  Plasmonic nanoantennas have been recently proposed to boost nonlinear optical
processes. In a metal dipole nanoantenna with a dielectric nanoparticle placed
in the gap, the linear field enhancement can be exploited to enhance third
harmonic emission. Since both metals and dielectrics exhibit nonlinearity, the
nonlinear far-field contains contributions from each, and the impossibility of
measuring these contributions separately has led to seemingly contradictory
interpretations about the origin of the nonlinear emission. We determine that
the origin of the third harmonic from metal-dielectric dipole nanoantennas
depends on nanoantenna design, and in particular, the width. We find that the
emission from gold dominates in thin threadlike nanoantennas, whereas the
emission from the gap material dominates in wider nanoantennas. We also find
that monopole nanoantennas perform better than dipoles having the same width,
and due to their simplicity should be preferred in many applications.
