Bose–Einstein condensation in plasmonic lattices

Plasmonics is the study of the interaction between light and metallic structures at the nanoscale. This dissertation explores metallic nanostructures which enable coupling photons to the electrons in the metal, thereby confining light in space smaller than the wavelength. This allows for observing macroscopic quantum-coherent phenomena at room temperature, such as the first Bose-Einstein condensate made of light and electrons observed in this dissertation. The research focuses on periodic arrays (lattices) of gold nanoparticles that are overlaid with organic fluorescent molecules. The molecules can be excited optically by an external laser. The molecules emit photons into the lattice, exciting optical resonances supported by the array structure. When the concentration of molecules is sufficiently high, the lattice resonances can be strongly coupled with the molecules, which modifies the energy states of both. At strong coupling, the lattice resonances and the molecules form new type of quasiparticles with properties of both light and matter. The dissertation consists of five research articles. In Publication I, we introduce the first Bose-Einstein condesate in a plasmonic system. The condensate is formed at room temperature in a picosecond timescale. In Publication II, we achieve the first plasmonic Bose-Einstein condensate at the strong coupling regime. The strongly coupled condensate is 100000 times more luminous than the first plasmonic condensate. Due to the room temperature operation and high luminosity, the strongly coupled plasmonic condensate provides a promising platform for fundamental studies of condensates of light and also for possible applications, for example, in the fields of sensing and optical communications. In Publication III, we study spatial and temporal coherence of the strongly coupled plasmonic Bose-Einstein condensate in large arrays. The condensate studied in this work is half a millimeter long, making it reportedly the largest luminous condensate to date. In Publication IV, we report our observations on the phase and polarization properties of the strongly coupled plasmonic Bose-Einstein condensate. We observe a non-trivial phase distribution, which allows for creating different polarization textures. In Publication V, we present a new theoretical model for strongly coupled organic systems. With the new model we compute, for instance, lasing phase diagrams both at the weak and the strong coupling regime and pinpoint the origin of effective interactions in strongly coupled organic systems.