  Unidirectional wave propagation in nonreciprocal structures enables exciting
opportunities to control and enhance wave-matter interactions in extreme ways.
Within this context, here we investigate the possibility of using terminated
unidirectional plasmonic waveguides to enhance typically weak nonlinear effects
by orders of magnitude. We theoretically demonstrate remarkable levels of
electric field enhancement and confinement (field hot-spots) when the
unidirectional waveguiding structure is terminated with a suitable boundary
that fully stops the one-way mode. Such a large field enhancement, originating
from a nonresonant effect, is fundamentally different from the narrow-band
field concentration effects in resonant plasmonic structures. Instead, it is
analogous to the broadband response of plasmonic tapers, but without the need
for any adiabatic impedance matching. We show that this effect can indeed lead
to a substantial boosting of nonlinear light-matter interactions, exemplified
by an improvement of several orders of magnitude in the
third-harmonic-generation efficiency, which is of large significance for
several applications. More broadly, our findings show the potential of extreme
nonreciprocal configurations for enhanced wave-matter interactions.
