  Silicon-based micro-devices are considered promising candidates for
consolidating several terahertz technologies into a common and practical
platform. The practicality stems from the relatively low loss, device
compactness, ease of fabrication, and wide range of available passive and
active functionalities. Nevertheless, typical device footprints are limited by
diffraction to several hundreds of micrometers, which hinders emerging
nanoscale applications of terahertz frequencies. While metallic gap modes
provide nanoscale terahertz confinement, efficiently coupling to them is
difficult. Here we present and experimentally demonstrate a strategy for
efficiently interfacing sub-terahertz radiation ({\lambda}=1 mm) to a waveguide
formed by a nanogap, etched in a gold film, that is 200 nm ({\lambda}/5000)
wide and up to 4.5 mm long. The design principle relies on phase matching
dielectric and nanogap waveguide modes, resulting in efficient directional
coupling between them when placed side-by-side. Broadband far field terahertz
transmission experiments through the dielectric waveguide reveal a transmission
dip near the designed wavelength due to resonant coupling. Near field
measurements on the surface of the gold layer confirm that such a dip is
accompanied by a transfer of power to the nanogap, with an estimated coupling
efficiency of ~10%. Our approach provides a pathway for efficiently interfacing
millimeter-wave and near-infrared photonic circuits, providing controlled and
tailored nanoscale terahertz confinement, with important implications for
on-chip nanospectroscopy, telecommunications, and quantum technologies.
