Temporal and spatial coherence of high order harmonics

We study coherence properties of high harmonics in three different situatio ns. 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 inter ferometry in the XUV range. With this technique, we demonstrate the feasibi lity of time-resolved plasma diagnostics in a fully ionized gas. Third, we use Fresnel's mirrors interferometer to measure the spatial coherence degre e of 13(th) harmonic. In the best conditions, we find high coherence (gamma > 0.5) in almost the full beam section. The studies emphasize the role of the intensity-dependent phase of the nonlinear polarization in the coherenc e properties of harmonics.