Time-reversal invariant topological modes in plasmonic metasurfaces

A prominent goal of nanophotonics is the efficient manipulation of light on the nanoscale. Topological nanophotonics and plasmonics offer a potential path towards realising this through protected modes of light on subwavelength scales at optical frequencies. Inspired by the success of topological modes for electrons, which are robust against a range of defects and disorder, there has been a push towards realising analogues of these modes in photonic systems. Typically these have employed time-reversal breaking components, such as strong magnetic fields or complex engineering of lattice elements, and often operate in narrow frequency regimes, making them difficult to miniaturise to the nanoscale. On the other hand, time-reversal invariant effects which are only reliant on the underlying crystalline symmetry of a lattice are more suitable candidates for hosting subwavelength topological modes. In this thesis, we study the emergence of these modes in the plasmonic metasurface: A two-dimensional lattice of metallic nanoparticles. Individual nanoparticles host localised surface plasmon resonances, which when placed close to other nanoparticles result in collective modes across the system. The strong dipolar coupling between nanoparticles separated by subwavelength distances means many ideas can be brought over from tight- binding models in condensed matter. We begin by examining one-dimensional propagating modes of two different lattice systems. Specifically, we link the crystalline symmetries of the respective bulk phases with topological properties and show how these result in the emergence of edge states. We compare the robustness of these by examining the energy propagation along the interface and in doing so also show how these phases can be used to realise directional propagation of energy. We then study higher-order topology and zero-dimensional localised corner modes in finite lattices. Again we show how these modes arise due to the topology of the bulk, and with this we test the robustness of the modes against disorder. Throughout the thesis, we develop the theoretical understanding of novel concepts in topological nanophotonics, as well as linking the theory to experimental signatures of topology. We do so by presenting results in the context of the experimental state of the art and as well as proposing methods for probing observables in topological plasmonics.Open Acces