Today's brightest coherent X-ray sources, X-ray free-electron lasers, produce ultrafast X-ray pulses for which full-width at half-maximum durations as short as 3 fs have been measured1. There has been a marked increase in the popularity of such short pulses now that optical timing techniques have begun to report an X-ray/optical delay below ∼10 fs r.m.s. errors. As a result, sub-10 fs optical pulses have been implemented at the Linac Coherent Light Source (LCLS) X-ray beamlines, thus warranting a push to reduce the error in X-ray/optical delay measurements to the 1 fs level. Here, we report a unique two-dimensional spectrogram measurement of the relative X-ray/optical delay. This easily scalable relative delay measurement already surpasses previous techniques by an order of magnitude with its sub-1 fs temporal resolution and opens up the prospect of time-resolved X-ray measurements to the attosecond community.
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The authors thank S. Durbin, D. Reis, A. Lindahl and R. Schoenlein for discussions of bulk material X-ray interactions. W.H. acknowledges financial support from a Marie Curie fellowship. T.F. acknowledges financial support from National Center of Competence in Research, Molecular Ultrafast Science and Technology. 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 US Department of Energy Office of Science by Stanford University.
D.J.K., head of Mesa Photonics, has referenced some of the results herein for SBIR proposals regarding the development of temporal X-ray diagnostics.
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Hartmann, N., Helml, W., Galler, A. et al. Sub-femtosecond precision measurement of relative X-ray arrival time for free-electron lasers. Nature Photon 8, 706–709 (2014). https://doi.org/10.1038/nphoton.2014.164
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