Temporal coherence of high-order harmonics

Systematic studies of the temporal coherence properties of high-order harmo nic radiation are presented. These complement our previous investigations [ Bellini et at, Phys. Rev. Lett. 81, 297 (1998)], where we showed the separa tion of the far-field pattern of high-order harmonics into two distinct spa tial regions with different coherence times. Here we show how the coherence time of the inner and outer regions changes as a function of the harmonic order, the laser intensity, and the focusing conditions. Good agreement wit h the predictions of the semiclassical model of harmonic generation is obta ined. [S1050-2947(99)10312-3].