  The incumbent technology for bringing light to the nanoscale, the near-field
scanning optical microscope, has notoriously small throughput efficiencies - of
the order of 10^(-4) - 10^(-5), or less. We report on a broadband, topological,
unidirectionally-guiding structure, not requiring adiabatic tapering and in
principle enabling near-perfect (ideally, ~100%) optical transmission through
an unstructured single (POTUS) arbitrarily-subdiffraction slit at its end.
Specifically, for a slit width of just lambda_eff / 72 (lambda_0 / 138) the
attained normalized transmission coefficient reaches a value of 1.52, while for
a unidirectional-only (non-topological) device the normalized transmission
through a lambda_eff / 21 (~lambda_0 / 107) slit reaches 1.14 - both, limited
only by inherent material losses, and with zero reflection from the slit. The
associated, under ideal conditions, near-perfect optical extraordinary
transmission (POET) has implications, among diverse areas in wave physics and
engineering, for high-efficiency, maximum-throughput nanoscopes and
heat-assisted magnetic recording devices.
