Microscopic theory of polariton group velocity renormalization

Cavity exciton-polaritons exhibit ballistic transport and can achieve 100 μm in one picosecond. This ballistic transport significantly enhances mobility compared to that of bare excitons, which often move diffusively and become the bottleneck for energy conversion and transfer devices. Despite being robustly reproduced in experiments and simulations, there is no microscopic theory available for describing the group velocity v_g of polariton transport and its renormalization. In this work, we derive an analytic expression for v_g renormalization. The theory suggests the v_g renormalization is caused by phonon-mediated transitions between the lower polariton (LP) states and the dark states. The theory predicts that the renormalization magnitude depends on both exciton-phonon coupling strength and temperature, which are in quantitative agreement with numerical quantum dynamics simulations. Our results provide theoretical insights and a predictive analytical theory for understanding cavity-enhanced exciton-polariton transport.