## 4 Discussion
The observation of the enhancement of the even (H18) harmonic and the odd (H17) harmonic, in the case of the TCP and SCP of two-component plasma, have demonstrated that the macroscopic properties of the plasma cannot be considered as the origin of the single harmonic enhancement. In other words, the co-existence of lead plasma does not destroy the resonance effect and single harmonic enhancement caused by tin, which suggests the diminished role of the macroscopic effect in the modification of the resonance-enhanced properties of harmonics in the latter component of plasma. We did not analyze the size-dependent effect of the harmonic yield in the case of nanoparticle-containing plasmas. Meanwhile, the assumption of the influence of a strong local field in the case of smaller-sized nanoparticles, like quantum dots, can predict the growth of harmonic yields in the latter case compared with the large nanoparticles. This assumption can be supported by the vision of HHG as a three-step process, when only atoms on the surface of nanoparticles efficiently participate in harmonic generation, while inner atoms cannot be considered as the emitters of harmonics due to their strong absorbance of generated radiation. The most probable scenario in determining the optimal sizes of atomic aggregates for HHG is the demonstration of the attractiveness of the species with sizes of 1-5 nm, when the quantum confinement effect allows for the increase of the local field-related nonlinearity while limiting the negative effects attributed to the large (20-100 nm) nanoparticles. This assumption has recently been confirmed during the application of metal sulfide quantum dots for HHG [48,49]. There are a few studies of HHG in the plasmas produced on the nanoparticle-coated targets. The aim of those studies was to elevate the nanoparticle cloud by laser ablation (like in our experiments) and to analyze the nonlinear conversion of infrared pulses towards the extreme ultraviolet range. Meanwhile, the most intriguing method would be the propagation of strong laser pulses through nanoparticle-coated thin films. Interest in HHG in solids has been increased dramatically, though this process was demonstrated using the mid-infrared laser sources. The difficulties related to the absorption of generated harmonics in UV and XUV ranges during propagation through solids can be diminished while using the thin films containing nanoparticles. The main difference of the present study compared to refs 42 and 43 is the analysis of resonance enhancement of the harmonics in the region of -47 nm during the propagation of the femtosecond pulses through the tin plasma. The analysis, particularly, includes the application of the tunable source in the near-infrared range, which allowed for the tuning of the order of the harmonic being enhanced (from the 28 th to the 29 th order),which allowed for the identification of the spectral range where the highest enhancement of single harmonics was achieved. The important finding in the case of the two-color pump study was the appearance of the exceptionally strong 18 th harmonic of 806 nm radiation notably exceeding the earlier analyzed 17 th harmonic achieved during the single-color pump of Sn laser-induced plasma. Our present study shows that the closeness of the wavelength of the harmonic with the ionic transition possessing large oscillator strength (as in the case of the almost perfect coincidence of the H17 (\(\lambda\) = 47.4 nm) and 44\({}^{10}\)5s\({}^{2}\)5p\({}^{2}\)P\({}_{3/2}\)\(\rightarrow\)4d\({}^{9}\)5s\({}^{2}\)5p\({}^{2}\) (\({}^{1}\)D)(\({}^{2}\)D\({}_{5/2}\)) transition (\(\lambda\) = 47.8 nm)) does not necessarily lead to the stronger enhancement compared with the neighbor harmonic (H18, in our case). Notice that the latter harmonic became available due to the interaction of the weak second field (403 nm) with the strong fundamental wave (806 nm).