Title: Thermalization, Coherence, and Critical Fluctuations in Exciton-Polariton Bose–Einstein Condensates: From Cryogenic to Room Temperature

Abstract: Exciton-polaritons are composite bosonic quasiparticles that arise from the strong coupling of a cavity photon state and a semiconductor exciton state. These hybrid particles combine the key properties of their constituents: the photonic component gives them very light mass and the excitonic component gives them interactions. Due to their light mass, polaritons can undergo Bose-Einstein condensation at relatively high temperatures. This dissertation focuses on the thermalization, coherence, and critical fluctuations of polaritons in GaAs/AlGaAs microcavities. At cryogenic temperatures, I demonstrated spatially homogeneous polariton condensation in thermal equilibrium. I measured the coherent fraction across a wide density range, from the classical to the degenerate regime, revealing a universal power-law dependence of coherence on density. I also studied polaritons in the high photonic fraction limit, where their behavior approaches that of a laser. Near the condensation threshold, I observed critical fluctuations characterized by single-mode number fluctuations and competition between modes. Extending this work to room temperature, I report direct measurements of the polariton dispersion in GaAs/AlGaAs microcavities, confirming strong coupling and nonlinearity. These findings demonstrate the potential for high-quality, room-temperature polaritonic devices in GaAs-based materials.