  In this article, the scattering of surface plasmon polaritons (SPPs) into
photons at holes is investigated. A local, electrically excited source of SPPs
using a scanning tunnelling microscope (STM) produces an outgoing circular
plasmon wave on a thick (200 nm) gold film on glass containing holes of 250,
500 and 1000 nm diameter. Fourier plane images of the photons from
hole-scattered plasmons show that the larger the hole diameter, the more
directional the scattered radiation. These results are confirmed by a model
where the hole is considered as a distribution of horizontal dipoles whose
relative amplitudes, directions, and phases depend linearly on the local SPP
electric field. An SPP-Young's experiment is also performed, where the
STM-excited SPP-wave is incident on a pair of 1 $\mu$m diameter holes in the
thick gold film. The visibility of the resulting fringes in the Fourier plane
is analyzed to show that the polarization of the electric field is maintained
when SPPs scatter into photons. From this SPP-Young's experiment, an upper
bound of $\approx$ 200 nm for the radius of this STM-excited source of surface
plasmon polaritons is determined.
