  Optical travelling wave antennas offer unique opportunities to control and
selectively guide light into a specific direction which renders them as
excellent candidates for optical communication and sensing. These applications
require state of the art engineering to reach optimized functionalities such as
high directivity and radiation efficiency, low side lobe level, broadband and
tunable capabilities, and compact design. In this work we report on the
numerical optimization of the directivity of optical travelling wave antennas
made from low-loss dielectric materials using full-wave numerical simulations
in conjunction with a particle swarm optimization algorithm. The antennas are
composed of a reflector and a director deposited on a glass substrate and an
emitter placed in the feed gap between them serves as an internal source of
excitation. In particular, we analysed antennas with rectangular- and
horn-shaped directors made of either Hafnium dioxide or Silicon. The optimized
antennas produce highly directional emission due to the presence of two
dominant guided TE modes in the director in addition to leaky modes. These
guided modes dominate the far-field emission pattern and govern the direction
of the main lobe emission which predominately originates from the end facet of
the director. Our work also provides a comprehensive analysis of the modes,
radiation patterns, parametric influences, and bandwidths of the antennas that
highlights their robust nature.
