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Cascaded four-wave mixing in the XUV region.

We present a detailed study of the wave-mixing process in the extreme ultraviolet (XUV) region (around 30 nm) by using two collinear multiple-cycle laser pulses with incommensurate frequencies (wavelengths 1400 and 800 nm). The experimental data provide evidence for the coherent accumulation of wave-mixing fields and a high third-order response of the medium in this spectral range. We show that the time evolution of the mixing fields can be used to study the coherence dynamics of the free-electron wave packet with a lifetime of 200-750 fs.