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Van der Waals (vdW) materials, weakly bound layered compounds, have receivedenormous interest as they offer a malleable playground for a wide range ofphysical properties in thermal, electronic and optical devices. In particular,owing to their inherent deep subwavelength light confinement, they support avariety of light-matter interactions phenomena such as plasmons, excitons andphonons conveyed as polaritonic modes. Specifically, semiconductor vdWmaterials such as WSe_2 are particularly attractive for photonic and quantumintegrated technologies since they sustain VIS-NIR exciton-polariton modes atroom temperature. In the quest to unravel the underlying physics of theseintriguing phenomena, advanced subdiffraction imaging techniques such as SNOMhas provided valuable insights on the nature of the EP coupling mechanism tothe waveguide modes sustained in vdW materials. While most of these worksfocused on the steady state of the EP, the spatio-temporal dynamics of EPformation, happening in the sub-picosecond regime, remains largely unexplored.Hence, a direct imaging at the femtosecond-nanoscale of the EP evolution iscritical to the understanding of these coupled light-matter states. Here wereport for the first time the ultrafast and deep-subwavelength imaging of EPformation and propagation in WSe2 waveguides. Our method, based on a novelultrafast pump-probe near-field imaging, allows to directly visualize the EPtime evolution at room temperature. More specifically, and in agreement withour time dependent model, we directly observe a significantly slow EP wavepacket group velocity of v_g~0.017c which is attributed to the bandgaprenormalization originating in the light coupling near the exciton transition.These findings suggest that vdW materials could be used for slow light withapplications in light storage for memory, enhanced optical nonlinearity,sensing and more.

Ultrafast near-field imaging of exciton-polariton dynamics in WSe_2 waveguides at room temperature