  We aim to clarify the role that absorption plays in nonlinear optical
processes in a variety of metallic nanostructures and show how it relates to
emission and conversion efficiency. We define a figure of merit that
establishes the structure's ability to either favor or impede second harmonic
generation. Our findings suggest that, despite the best efforts embarked upon
to enhance local fields and light coupling via plasmon excitation, nearly
always the absorbed harmonic energy far surpasses the harmonic energy emitted
in the far field. Qualitative and quantitative understanding of absorption
processes is crucial in the evaluation of practical designs of plasmonic
nanostructures for the purpose of frequency mixing.
