## 3 Results
### Tunable NIR Pump  
One can distinguish the importance of tuning the wavelength of the driving pulses to determine the variations of harmonic yields in the case of resonance-related processes. Therefore, the application of OPA-based tunable sources offers large perspectives in these studies. Below, we apply the tunable NIR pulses from the OPA and their second harmonics for the analysis of resonance enhancement in Sn LIP, as well as in Sn NP LIP. High-order harmonic generation in two media using TCP pulses was optimized by different means (i.e., the fluence of HP, the intensity of DP, the delay between the HP and the DP, the application of BBO crystals of different thickness, etc.). Particularly, relatively stronger ablation of the Sn NP target allowed for the observation of the enhancement of odd and two neighboring harmonics in the vicinity of a strong ionic transition of tin, contrary to the case of the weak ablation of this nanoparticle-containing target. Our TEM studies of Sn NP debris deposited on nearby glass slides showed the presence of the small NPs ranging from 4 to 14 nm. These NPs appeared in the TEM images at the optimal fluencies of HP (F = 1.4-1.7 J cm\({}^{-2}\)), when the maximal harmonic yield was achieved. The harmonics in the case of the NIR pulses (1 mJ, 1300 nm) were significantly weaker than in the case of the 3 mJ, 806 nm pump, due to a higher intensity of DP in the latter case, and the \(\mathrm{I_{harm}}\)\(\infty\)\(\lambda^{-5}\) rule. The comparison of these two pumps at similar energies of pulses (1 mJ) showed a four-fold decrease of harmonic yield from the plasma produced on the bulk Sn target in the case of 1320 nm pulses compared with the 806 nm pulses, which, to some extent, corresponds to the theoretical prediction of this ratio (11), taking into account a difference in the spatial distributions of two focused beams in the plasma area. Meanwhile, the harmonic cutoff in the case of the NIR pulses was larger than in the case of 806 nm pulses, in accordance with the expectations in the extension of this parameter for the longer-wavelength pump (\(\mathrm{E_{cutoff}}\)\(\infty\)\(\lambda^{2}\)). The harmonic cutoffs in the case of the 806 nm and NIR pump (\(\sim\)1300 nm) pulses were in the range of 42 nm (E = 29.5 eV) and 38 nm (E = 32.6 eV), respectively, while the expected cutoff was calculated to be 76 eV. However, the very small conversion efficiency of higher-order harmonics in the case of NIR pulses did not allow for the observation of harmonics well below the 40 nm spectral region. Thus, the use of 1-mJ NIR pulses for plasma harmonic studies caused the generation of weak odd harmonics in the longer-wavelength range of the XUV spectrum. For this reason, we used the TCP (NIR and H2) of plasma, which proved to enhance the odd harmonic yield. The extension of the harmonic cutoff compared with the single-color pump, the ten-fold growth of the yield of odd harmonics, the comparable harmonic intensities for the odd and even orders along the whole range of generation in spite of using the 0.5 mm-long crystal, contrary to the case of the 806 nm pump, and the tuning of harmonics that allowed for the optimization of resonance-induced single harmonic generation were among the attractive features of these experiments. The raw images of the harmonic spectra presented in Figure 4 were obtained using NIR pulses (\(\lambda\) = 1370 nm) and the second harmonic of this radiation (\(\lambda\) = 685 nm) during their propagation through the plasma produced on the bulk tin and tin NP targets. As in the case of the 806 nm pump of the Sn LIP, we observed the enhanced harmonic in the region of 47 nm (Figure 4, upper panel). At the used pump conditions, it was the 29 th harmonic (H29) of 1370 nm radiation.