Tunable Dirac Polaritons in Cavity-Embedded Metasurfaces

Abstract

The physics of graphene has been aptly described as ``QED in a pencil trace'' because the low-energy electrons behave like massless Dirac fermions -- these exhibit pseudo-relativistic phenomena that were once thought to be exclusive to the realm of high-energy physics. Inspired by the rise of graphene, in this thesis we theoretically explore a range of novel phenomena that can emerge in 2D hexagonal metasurfaces composed of subwavelength arrays of dipole emitters/antennas. These metasurfaces exhibit Dirac polaritons whose properties are not solely determined by the intrinsic structure of the emitters and the metasurface geometry; in fact, their properties are also inextricably tied to the local photonic environment. Exploiting this hybrid light-matter nature of the Dirac polaritons, we unveil that one can dramatically alter their fundamental properties by structuring the photonic environment via a cavity waveguide. First, we show that a honeycomb metasurface is not merely a simple analog of graphene, despite sharing the same underlying lattice structure. In particular, the metasurface exhibits two distinct species of massless Dirac polaritons, type-I and type-II, where the latter emerge from a non-trivial winding in the light-matter interaction. Moreover, by varying the cavity width one can induce the multi-merging of the type-I and type-II Dirac points and the subsequent annihilation of the type-II Dirac points. Consequently, we unveil a morphing between a linear and a quadratic spectrum which is accompanied by a change in the topological winding number and an inversion of the chirality. Unfortunately, because the polaritons are neutral particles they do not experience a Lorentz force when subjected to a real magnetic field. Despite this fundamental drawback, we show that one can generate a pseudo-magnetic field for the polaritons by straining the honeycomb metasurface. Interestingly, by varying the cavity width one can tune the strength of the pseudo-magnetic field and even switch it off entirely without modifying the strain configuration. This enables one to emulate phenomena such as Landau quantization, where varying the cavity width can induce a collapse and revival of the polariton Landau levels. Finally, we show that one can generate non-trivial Berry curvature in momentum space by breaking the inversion symmetry of a kagome metasurface. Crucially, the geometrical and topological properties of the polaritons are not fixed by the symmetry-breaking perturbation but also depend qualitatively on the local photonic environment in which the dipoles are embedded. Specifically, we show that one can invert the valley-Chern numbers and thus switch the chirality of the polariton valley-Hall edge states by varying the cavity width.