Abstract
We characterize the spatial and temporal coherence properties of hard X-ray pulses from the Pohang Accelerator Laboratory X-ray Free Electron Laser (PAL-XFEL, Pohang, Korea). The measurement of the single-shot speckle contrast, together with the introduction of corrections considering experimental conditions, allows obtaining an intrinsic degree of transverse coherence of 0.85 ± 0.06. In the Self-Amplified Spontaneous Emission regime, the analysis of the intensity distribution of X-ray pulses also provides an estimate for the number of longitudinal modes. For monochromatic and pink (i.e. natural bandwidth provided by the first harmonic of the undulator) beams, we observe that the number of temporal modes is 6.0 ± 0.4 and 90.0 ± 7.2, respectively. Assuming a coherence time of 2.06 fs and 0.14 fs for the monochromatic and pink beam respectively, we estimate an average X-ray pulse duration of 12.6 ± 1.0 fs.
Highlights
We characterize the spatial and temporal coherence properties of hard X-ray pulses from the Pohang Accelerator Laboratory X-ray Free Electron Laser (PAL-X-ray Free Electron Lasers (XFELs), Pohang, Korea)
In combination with the recent developments of generating two XFEL pulses separated in time these sources offer the promise for probing dynamics in amorphous and disordered systems on length- and time-scale of interest by means of X-ray Speckle Visibility Spectroscopy[13,16]
The speckle contrast is evaluated in concentric regions of interests (ROIs) by fitting the intensity distribution from the pixels contained in these areas with the intensity distribution described in Eq (1)
Summary
We characterize the spatial and temporal coherence properties of hard X-ray pulses from the Pohang Accelerator Laboratory X-ray Free Electron Laser (PAL-XFEL, Pohang, Korea). X-ray Free Electron Lasers (XFELs) deliver orders of magnitude more brilliant, nearly fully transversely coherent and ultrashort X-ray pulses than previously available at synchrotron storage-ring based sources[1,2] These unique properties enable probing complex structural dynamics down to femtosecond time-scales by means of optical-pump X-ray-probe scattering[3,4], X-ray photon correlation spectroscopy[5,6,7], and single-pulse coherent diffraction imaging[8,9,10]. By utilizing the short pulse duration and nearly full transverse coherence of the beam, high-contrast speckles have been measured from atomic-scale ordering[11] Such properties allow, for example, capturing instantaneous snap-shots of ferromagnetic, nanoscale spin order via single-pulse resonant X-ray holography techniques[12]. Extending to the hard X-ray regime www.nature.com/scientificreports/
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