  Artifical superlattice (SL) potentials have been employed extensively for
band structure engineering of two-dimensional (2D) Dirac electron gas in
graphene. While such engineered electronic band structures can modify optical
or plasmonic properties of graphene, an emergent polaritonic behavior beyond
weak perturbative effects (e.g. anisotropic Drude weights) has not been
reported. Here, we show that an extreme modulation of one-dimensional (1D) SL
potentials in monolayer graphene deforms the underlying Dirac band dispersion
and introduces ladder-like energy levels near the Fermi surface, which result
in emergent intersubband polaritonic responses in optical conductivity. In our
proposed system, hBN-encapsulated graphene is placed on top of a 1D periodic
metagate. In addition, a backgate placed beneath the metagate is used as the
second gate, further modulating carrier density on regions in graphene that are
not directly screened by the metagate. With a strong carrier density
modulation, graphene electrons experience an array of deep potential wells, and
at large enough momenta perpendicular to the modulation direction, Dirac
electrons are waveguided via total internal reflections. These waveguided modes
appear as flat subbands with nearly equispaced energy levels. As a result,
there arise hybrid intersubband-polaritons with ultra-strong coupling in
plasmonic dispersions. Our study opens up an avenue for exploring emergent
polaritons in two-dimensional materials with gate-tunable electronic band
structures.
