Attosecond time–energy structure of X-ray free-electron laser pulses


The time–energy information of ultrashort X-ray free-electron laser pulses generated by the Linac Coherent Light Source is measured with attosecond resolution via angular streaking of neon 1s photoelectrons. The X-ray pulses promote electrons from the neon core level into an ionization continuum, where they are dressed with the electric field of a circularly polarized infrared laser. This induces characteristic modulations of the resulting photoelectron energy and angular distribution. From these modulations we recover the single-shot attosecond intensity structure and chirp of arbitrary X-ray pulses based on self-amplified spontaneous emission, which have eluded direct measurement so far. We characterize individual attosecond pulses, including their instantaneous frequency, and identify double pulses with well-defined delays and spectral properties, thus paving the way for X-ray pump/X-ray probe attosecond free-electron laser science.

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Fig. 1: Angular streaking resolves the X-ray pulse structure via angle-dependent kinetic energy changes of photoelectrons.
Fig. 2: X-ray pulse time–energy characterization of three exemplary SASE XFEL shots.
Fig. 3: X-ray pulse statistics.
Fig. 4: Attosecond X-ray double pulses from an XFEL.


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We would like to thank C. Bostedt for his extensive and skilful support during the beam time. This research was carried out at the Linac Coherent Light Source (LCLS) at the SLAC National Accelerator Laboratory. LCLS is an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science by Stanford University. W.H. acknowledges financial support from a Marie Curie International Outgoing Fellowship. W.H., R.H., M.S.W. and R.K. acknowledge financial support by the BaCaTeC program, the German Cluster of Excellence MAP and the European I3-Initiative 'LASERLAB-Europe IV'. R.K. acknowledges an ERC Consolidator Grant 'AEDMOS'. J.K., A.A.L, A.M., T.M., S.M., R.N.C. and J.R. acknowledge DOE support under contract DE-AC02-76SF00515. N.M.K. acknowledges financial support from the theory group in cooperation with the SQS work package of European XFEL. A.K.K. acknowledges financial support from the project FIS2016-76617-P of MINECO. T.F. acknowledges financial support from the National Center of Competence in Research, Molecular Ultrafast Science and Technology. M.I. acknowledges funding of the Volkswagen Foundation within a Peter Paul Ewald-Fellowship.

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N.H., M.I., A.O.L., R.N.C. and W.H. conceived and coordinated the experiment. N.H., R.H., M.S.W., C.B., J.L., A.A.M., S.P.M., J.R., R.N.C. and W.H. built the vacuum system and the optical set-up, and supervised them during the measurement. M.I, J.B., A.O.L., J.G., M.P. and J.V. were responsible for constructing, installing and operating the photoelectron detector. J.K., A.A.L., A.M. and T.M. carried out accelerator control and XFEL characterization. N.H., G.H., R.H., M.S.W., J.B., A.O.L., J.L., R.N.C. and W.H. performed data analysis. N.H., G.H., M.I., T.F., R.K., R.N.C. and W.H. interpreted data and oversaw manuscript production. A.K.K., N.M.K. and T.F. provided simulations for comparison with experimental data. N.H., G.H. and W.H. wrote the paper with extensive contributions from all authors.

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Correspondence to N. Hartmann or W. Helml.

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Supplementary Information

Supplementary Methods; Supplementary Discussion; Supplementary Figures 1–8; Supplementary Table 1; Supplementary References 1–8.

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Hartmann, N., Hartmann, G., Heider, R. et al. Attosecond time–energy structure of X-ray free-electron laser pulses. Nature Photon 12, 215–220 (2018).

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