Seminarium Fizyki Materii Skondensowanej

Metamaterials are notorious for providing fascinating optical properties not found in nature, by virtue of designing the underlying geometry of arrays of optical components. However it is not quite as well appreciated that one can also induce non-trivial, even topological, changes within an optical medium simply by tuning the strength of the light-matter coupling. We highlight this key fact with a very simple example. First, we consider a dimerised chain of meta-atoms that experience Coloumb interactions with their nearest neighbours. In this case, the famous bulk-edge correspondence holds: there is a direct correspondence between the topological Zak phase (a certain integral over eigenstates performed in the continuum), and the presence of edge states in a finite chain. This correspondence holds when we include next-nearest neighbours. It holds when we include all neighbours. It holds when we include non-resonant terms, when we break chirality, when we include retardation effects and radiation damping, and when we include disorder. Thus the bulk-edge correspondence indeed appears to be a very useful predictor of edge states. However, strong light-matter coupling spoils it all. In the strong light-matter coupling limit, we show that by simply changing the height of an enclosing photonic cavity, we can reach a critical coupling strength where we break the bulk-edge correspondence. Namely, we observe an apparently topologically nontrivial phase, which unexpectedly does not exhibit edge states.