Light-matter interactions and topological effects in ensembles of plasmonic nanoparticles

The cornerstone of plasmonics is the fundamental coupling between photons and electron oscillations on the surface of a metal, which allows light to be trapped in subwavelength volumes. The electromagnetic fields of plasmonic excitations are highly confined to the metal-dielectric interface, leading to extreme field enhancement and enabling strong light-matter interactions. Enforcing this effect in carefully designed nanostructures allows the creation of high-quality optical modes that provide efficient coupling between photons and molecular emitters. This dissertation studies lasing and condensation as light-matter phenomena in plasmonic nanoparticle lattices and their underlying connections to topology, which is concerned with properties that are invariant under continuous deformations. The optical modes supported by different nanoparticle structures and the phenomena enabled by them are investigated experimentally: samples are fabricated using an electron beam lithography process, and an angle-resolved spectroscopy setup is used to induce the light-matter effects and to characterize their properties. The experimental methods used in the research are discussed in-depth to enhance reproducibility and to provide tools for future implementations. In Publication I, the polarization and phase properties of a strongly coupled plasmon condensate are studied in a square nanoparticle lattice. Polarization-resolved images of the condensate and its far-field emission pattern are used in a phase-retrieval algorithm. The resulting nonuniform condensate phase is shown to host a topologically trivial winding of the polarization vector, which is treated as a pseudo-spin property of the system. In Publication II, the quantum metric and Berry curvature are measured in a plasmonic lattice constituting the first observation of the quantum geometric tensor in a non-Hermitian system. Nonzero components of the tensor are discovered around high-symmetry lines of the Brillouin zone, explained by the non-hermiticity of the system and pseudospin-orbit coupling. The experimental findings are verified qualitatively by a two-band model. In Publication III, a pattern design method based on the lossy nature of metallic nanoparticles is utilized in creating plasmonic quasicrystals that host modes with polarization vortices. The fabricated samples are combined with dye molecules to demonstrate lasing with extremely high topological charges, verified by polarization-resolved measurements and theoretical considerations.