Distinguishing monomer and nanoparticle contributions to high-harmonic emission from laser-ablated plumes

Nano-clusters and nano-particles (NPs) are attractive media for high-harmonic generation (HHG) since they combine the advantages of using atomic media (for the low average density) and bulk solid media (for the high local density). Recently, laser ablated plumes from metal nano-powders have been used as HHG media and it has been often assumed that the harmonics mainly come from the NPs in the plumes but not by the isolated atoms/ions. However, this assumption is yet to be fully justified. Here, we show that in fact both NPs and isolated monomers could dominate the harmonic spectrum, depending on which part of the plume is interacting with the driving laser. From the ablated plume of indium NPs, it is found that the harmonic spectra from the region where monomers dominate are distinctively different from the region where NPs dominate. Our results demonstrate that accurately capturing the contribution of NPs in HHG processes requires precise selection of the laser-plasma interaction region, a factor that had not been carefully considered in previous studies.