  Optical antenna is a nanoscale miniaturization of radio or microwave antennas
that is also governed by the rule of plasmonics. We introduce various types of
optical antenna and make an overview of recent developments in optical antenna
research. The role of local and surface plasmons in optical antenna is
explained through antenna resonance and resonance conditions for specific metal
structures are explicitly obtained. Strong electric field is shown to exist
within a highly localized region of optical antennas such as antenna feed gap
or apertures. We describe physical properties of field enhancement in
apertures(circular and rectangular holes) and gaps(infinite slit and feed gap),
as well as experimental techniques measuring enhanced electric vector field. We
discuss about analogies and differences between conventional and optical
antennas with a projection of future developments.
