Long-wavelength reshaping of the high-order harmonic spectrum generated by an intense multicycle near-IR field in gas

We experimentally demonstrate the control of high-harmonic spectrum discreteness in the vicinity of 100 eV under modulation of multi-cycle driving laser field (1.24 \textmu{}m, 100 fs) by intense mid-IR radiation (4.5 \textmu{}m, 200 fs) and a strong terahertz field (100 \textmu{}m, 530 fs). The observed transition between the discrete harmonic spectrum and broadband quasicontinuum is explained by the modification of attosecond electron dynamics, which can be controlled by the strength and polarization direction of copropagating long-wavelength field. The presented research reveals the underlying physical mechanisms and demonstrates the prospectives of long-wavelength radiation for the generation of quasicontinuous spectrum in the extreme ultraviolet and soft-x-ray regions.