  Here we explore the radiation features of optical nanoantennas, analyzing the
concepts of input impedance, optical radiation resistance, impedance matching
and loading of plasmonic nanodipoles. We discuss how the concept of antenna
impedance may be applied to optical frequencies, and how its quantity may be
properly defined and evaluated. We exploit these concepts in optimization of
nanoantenna loading by optical nanocircuit elements, extending classic concepts
of radio-frequency antenna theory to the visible regime for the proper design
and matching of plasmonic nanoantennas.
