Transient exciton-polariton dynamics in WSe ₂ by ultrafast near-field imaging

Van der Waals (vdW) materials offer an exciting platform for strong light-matter interaction enabled by their polaritonic modes and the associated deep subwavelength light confinement. Semiconductor vdW materials such as WSe ₂ are of particular interest for photonic and quantum integrated technologies because they sustain visible–near-infrared (VIS-NIR) exciton-polariton (EP) modes at room temperature. Here, we develop a unique spatiotemporal imaging technique at the femtosecond-nanometric scale and observe the EP dynamics in WSe ₂ waveguides. Our method, based on a novel ultrafast broadband intrapulse pump-probe near-field imaging, allows direct visualization of EP formation and propagation in WSe ₂ showing, at room temperature, ultraslow EP with a group velocity of v _g ~ 0.017c. Our imaging method paves the way for in situ ultrafast coherent control and extreme spatiotemporal imaging of condensed matter.