  We employ tip-enhanced infrared near-field microscopy to study the plasmonic
properties of epitaxial quasi-free-standing monolayer graphene on silicon
carbide. The near-field images reveal propagating graphene plasmons, as well as
a strong plasmon reflection at gaps in the graphene layer, which appear at the
steps between the SiC terraces. When the step height is around 1.5 nm, which is
two orders of magnitude smaller than the plasmon wavelength, the reflection
signal reaches 20% of its value at graphene edges, and it approaches 50% for
step heights as small as 5 nm. This intriguing observation is corroborated by
numerical simulations, and explained by the accumulation of a line charge at
the graphene termination. The associated electromagnetic fields at the graphene
termination decay within a few nanometers, thus preventing efficient plasmon
transmission across nanoscale gaps. Our work suggests that plasmon propagation
in graphene-based circuits can be tailored using extremely compact
nanostructures, such as ultra-narrow gaps. It also demonstrates that
tip-enhanced near-field microscopy is a powerful contactless tool to examine
nanoscale defects in graphene.
