Low-dimensional polaritonics: Emergent non-trivial topology on exciton-polariton simulators

Abstract: Polaritonic lattice configurations in dimensions $D=2$ are used as simulators of topological phases, based on symmetry class A Hamiltonians. Numerical and topological studies are performed in order to characterise the bulk topology of insulating phases, which is predicted to be connected to non-trivial edge mode states on the boundary. By using spectral flattened Hamiltonians on specific lattice geometries with time reversal symmetry breaking, e.g. Kagome lattice, we obtain maps from the Brillouin zone into Grassmannian spaces, which are further investigated by the topological method of space fibrations. Numerical evidence reveals a connection between the sum of valence band Chern numbers and the index of the projection operator onto the valence band states. Along these lines, we discover an index formula which resembles other index theorems and the classical result of Atiyah-Singer, but without any Dirac operator and from a different perspective. Through a combination of different tools, in particular homotopy and homology-cohomology duality, we provide a comprehensive mathematical framework, which fully addresses the source and structure of topological phases in coupled polaritonic array systems. Based on these results, it becomes possible to infer further designs and models of two-dimensional single sheet Chern insulators, implemented as polariton simulators.