## 4 Discussion  
The most important feature of these studies was the availability in the analysis of the excited states influencing the harmonic distribution at different excitation conditions of the ablating target. We presented the results of the harmonic spectra modification using tunable driving two-color (NIR and H2) orthogonally polarized pulses, which led to the observation of the resonance enhancement of some harmonic orders caused by different states of plasmas. Studying resonance-induced enhancement of harmonics using tunable NIR and H2 pulses provides the opportunity to fine-tune this high-order nonlinear optical process for wavelength-dependent enhancement of the harmonic yield. This advantage underlines the attractiveness of the analysis of metals, semiconductors, multi-particle systems, and so forth, and, particularly, of the oscillator strengths of some ionic transitions using the plasma HHG technique. Thus, this approach could be considered as a new method of nonlinear spectroscopy. The second field (403 nm or 685 nm) breaks the inversion symmetry, which allows for the observation of even harmonics generation forbidden in the case of the 806 nm and 1370 nm pumps. The interaction of a strong fundamental wave (806 nm or 1370 nm) and a weak second harmonic wave (403 nm or 685 nm) in the LIP becomes a sufficient factor to drastically change the spectral pattern of harmonic emissions. Notice that, in this case, the yields of even harmonics were comparable with those of odd ones, in spite of a ~30:1 ratio of the energies of two pumps. The demonstrated effect shows that aweak second field serves as a trigger for the notable change of this nonlinear optical process. Most of our studies were performed at the fluence F = 1.6 J cm\({}^{-2}\). As we mentioned during the description of the results shown in Figure 4, the growth of the fluence of the 370 ps heating pulses on the target containing Sn NPs allowed for achieving the conditions when the resonance-induced enhancement of the harmonic in the vicinity of strong ionic transitions of Sn becomes pronounced. The disintegration of large NPs into smaller ingredients (atoms/ions, clusters, and tiny NPs) at these conditions of ablation resulted in the involvement of those ionized species in the process of HHG, followed by the observation of the enhancement of some specific harmonic (either H28 or H29) of the used tunable NIR radiation. The significant suppression of the \"resonant\"-induced harmonic gain (either the 17 th order of the 806 nm pump or 28 th and 29 th orders of the NIR pump) could have been caused by the shift of the above-discussed transition. However, our experiments using tunable NIR pulses at the fluence of the HP at F = 1.6 J cm\({}^{-2}\) did not reveal the resonance-related processes lead to the enhancement of the harmonic yield. This observation suggests that the tuning of the ionic transition should not be the main factor in leading to the cancellation of the resonance-related effect. Another factor may be a decrease in the oscillator strength of this transition. The transitions of Sn II and Sn III can be either detuned or weakened once the original species become presented in the form of nanoparticles. The reduction mechanisms of harmonic amplification can be related to either the reduction of the oscillator strength of this transition or the detuning from the wavelength of \"resonant\" harmonics, or both. The macroscopic dispersion properties of the plasma could be another key factor for the formation of the resonance-induced enhancement of a single harmonic in tin plasma, as was suggested in [43]. Recent studies of two-component plasmas allowed for analyzing this opportunity in the case of one of the components (Sn) allowing for the generation of enhanced harmonics and the other component (Pb) causing the modification of plasma dispersion properties [47].