Ultrafast imaging of coherent polariton propagation and interactions

Semiconductor excitations can hybridize with cavity photons to form exciton-polaritons (EPs) with remarkable properties, including light-like energy flow combined with matter-like interactions. To fully harness these properties, EPs must maintain long-range coherence despite exciton-mediated interactions with lattice phonons. To address this open question, we develop a nonlinear momentum-resolved optical approach that images EPs in real space on femtosecond scales. We directly visualize EP wavepacket propagation in two-dimensional halide perovskite microcavities, allowing precise quantification of EP interactions with lattice phonons and reservoir exciton states throughout their lifetimes. We show that EP-lattice interactions strongly renormalize EP group velocities at high exciton content without sacrificing long-range coherence, enabling ballistic propagation to be maintained for up to half-exciton EPs. These results concur with our quantum simulations of dynamic disorder shielding through light-matter hybridization. We extend our measurements to molecular polaritons and self-hybridized EPs in two-dimensional materials, testifying to the remarkable generalizability of our approach.