Phase-matched four-wave mixing in the extreme ultraviolet region

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.