Dirac Plasmon Polaritons

Abstract

We study theoretically graphene-like plasmonic metamaterials, in particular a honeycomb structured array of identical metallic nanoparticles, and examine the collective plasmonic modes that arise due to the near-field dipolar coupling between the localised surface plasmons of each individual nanoparticle. An analysis of the band structure of these eigenmodes reveals a phenomenal tunability granted by the polarisation of the dipole moments associated with the localised surface plasmons. As a function of the dipole orientation we uncover a rich phase diagram of gapped and gapless phases, where remarkably every gapless phase is characterised by the existence of collective plasmons that behave as massless chiral Dirac particles, in analogy to electrons in graphene. We consider lattices beyond the perfect honeycomb structure in two ways. Firstly, we break the inversion symmetry which leads to collective plasmons described as massive chiral modes with an energy dependent Berry phase. Secondly, we break the three-fold rotational symmetry and investigate generic bipartite lattices. In this scenario we progressively shift one sublattice away from the original honeycomb arrangement and observe a sequence of topological phase transitions in the phase diagram, as well as the merging and annihilation of Dirac points in the dispersions. After examining the purely plasmonic response we wish to address the true eigenmodes responsible for transporting electromagnetic radiation. For this reason we examine plasmon polaritons that arise from the strong light-matter coupling between the collective plasmons in a honeycomb array of metallic nanoparticles and the fundamental photonic mode of an enclosing cavity. Here we identify that the Dirac point remains robust and fixed in momentum space, irrespective of the light-matter coupling strength. Moreover, we demonstrate a qualitative modification of the polariton properties through modulation of the photonic environment, including order-of-magnitude renormalisation of the group velocity and the intriguing ability to invert the chirality of Dirac polaritons.