We review nonlinear multiphoton processes induced by intense attosecond XUV pulses. Phase-matched high harmonics by a loosely focusing geometry produce high focused intensity sufficient to induce nonlinear optical phenomena in the XUV region. Such nonlinear phenomena not only have been attracting much attention in the field of quantum electronics, but also are of great importance for direct measurement of temporal shapes of the XUV pulses. Nonresonant two photon processes in atoms and molecules in the XUV region are employed for the direct observation of a train of attosecond pulses by means of autocorrelation. A train of attosecond pulses is characterized by energy-resolved autocorrelation with photoelectrons produced by two-photon above threshold ionization in Ar. Molecular Coulomb explosion induced by two-photon double in N2 is demonstrated for the first time and also is found to be useful for efficient measuring a train of attosecond pulses
