  Optical antennas link objects to light. Here, we analyze metal nanorod
antennas as cavities with variable reflection coefficients to derive the
interaction of dipolar transitions with radiation through the antenna modes.
The presented analytical model accurately describes the complete emission
process, and is summarized in a phase-matching equation. We show how antenna
modes evolve as they become increasingly more bound, i.e. plasmonic. The
results illustrate why efficient antennas should not be too plasmonic, and how
subradiant even modes can evolve into weakly-interacting dark modes. Our
description is valid for the interaction of nanorods with light in general, and
is thus widely applicable.
