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The high-order harmonic spectrum generated in two-color intense laser fieldunder certain conditions has a pronounced maximum caused by the so calledspectral caustic. Using a numerical solution of the 3D time-dependentSchr\"odinger equation we study the width of the maximum and the degree of thegeneration enhancement due to the caustic as a function of the fundamentalintensity, wavelength, and atomic ionization potential. It is shown that thedegree of this enhancement can be well quantitatively characterized by a singleparameter: the ratio of the radius of the free electronic oscillation in thelaser field to the atomic size. This behavior can be explained by the Coulombattraction of the photoelectron to the parent ion, whereas the effect of thisattraction on the width of the maximum is negligible. Choosing the fieldparameters providing a pronounced enhancement, we calculate the high harmonicmacroscopic response taking into account the transient phase-matching. We showthat the caustic feature can be used to provide a quasi-monochromatic XUVsource with an almost linear frequency vs. time dependence. Such source can beused, in particular, for the time-resolved single-shot XUV imaging.

Spectral caustic in two-color high harmonic generation: role of Coulomb effects