  An effective approach to improve the detection efficiency of nanoscale light
sources relies on a planar antenna configuration, which beams the emitted light
into a narrow cone. Planar antennas operate like optical Yagi-Uda antennas,
where reflector and director elements are made of metal films. Here we
introduce and investigate, both theoretically and experimentally, a scanning
implementation of a planar antenna. Using a small ensemble of molecules
contained in fluorescent nanobeads placed in the antenna, we independently
address the intensity, the radiation pattern and the decay rate as a function
of distance between a flat-tip scanning gold wire (reflector) and a thin gold
film coated on a glass coverslip (director). The scanning planar antenna
changes the radiation pattern of a single fluorescent bead and it beams light
into a narrow cone down to angles of 45{\deg} (full width at half maximum).
Moreover, the collected signal compared to the case of a glass coverslip is
larger than a factor of 3, which is mainly due to the excitation enhancement.
These results offer a better understanding of the modification of light-matter
interaction by planar antennas and they hold promise for applications, such as
sensing, imaging and diagnostics.
