Light-Matter Interactions in DielectricNanocavities and 2D Transition-Metal Dichalcogenides

Photonic technologies, such as lasers, LEDs, and detectors, are essential for modernsociety, as they enable information and communication technologies. Unfortunately,energy consumption from these technologies increases rapidly. At the same time,photonic technologies hold great promise for quantum information processing, suchas quantum computing and quantum communication. This demands research fornovel photonic components for both energy-efficient data processing and quantumtechnologies.Many of the photonic components rely on optical cavities that confine light andenhance light-matter interactions. It was only recently demonstrated that light canbe confined by a dielectric cavity on length scales smaller than the wavelength oflight. The first demonstrations of this extreme dielectric confinement, which exploitedsilicon structures, combine small losses with sub-wavelength mode volumes, which areexpected to give rise to unprecedented light-matter interactions.In this thesis, extreme dielectric confinement cavities and their light-matter interactions with a monolayer transition metal dichalcogenide are investigated. At first,tight field confinement in indium phosphide-based structures is experimentally verified with a scattering-type scanning near-field optical microscope. Moreover, the spectral properties of these nanocavities are investigated with position- and polarizationresolved reflection measurements in a confocal geometry. It is shown that the background in these measurements can be completely suppressed by carefully choosing thepolarization projection of the detection signal. The approach reveals another modepresent in the cavity, which is confirmed with finite element method simulations inthe quasinormal mode framework.After investigation of the passive structures, light-matter interactions are studied. To achieve strong coupling, where excitons and photons hybridize and formexciton-polaritons, the light-matter interaction needs to overcome the system’s losses.Exploiting cryogenic temperatures, phonon-induced losses are strongly reduced, whilethe dielectric cavity minimizes Ohmic and absorption losses. At the same time, thelarge oscillator strength of excitons in monolayer transition metal dichalcogenidesgives rise to efficient light-matter coupling. As one of the main achievements ofthis Ph.D. project, strong coupling between a dielectric nanocavity and excitons in amonolayer transition metal dichalcogenide is demonstrated. The light-matter interaction strength of (5.3 ± 0.3) meV, obtained from photoluminescence measurements,is in excellent agreement with the theoretical prediction of (5.2 ± 0.7) meV, therebyii Summarycorroborating an exciton-reaction coordinate formalism. The system might serve as anovel testbed for studying nanoscale light-matter interactions. Importantly, the polaritons are confined to sub-wavelength scales by the strict out-of-plane confinementof the excitons in the monolayer and the tight in-plane confinement of the cavitymode. This is expected to enhance exciton-exciton interactions and, together withthe small losses, could enable single-photon nonlinearities. Moreover, the system is apromising platform for studying condensation effects.