Enhanced high-order harmonic generation from Argon-clusters

High-order harmonic generation (HHG) in clusters is of high promise because clusters appear to offer an increased optical nonlinearity. We experimentally investigate HHG from Argon clusters in a supersonic gas jet that can generate monomer-cluster mixtures with varying atomic number density and associated liquid mass fraction, g, and average cluster size. To obtain the single-atom response from atoms within the cluster, we use two models to theoretically obtain both the expected high-order harmonic yield from monomers and g. Our analysis shows an enhanced single-atom response from atoms in clusters of small size (<3000 atoms) with an enhancement up to a factor of 100 for clusters containing fewer than 500 atoms. Our observations agree with the atom-to-cluster picture where, for small clusters, spreading of the bound part of the electron wave function after ionization over the whole cluster allows coherent recombination with any ion inside the cluster leading to an enhanced polarizability.