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On- and off-axis quasi-phase-matching of the harmonics generated in multi-jet laser-produced plasmas

We analyze the spatial conditions of the quasi-phase-matching in multi-jet laser-produced silver plasma. Our studies of the off-axis and on-axis spatial components of harmonics allow the demonstration of a significant enhancement of a group of harmonics in the latter case. The divergence of enhanced radiation was a few times lower compared with the case of harmonic generation in the extended homogeneous plasma. We show the appearance of plasma emission, together with phase-mismatch, that deteriorate the conditions of high-order harmonic generation (HHG) due to significant phase distortion between the interacting waves. Our two-color-pump-induced HHG at the conditions of multi-jet plasma showed the complexity of the phase relations between the two orthogonally polarized pumps and the harmonic wave, which led to variable conditions of the quasi-phase-matching in the cases of shorter- and longer-wavelength pumps.