## 2 Experiment
### QPM in LPPs  
Figure 2 shows the raw images of the harmonic spectra from different LPPs. The saturated images of harmonics are intentionally chosen to present the spectra for better viewing. Note that for the line-outs of the HHG spectra, unsaturated images are used. The difference between the so-called plateulike harmonics and the group of enhanced harmonics in the plateau range is clearly displayed. Figure 2a shows the plateulike shapes of the odd harmonics generated in indium, silver and gold plasmas in the case of 810 nm, 50 fs driving pulses. The harmonics between the 17 th (H17) and 33 rd (H33) orders are shown. Nearly similar distributions, with almost equal intensities of these harmonics, are observed. The plateaulike shape of harmonic spectra drastically changed once the MSM was inserted in the path of the heating picosecond radiation. The examples of three strongly modulated and enhanced spectra of the groups of odd and even harmonics of two-color NIR pump (1320 nm + 660 nm, 70 fs pulses) compared to the case of extended homogeneous indium plasma are shown in Figure 2b. The three upper spectra containing significantly enhanced groups of harmonics centered at H34, H31, and H28 were observed when tilting the MSM at angles of 35\({}^{\mathrm{0}}\), 20\({}^{\mathrm{0}}\) and 0\({}^{\mathrm{0}}\) and creating the 11-, 9-, and 8-jet indium plasmas, with the sizes of single jet of 0.23, 0.28, and 0.31 mm, respectively. Meanwhile, the bottom spectrum shows the HHG from the homogeneous extended indium plasma produced at similar conditions. The collection times of these four spectra and the energy fluencies of target ablation were similar to each other. When the MSM was used, the whole energy of heating pulses  
Figure 2: Raw images of harmonic spectra from (**a**) extended homogeneous (top to bottom) indium, silver and gold laser-produced plasmas (LPPs) pumped by 810 nm, 50 fs pulses and (**b**) multi-jet indium LPP produced using tilted MSM with the single plasma jet sizes of (top to bottom) 0.23, 0.28, and 0.31 mm pumped by two-color NIR + visible (1320 nm + 660 nm), 70 fs pulses. The peak intensities of 800 and 1320 nm pumps in the plasma area were \(5\times 10^{14}\) and \(1.4\times 10^{14}\) W cm\({}^{-2}\), respectively, while the intensity of the second harmonic of 1320 nm pump was \(\sim\)\(1\times 10^{13}\) W cm\({}^{-2}\). Maximally enhanced harmonics (H34, H31, and H28) are clearly distinguished for each of these plasma configurations. Bottom spectrum corresponds to the HHG from the homogeneous extended indium plasma produced at similar conditions. interacting with target was two times smaller when compared with the case of extended homogeneous plasma, while the energy fluencies of heating radiation on the target surface were equal to each other. The enhancement factors of these harmonics (three upper spectra) with respect to those generated in the case of imperforated plasma (bottom spectrum) were measured to be 38\(\times\), 36\(\times\), and 27\(\times\) respectively. A two-fold decrease of the whole length of multi-jet plasma compared with extended imperforated plasma (2.5 and 5 mm respectively) did not play an important role in the harmonic yield, which underlines the importance of the collective processes over single-atom ones. The bottom spectrum of Figure 2b, as well as the three upper spectra, shows the strong resonance-induced enhancement of H21. On the other hand, the formation of QPM conditions (three upper spectra) demonstrates the significant enhancement of the groups of harmonics surrounded the one (\(H_{\mathrm{qpm}}\)) for which conditions of QPM became most favorable. Similar pattern, though without resonance-induced enhancement of harmonics, was observed in the case of the silver plasma pumped by 1310 nm + 655 nm radiation (Figure 3). Again, in the case of imperforated 5-mm-long Ag plasma, plateaulike harmonics were barely seen (bottom image). The groups of strong harmonics appeared once the MSM was inserted between the target and heating pulse.