Temporal and spatial coherence of high order harmonics

We study coherence properties of high harmonics in three different situations. First, we measure spectral profiles for 19^th and 21^th harmonics in argon and neon as a function of the focusing, and give reasonable evidence of a harmonic chirp. Second, we produce two phase-locked harmonic sources, separated in time, and we perform for the first time frequency-domain interferometry in the XUV range. With this technique, we demonstrate the feasibility of time-resolved plasma diagnostics in a fully ionized gas. Third, we use Fresnel's mirrors interferometer to measure the spatial coherence degree of 13^th harmonic. In the best conditions, we find high coherence (γ > 0.5) in almost the full beam section. The studies emphasize the role of the intensity-dependent phase of the nonlinear polarization in the coherence properties of harmonics.