Strong coupling of phonon polaritons in h-BN nanowires with graphene plasmons

Abstract

The phonon polaritons in hexagonal boron nitride (h-BN) can couple with graphene plasmons, forming new hybrid modes which are called plasmon-phonon polaritons. In this paper, we point out that such coupling can occur within the strong coupling regime, where the dispersion relations of the graphene plasmons and the phonon polaritons avoid crossing with each other. We have investigated graphene-wrapped h-BN nanowires, and found that such strong couplings exist between the graphene plasmons and the fundamental as well as the high-order phonon polariton modes, even if the graphene layer is far away from the h-BN. This is interesting because the high-order phonon polariton modes are mostly confined within the bulk h-BN with little fields outside the nanowire, i.e., the interactions of these modes with the graphene plasmons should be rather weak; however, our calculations disagree with such judgements. Due to the dispersive permittivity of h-BN, the graphene plasmons can couple simultaneously with two branches of phonon polariton modes under carefully chosen electron doping levels. Our investigations may open the way to explore the plasmon-phonon polaritons in the strong coupling regime.