XUV multiphoton processes with intense high-order harmonics

We review generation of intense high-order harmonics and its application to nonlinear multiphoton processes in atoms and molecules. Phase-matched high harmonics by a loosely focusing geometry produce highly focusable intensity with fully spatiotemporal coherence, which is sufficient to induce nonlinear optical phenomena in the extreme ultraviolet and soft X-ray (XUV) region where no intense coherent sources had previously existed. Observation of two-photon double ionization as well as two-photon above-threshold ionization in He is demonstrated with 42-eV high-harmonic photons. These nonlinear processes allow us to measure directly the temporal shapes of XUV high-harmonic pulses of which duration should be shorter than 1 fs. By combining intensity autocorrelation and photoelectron spectroscopy in two-photon above threshold ionization in Ar, temporal characterization of a train of attosecond pulses is achieved. On the other hand, when intense high harmonics around 20 eV is subjected to N2 molecules, occurrence of Coulomb explosion following two-photon double ionization is observed in attosecond temporal precision. Taking advantage of larger cross section of two-photon ionization in molecules, we successfully perform the interferometric autocorrelation of an attosecond pulse train with the ion signals produced by Coulomb explosion of nitrogen molecules. The result reveals the phase relation between attosecond pulses in the train.