Effects of interactions on correlation, thermalization, and transport of exciton-polaritons

Abstract

Light-matter interactions are fundamental processes that allow us not only to interrogate material properties but also to coherently control material phases that cannot be reached otherwise. Matter-matter interactions, on the other hand, result in strong correlations and emergent behavior that cannot be explained in terms of single-particle physics. Exciton-polaritons (hereafter "polaritons") are hybrid quasiparticles in a semiconductor quantum-well microcavity that exhibit both light-matter and matter-matter interactions. Polaritons have the effective mass inherited from the ultralight cavity photon mass, which sets polariton transport phenomena to be photon-like and allows macroscopic quantum phenomena such as Bose-Einstein condensation and superfluidity up to room temperature. Meanwhile, the effect of photon dressing only reduces the exciton-exciton interaction strength by the Hopfield coefficient, which sets the polariton-polariton interaction strength to be exciton-like.

Along with the narrow linewidth protected from inhomogeneously broadening, polaritons are an excellent platform to study interaction-induced physics and nonlinear device applications such as ultralow-power optical switches. In this thesis, we investigated the effects of light-matter and matter-matter interactions on various aspects of polaritons. In the first part, we first measured the polariton-polariton interaction strength by tracking the energy blueshift as a function of polariton density. This was enabled by separating and trapping polaritons away from a pumped region, where the measurement of polariton interactions can be obscured by much heavier particles such as a dark exciton reservoir. We provided possible mechanisms that explain the observed anomalously large blueshifts. In the second part, we addressed a long-standing debate on whether polaritons can reach thermal equilibrium.

We used a long-lifetime microcavity structure to achieve Bose-Einstein distributions of polaritons, which was the first demonstration of polaritons in equilibrium. This allowed us to measure equilibrium properties, such as temperature and chemical potential, and to map out the phase diagram of Bose-Einstein condensation in a quasi-two-dimensional system. We further investigated how all-optical trapping and polariton interactions enhance relaxation and thermalization processes. In particular, we found that a significant redistribution of polaritons occurs through the reduced density of states and polariton interactions. In the third part, we studied trapped eigenstates and interference patterns of polariton condensates in various trapping and pump geometries. Competition between eigenstates and selection of one of them have been well explained by the overlap of real-space, monientum-space, and energy distributions between the pump and the eigenstate.

A mismatch between the pump-induced potential profile and the polariton source profile was a key factor in determining the distribution of transported polaritons. In the last part, we extended the polariton physics to study topological and cooperative effects in open quantum systems. We demonstrated bulk Fermi arcs by connecting two exceptional points arising from the engineered non-Hermitian properties of a photonic crystal. In addition, we theoretically showed that a cascaded-cavity system can outperform a single-cavity system in terms of the single-photon indistinguishability and efficiency, which works even with bad quantum emitters and practical cavity quality factors. Our work provides invaluable insights into the fundamental light-matter and matter-matter interactions, as well as many-body physics of condensed matter and photonic systems.