  Plasmon polaritons are formed by coupling light with delocalized electrons.
The half-light and half-matter nature of plasmon polaritons endows them with
unparalleled tunability via a range of parameters, such as dielectric
environments and carrier density. Therefore, plasmon polaritons are expected to
be tuned when in proximity to polar materials since the carrier density is
tuned by an electrostatic potential; conversely, the plasmon polariton response
might enable the sensing of polarization. Here, we use infrared nano-imaging
and nano-photocurrent measurements to investigate heterostructures composed of
graphene and twisted hexagonal boron nitride (t-BN), with alternating
polarization in a triangular network of moir\'e stacking domains. We observe
that the carrier density and the corresponding plasmonic response of graphene
are modulated by polar domains in t-BN. In addition, we demonstrate that the
nanometer-wide domain walls of graphene moir\'es superlattices, created by the
polar domains of t-BN, provide momenta to assist the plasmonic excitations.
Furthermore, our studies establish that the plasmon of graphene could function
as a delicate sensor for polarization textures. The evolution of polarization
textures in t-BN under uniform electric fields is tomographically examined via
plasmonic imaging. Strikingly, no noticeable polarization switching is observed
under applied electric fields up to 0.23 V/nm, at variance with transport
reports. Our nano-images unambiguously reveal that t-BN with triangular domains
acts like a ferrielectric, rather than ferroelectric claimed by many previous
studies.
