Characterizing temporal coherence of visible synchrotron radiation with heterodyne near field speckles

In this paper we extensively describe the heterodyne near field speckle method (HNFS) to characterize both spatial and temporal coherence of synchrotron radiation (SR). The method relies on Fourier analysis of near field speckles generated by scattering from nanoparticles suspended in a liquid. A criterion based on master curves of power spectra is introduced and validated by measurements on the visible light produced by the ALBA bending dipole. While spatial coherence measurements with HNFS have been reported, we present for the first time measurements of the temporal coherence of SR wavefronts with the HNFS method both for narrowband and white light beams. In the former case, using a band-pass filter, a coherence time of 40±10 fs is measured, in good agreement with the expected value of 43 fs for the filter inverse linewidth. Moreover, by exploiting the self-reference scheme of the technique, we show that coherence areas propagate carrying nonvanishing curvature. In the latter case, the measured coherence time of the incident SR without any monochromator is 1.6±0.4 fs, corresponding to a bandwidth of 240 nm at a peak wavelength of 350 nm. Exploiting the Wiener-Kintchine theorem, we also retrieve the SR power spectral density at the sample position from the measured temporal coherence function. Results are in good agreement with the measurements performed using a standard spectrometer, yielding a coherence time of 1.4 fs.