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


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.

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Figure 1: Experimental set-up and measurement principle at the LCLS.
Figure 2: Dressed single-shot X-ray photoelectron spectra and correlation plots.
Figure 3: Streaking of few-femtosecond X-ray pulses.
Figure 4: Average FEL pulse duration upper limit.
Figure 5: Measurement of an attosecond FEL X-ray pulse.


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Portions of this research were carried out at the Linac Coherent Light Source (LCLS) at SLAC National Accelerator Laboratory. LCLS is an Office of Science User Facility operated for the US Department of Energy (DOE) Office of Science by Stanford University. The work was partly supported from the German cluster of excellence ‘Munich-Centre for Advanced Photonics’. W.H., W.S. and R.K. acknowledge financial support by the BACATEC programme. W.H., A.R.M. and W.S. acknowledge ‘The International Max Planck Research School on Advanced Photon Science’ for funding and continued inspiring support. W.H. acknowledges financial support from a Marie Curie fellowship. A.R.M. thanks S. Reiche for fruitful discussions and help with GENESIS simulations. C.R. and G.D. acknowledge partial support from the US DOE (DE-FG02-04ER15614) and the National Science Foundation (NSF; PHY-1004778). G.D. also acknowledges support from US Department of Energy/Basic Energy Sciences (US DOE/BES; DE-AC02-06CH11357). J.T.C. acknowledges support from Science Foundation Ireland (grant no. 12/IA/1742). R.K. acknowledges funding from an European Research Council Starting Grant. The authors thank T. Schätz for comments on the manuscript. The authors thank P. Emma, Y. Ding, J.B. Hastings and W. White for their support, C. Behrens and H.-D. Nuhn for their expertise and advice on FEL-related questions, and to the whole scientific and technical team at LCLS for their dedication and unrelenting work during our beam time, in particular to the machine operators.

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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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Correspondence to R. Kienberger.

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The authors declare no competing financial interests.

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Helml, W., Maier, A., Schweinberger, W. et al. Measuring the temporal structure of few-femtosecond free-electron laser X-ray pulses directly in the time domain. Nature Photon 8, 950–957 (2014).

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