Abstract
Temporal coherence is one of the most fundamental characteristics of light, connecting to spectral information through the Fourier transform relationship between time and frequency. Interferometers with a variable path-length difference (PLD) between the two branches have widely been employed to characterize temporal coherence properties for broad spectral regimes. Hard X-ray interferometers reported previously, however, have strict limitations in their operational photon energies, due to the specific optical layouts utilized to satisfy the stringent requirement for extreme stability of the PLD at sub-ångström scales. The work presented here characterizes the temporal coherence of hard X-ray free-electron laser (XFEL) pulses by capturing single-shot interferograms. Since the stability requirement is drastically relieved with this approach, it was possible to build a versatile hard X-ray interferometer composed of six separate optical elements to cover a wide photon energy range from 6.5 to 11.5 keV while providing a large variable delay time of up to 47 ps at 10 keV. A high visibility of up to 0.55 was observed at a photon energy of 10 keV. The visibility measurement as a function of time delay reveals a mean coherence time of 5.9 ± 0.7 fs, which agrees with that expected from the single-shot spectral information. This is the first result of characterizing the temporal coherence of XFEL pulses in the hard X-ray regime and is an important milestone towards ultra-high energy resolutions at micro-electronvolt levels in time-domain X-ray spectroscopy, which will open up new opportunities for revealing dynamic properties in diverse systems on timescales from femto-seconds to nanoseconds, associated with fluctuations from ångström to nanometre spatial scales.
Highlights
Optical interferometry using visible light is one of the most powerful methods for high-precision metrology owing to its high sensitivity to the phase of the light (Hariharan, 2007)
The temporal coherence connects to the spectral information through the Fourier transform (FT) relationship between time and frequency, and a path length difference (PLD) of 100 mm yields an ultra-high energy resolution of approximately 10 meV, which is beyond the resolution achieved with state-of-the-art X-ray monochromators/spectrometers made of perfect crystals (Yabashi et al, 2001; Shvyd’ko et al, 2003)
They obtained interferograms with a high visibility, the PLD range was limited to about 100 nm, which corresponds to an energy resolution of approximately 10 eV in the FT analysis
Summary
Optical interferometry using visible light is one of the most powerful methods for high-precision metrology owing to its high sensitivity to the phase of the light (Hariharan, 2007). In 1965, Bonse and Hart achieved a marked advance in this field by developing a triple-Laue (LLL) crystal interferometer that consists of three blades acting as a splitter, mirror and analyser arranged in a monolithic block made of a perfect crystal of silicon (Bonse & Hart, 1965) This monolithic design significantly facilitates the stabilization of the PLD, the development of X-ray interferometers with a variable PLD is still in great demand for expanding the range of interferometry applications. Appel & Bonse (1991) first demonstrated an X-ray Michelson interferometer in which the LLL interferometer was combined with weakly linked Bragg-case channel-cut crystals placed on a common rotational stage They obtained interferograms with a high visibility, the PLD range was limited to about 100 nm, which corresponds to an energy resolution of approximately 10 eV in the FT analysis. The validity of this scheme was verified through characterization of the temporal coherence of XFEL pulses at the SPring-8 Angstrom Compact Free-Electron Laser (SACLA) (Ishikawa et al, 2012)
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