  We report here a detailed study of the four-wave mixing process in the
extreme ultraviolet (XUV) region around 30 nm by using two collinear
incommensurate frequency laser pulses. The experimental results reveal evidence
of the coherent accumulation of the wave-mixing fields and low-order
(third-order and fifth-order) nonlinear response of an argon medium. The
dependence of the intensities of the mixing fields on the intensity of a weak
control field, on the argon pressure and on the interaction length is analyzed
to show that the four-wave mixing fields in this spectral range are generated
under the phase-matched condition.
