Investigations of Strong Light-Matter Interactions in Nanophotonic Systems

Sammanfattning: Noble metal nanoparticles can support localized surface plasmon resonances (LSPR), thus behaving as open optical resonators. Outstanding optical properties as well as a subwavelength mode volume make plasmonic nanoparticles a promising platform for enhanced light-matter interactions. Light is focused to a nanoscale volume, so called ‘hot-spots’, resulting in strong electromagnetic field amplification in that region. An emitter placed in such a hot-spot can couple with the LSPR of the nanoparticle and thereby experience a dramatic change in its properties. When the light-matter interaction becomes strong enough, the system enters a special regime, so-called strong coupling. In this regime, the cavity and emitter exchange their energy in a coherent manner on time scales that are faster than their respective dissipation rates. This leads to the formation of new hybrid light-matter states, referred to as polaritons. In this strong light-matter coupling regime, not only the optical but also material-related properties of the system can be modified. The aim of this thesis is to show and discuss room temperature strong light-matter coupling as well as beneficial and limiting factors of the coupling process. Excitons in transition metal dichalcogenides (TMDC) are coupled to plasmonic resonances of individual gold nanobipyramids (BPs). Strong coupling of excitons and BPs in a single hot-spot is demonstrated. Subsequentially, the asymmetric photoluminescence (PL) emission behavior of this hybrid system is investigated and discussed. Moreover, an interesting case of strong coupling arises when the TMDC material itself is made thick enough to support resonant Fabry-Pérot optical modes in the same frequency range as the exciton resonance. In such circumstances the excitons can be self-hybridized with the optical resonator made of the same material and thereby modify the absorption of the TMDC material over the whole visible spectrum. In addition to the above-mentioned studies of strong coupling, nonlinear laser microscopy has been employed to study plasmonic, as well as biological samples. And finally, the effect of temporal PL coherence from a single plasmonic nanoparticle is demonstrated as well as different methods for sample analysis and understanding their limitations are discussed.