  The phase picked up by a graphene plasmon upon scattering by an abrupt edge
is commonly assumed to be $-\pi$. Here, it is demonstrated that for high
plasmon momenta this reflection phase is $\approx -3\pi/4$, virtually
independent on either chemical potential, wavelength or dielectric substrate.
This non-trivial phase arises from a complex excitation of highly evanescent
modes close to the edge, which are required to satisfy the continuity of
electric and magnetic fields. A similar result for the reflection phase is
expected for other two-dimensional systems supporting highly confined plasmons
(very thin metal films, topological insulators, transition polaritonic layers,
etc.). The knowledge of the reflection phase, combined with the phase picked up
by the plasmon upon propagation, allows the estimation of resonator properties
from the dispersion relation of plasmons in the infinite monolayer.
