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Emergent intersubband-plasmon-polaritons of Dirac electrons under one-dimensional superlattices

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.