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 (1st scheme) or two spatially separated, phase-locked harmonic sources (2nd scheme). In the first scheme using a Fresnel bimirror interferometer, we measure the degree of spatial coherence of the 13 ^th harmonic generated in xenon, as a function 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. Then, we demonstrate that the second scheme can be used for interferometry measurements with an ultrahigh time resolution. The 11th 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.