Nonlinear exciton-polaritons from infrared to ultraviolet in photonic microstructures based on transition metal dichalcogenides and GaN materials

Being a superposition of photons and excitons in the strong coupling regime, exciton-polaritons inherit properties from both the constituents. We are interested in the interaction arising from their matter component, since the enhancement of the interaction strength compared to that in nonlinear optical materials in the weak coupling regime might lead to the development of new technological platforms, where light can be controlled by light thanks to Kerr-like nonlinearity. In Chapter 1, we define exciton-polaritons and introduce the fundamental many-body phenomena in which they are involved, like Bose-Einstein condensation and solitons. In Chapter 2, we describe the active media used in polaritonics, especially the two employed in this thesis, i.e. transition-metal dichalcogenide (TMDC) monolayers and GaN. In Chapter 3, we outline the different geometries utilised to confine the light and make it interacts with excitons, specifically microcavities and waveguides. In Chapter 4, we detail the experimental setups we used to study exciton-polaritons in TMDC-microcavities and GaN-waveguides. In Chapter 5, we report on the preliminary results obtained on bare MoSe2 monolayers and when we embedded them in our open-access microcavity. In Chapter 6, we narrate about the interesting results achieved on TMDC-polaritons, focusing on the giant trion-polariton interaction strength, which lays the foundation for a new generation of nonlinear devices in the IR spectral range, foreshadowing highly efficient polariton blockade. In Chapter 7, we observe for the first time nonlinear self-modulation of UV pulses operating up to room temperature, opening the way for potential technological platform for ultrafast nonlinear optics without cryogenics in the UV spectral range. Finally, in Chapter 8, we summarise all the results obtained and trace possible future outcome in polaritonics, ranging from topological studies to superconductivity. Appedices A and B are the theoretical models underneath the two experimental Chapters 6 and 7, respectively.