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

March 5, 2018

Paving the way for X-ray pump/X-ray probe attosecond free-electron laser science

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. The authors characterize individual attosecond pulses, including their instantaneous frequency, and identify double pulses with well-defined delays and spectral properties.

Fig. 4: Attosecond X-ray double pulses from an XFEL. a,b, Identification of double-spike pulses with variable temporal separation, showing four exemplary delay steps between 1.0 fs and 2.3 fs in b. The individual spikes have a duration of 600–800 as and a pulse energy of approximately 30 µJ each. These carefully selected attosecond double pulses amount to more than 0.3% of all shots in the evaluated dataset. In a, a delay-sorted three-dimensional scan image of attosecond X-ray double pulses is plotted, defining the accessible delays for attosecond X-ray pump/probe measurements in the run. The temporal resolution for defining these delays is set to 0.2 fs. By changing the overall XFEL pulse duration, one can statistically syntonize to different delay ranges.

The angular streaking method requires minimal X-ray pulse energy with near-unity X-ray transmission. It is therefore ideally suited for single-shot X-ray pulse tagging in parallel with independently ongoing experiments, which allows for post-selection of specific pulse durations and photon energies, identification of isolated attosecond pulses, or the detection and delay-sorting of double spikes for attosecond pump/probe experiments. Finally, the authors note that TOF spectrometers as used here are applicable to high-repetition rate FELs such as the 100 kHz–1 MHz European XFEL and LCLS-II. The high repetition rate compensates the potentially low fraction of selected shots, while the sorting simultaneously alleviates the need for full-rate detector and data systems. Thus, angular streaking sets the stage for advanced X-ray pump/probe experiments, enabling site-specific, time-and-energy-resolved measurements on the natural timescale of electronic motion.

Reference:  Hartmann, N., G. Hartmann, R. Heider, M. S. Wagner, M. Ilchen, J. Buck, A. O. Lindahl, C. Benko, J. Grünert, J. Krzywinski, J. Liu, A. A. Lutman, A. Marinelli, T. Maxwell, A. A. Miahnahri, S. P. Moeller, M. Planas, J. Robinson, A. K. Kazansky, N. M. Kabachnik, J. Viefhaus, T. Feurer, R. Kienberger, R. N. Coffee and W. Helml (2018). Attosecond time–energy structure of X-ray free-electron laser pulses. Nat. Photonics. (10.1038/s41566-018-0107-6) Hartmann-2018.

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