  Excitons in atomically-thin semiconductors interact very strongly with
electromagnetic radiation and are necessarily close to a surface. Here, we
exploit the deep-subwavelength confinement of surface plasmon polaritons (SPPs)
at the edge of a metal-insulator-metal plasmonic waveguide and their proximity
of 2D excitons in an adjacent atomically thin semiconductor to build an
ultra-compact photodetector. When subject to far-field excitation we show that
excitons are created throughout the dielectric gap region of our waveguide and
converted to free carriers primarily at the anode of our device. In the
near-field regime, strongly confined SPPs are launched, routed and detected in
a 20nm narrow region at the interface between the waveguide and the monolayer
semiconductor. This leads to an ultra-compact active detector region of only
~0.03$\mu m ^2$ that absorbs 86% of the propagating energy in the SPP. Due to
the electromagnetic character of the SPPs, the spectral response is essentially
identical to the far-field regime, exhibiting strong resonances close to the
exciton energies. While most of our experiments are performed on monolayer
thick MoSe$_2$, the photocurrent-per-layer increases super linearly in
multilayer devices due to the suppression of radiative exciton recombination.
These results demonstrate an integrated device for nanoscale routing and
detection of light with the potential for on-chip integration at
technologically relevant, few-nanometer length scales.
