We theoretically demonstrate the two-color-driven selective enhancement of high-order harmonic generations (HHGs) across a broad frequency range, spanning from the extreme ultraviolet (EUV) to the near-soft x-rays. The enhancement of harmonics, particularly at a central wavelength of ${\lambda }_{\text{H}118}=13.5\mathrm{nm}$, is of significant interest in the field of lithography. We identify two distinct enhancement mechanisms for HHGs in argon (Ar) and neon (Ne) gases: the enhancement of the 42nd harmonic (H42) in Ar is attributed to a coherent superposition of enhanced single-atom response harmonics in the off-axis region, whereas the enhancement of H118 in Ne is due to phase-matching selection of harmonics in the off-axis region. Through quantum orbit analysis, we demonstrate that phase matching is primarily influenced by the control of the amplitudes of the most relevant orbits in the off-axis region, which is associated with the modulation of the third strong peak of the driving field within the gaseous medium. This work presents a purely optical methodology for producing table-top tunable coherent sources of EUV or soft x-ray light.

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• Atomic & molecular processes in external fields
• Ultrafast optics