High-order harmonic generation (HHG) is investigated in rare-gas clusters as a function of the cluster size using 0.8 and $1.3\mu \mathrm{m}$ femtosecond lasers. A characteristic, species-dependent knee structure in the single particle response is observed. A 1D recollision model qualitatively reproduces this behavior and associates it to the degree of delocalization of the initial wave function. Small clusters are observed to have a higher efficiency than monomers but rapidly lose this advantage as the size increases. The implications of these findings on the HHG mechanism in clusters are discussed.

• Received 28 August 2014

DOI:https://doi.org/10.1103/PhysRevLett.113.263401