Transient exciton-polariton dynamics in WSe2 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 WSe2 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 WSe2 waveguides. Our method, based on a novel ultrafast broadband intrapulse pump-probe near-field imaging, allows direct visualization of EP formation and propagation in WSe2 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.