Coherence properties of high-order harmonics: Application to high-density laser-plasma diagnostic

We present two interferometry schemes in the extreme ultraviolet, based on either the wave-front division of a unique harmonic beam (1(st) scheme) or two spatially separated, phase-lacked harmonic sources (2(nd) scheme). In t he first scheme using a Fresnel bimirror interferometer, we measure the deg ree of spatial coherence of the 13(th) harmonic generated in xenon, as a fu nction of different parameters. A high degree of coherence,larger than 0.5, is found for the best conditions in almost the full section of the beam. T hen, we demonstrate that the second scheme can be used for interferometry m easurements with an ultrahigh time resolution. The 11(th) harmonic is used to study the spatial variation of the electron density of a laser-produced plasma. Electronic densities higher than 2.10(20) cm(-3) are measured.