Evidence of strong contribution from neutral atoms in intense harmonic generation from nanoparticles

We show experimental evidence that, for the intense high-order harmonics from nanoparticles, there is a strong contribution from neutral atoms. We present the results of studies on the harmonics generated in laser-produced plasmas containing various nanoparticles, including Cr2O3, In2O3, Ag, MnTiO3, Sn, Cu, and Au. These results are compared with the harmonics generated from plasma produced on the surface of bulk targets. The harmonic spectrum from nanoparticle and bulk In2O3 show that there is a lack in the resonant enhancement of the 13th harmonic for the former. Along with the relatively low cut-off for nanoparticle harmonics, these results show that it is the neutral atom in the nanoparticle that emits the intense harmonics. Structural studies of plasma debris confirm the presence and integrity of nanoparticles in the plasma plumes.