  Nanoantennas are efficient devices exhibiting large confined electric field
enhancements. So far, they have been extensively researched mainly in the
receiving mode, which means that the illuminating field is essentially a plane
wave. In this paper, we consider the problem of designing an efficient and
highly directive transmitting Nanoantenna where the system is energized by a
non-plane wave field, a subwavelength laser excitation. Including
short-wavelength components allowed us to achieve a 200-nm spot radius, which
is a quarter of its incident wavelength (800 nm). Near- and far-field antenna
quantities are introduced and calculated using an efficient full-wave
multiphysics solver. A nano-scale optical antenna is then presented with
optimized dimensions and material settings. Various design curves and insights
are also discussed in connection with how issues such as how to define
efficiency and determine whether the system is radiating properly.
