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High harmonic generation (HHG) is an established means of producing coherent,short wavelength, ultrafast pulses from a compact set-up. Table-tophigh-harmonic sources are increasingly being used to image physical andbiological systems using emerging techniques such as coherent diffractionimaging and ptychography. These novel imaging methods require coherentillumination, and it is therefore important to both characterize the spatialcoherence of high-harmonic beams and understand the processes which limit thisproperty. Here we investigate the near- and far-field spatial properties ofhigh-harmonic radiation generated in a gas cell. The variation with harmonicorder of the intensity profile, wavefront curvature, and complex coherencefactor is measured in the far-field by the SCIMITAR technique. Using theGaussian-Schell model, the properties of the harmonic beam in the plane ofgeneration are deduced. Our results show that the order-dependence of theharmonic spatial coherence is consistent with partial coherence induced by bothvariation of the intensity-dependent dipole phase as well as finite spatialcoherence of the driving radiation. These findings are used to suggest ways inwhich the coherence of harmonic beams could be increased further, which wouldhave direct benefits to imaging with high-harmonic radiation.

Gaussian-Schell analysis of the transverse spatial properties of high-harmonic beams