  Probing strongly coupled quasiparticle excitations at their intrinsic length
scales offers unique insights into their properties and facilitates the design
of devices with novel functionalities. In this work, we investigate the
formation and emission characteristics of plexcitons, arising from the
interaction between surface plasmons in narrow gold nanotrenches and excitons
in monolayer WSe2. We study this strong plasmon-exciton coupling in both the
far-field and the near-field. Specifically, we observe a Rabi splitting in the
far-field reflection spectra of about 80 meV under ambient conditions,
consistent with our theoretical modeling. Using a custom-designed near-field
probe, we find that plexciton emission originates predominantly from the
lower-frequency branch, which we can directly probe and map the local field
distribution. We precisely determine the plexciton extension, similar to the
trench width, with nanometric precision via collecting spectra at controlled
probe locations. Our work opens exciting prospects for nanoscale mapping and
engineering of plexcitons in complex nanostructures with potential applications
in nanophotonic devices, optoelectronics, and quantum electrodynamics in
nanoscale cavities.
