Measurement of the degree of spatial coherence of high-order harmonics using a Fresnel-mirror interferometer - art. no. 043802

We have studied in detail the spatial coherence of the far field of the 13t h harmonic from a Ti:sapphire laser generated in xenon, as a function of th e generation parameters. Experimentally, we use Fresnel mirrors to produce two-dimensional interferograms. This technique allows us to probe the coher ence at different scales d=1-3 mm between the interfering rays. i.e., throu ghout the full section of the incident beam. A high uniform degree of mutua l coherence gamma(d) , larger than 0.5 in most cases, is measured us a func tion of the position of the jet relative to the focus, and pressure in the jet. It confirms the high intrinsic spatial coherence already reported for the extreme-ultraviolet harmonics. which is much larger than the one produc ed from x-ray lasers, Spatial coherence decreases when the laser focus is m oved toward the jet, and when the pressure is increased: the onset of ioniz ation, as well as the intensity-dependent phase of the nonlinear polarizati on, are rapidly varying factors in time and space which degrade the correla tion between the fields at two different points. Simulation of the coherenc e degree emphasizes the role of the intensity-dependent phase in the evolut ion of the coherence degree.