Unconventional Dirac Polaritons in Cavity-Embedded Honeycomb Metasurfaces

Pseudorelativistic Dirac quasiparticles have emerged in a plethora of artificial graphene systems that mimic the underlying honeycomb symmetry of graphene. However, it is notoriously difficult to manipulate their properties without modifying the lattice structure. Here we theoretically investigate polaritons supported by crystalline honeycomb metasurfaces. Despite the trivial dipolar nature of individual resonant elements, we unveil rich Dirac physics stemming from a non-trivial winding in the light-matter interaction. As a result, a new kind of type-II Dirac point emerges which exists simultaneously with its conventional type-I counterpart. By modifying only the photonic environment via an enclosing cavity, one can manipulate the location of the type-II Dirac points, giving rise to distinct polariton phases. This striking tunability enables one to alter the fundamental properties of the emergent Dirac polaritons while preserving the lattice structure - a unique scenario which has no analog in real or artificial graphene systems. This new paradigm of exploiting the photonic environment will markedly expand the realm of Dirac physics at the subwavelength scale.