    • R. Kienberger
  3. Center for Free-Electron Laser Science (CFEL), Notkestraße 85, 22607 Hamburg, Germany

    • A. R. Maier,
    • I. Grguraš,
    • F. Grüner &
    • A. L. Cavalieri
  4. Institut für Experimentalphysik, Gruppe Beschleunigerphysik, University of Hamburg and Center for X-ray Free-Electron Laser Science, Luruper Chaussee 149, 22761 Hamburg, Germany

    • A. R. Maier &
    • F. Grüner
  5. Max-Planck Institute for the Structure and Dynamics of Matter, Luruper Chaussee 149, 22761 Hamburg, Germany

    • I. Grguraš &
    • A. L. Cavalieri
  6. European XFEL, Albert-Einstein-Ring 19, 22761 Hamburg, Germany

    • P. Radcliffe,
    • Th. Tschentscher &
    • M. Meyer
  7. Argonne National Laboratory, Argonne, Illinois 60439, USA

    • G. Doumy
  8. Department of Physics, The Ohio State University, Columbus, Ohio 43210, USA

    • G. Doumy,
    • C. Roedig &
    • L. F. DiMauro
  9. Linac Coherent Light Source, 2575 Sand Hill Road, Menlo Park, California 94025, USA

    • M. Messerschmidt,
    • S. Schorb,
    • C. Bostedt,
    • J. D. Bozek &
    • R. Coffee
  10. Institut des Sciences Moléculaires d'Orsay, UMR 8214, CNRS–Université Paris Sud, Bâtiment 350, 91405 Orsay Cedex, France

    • D. Cubaynes &
    • M. Meyer
  11. School of Physical Sciences and National Center for Plasma Science and Technology (NCPST), Dublin City University, Glasnevin, Dublin 9, Ireland

    • J. T. Costello
  12. DESY, Notkestraße 85, 22607 Hamburg, Germany

    • S. Düsterer


W.H. and A.R.M. contributed equally to the work. R.K. developed the concept. W.H., A.R.M., W.S., I.G., P.R., G.D., C.R., J.G., Ma.M., S.S., C.B., L.F.D., J.D.B., R.C., A.L.C. and R.K. designed the experiment and contributed to the preparation of the experimental set-up. W.H., A.R.M., W.S., I.G., P.R., G.D., C.R., Ma.M., S.S., C.B., D.C., J.D.B., Th.T., J.T.C., Mi.M., R.C., S.D., A.L.C. and R.K. performed the experiment. W.H., A.R.M., W.S., I.G., P.R., G.D., J.G., A.L.C. and R.K. analysed the data. W.H., A.R.M., G.D., J.G., F.G., L.F.D., Th.T., J.T.C., Mi.M., S.D., A.L.C. and R.K. wrote the paper.

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