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Over the recent years, van der Waals (vdW) materials, a class of materials composed of weakly bound two-dimensional (2D), atomically thin, crystalline layers, have attracted great interest due to their ability to deeply confine light and therefore significantly enhance its interaction with matter. This interaction is embodied in coupled states between light and the polarization of the media, polaritons. The most studied type of polaritons, plasmon polariton, stems from the collective oscillations of conduction electrons. These, however, suffer great losses and therefore offer limited applications. Recently, among the several other types of polaritons supported by vdW materials, the exciton polariton (EP) has stimulated intense research efforts because it can sustain both strong light– matter interactions and long-distance propagation that is necessary for applications associated with energy harvesting or information manipulation and transfer. In this context, WSe2 is of particular interest for integrated applications since it supports EP modes in the Visible- Near Infrared (VIS-NIR) spectral region at room temperature due to its tightly bond excitonic state. In the quest to unravel the underlying physics, scanning near field optical microscope (SNOM) has provided valuable insights on the nature of the steady state EP modes sustained in vdW and in WSe2 in particular. However the dynamics of the EP formation, happening in the first few hundreds of femtoseconds subsequent to light absorption, remains largely unexplored. Here we employ a unique broadband ultrafast near-field pump-probe imaging method and observe for the first time, at femtosecond and nanometric spatiotemporal scale, the dynamics of the EP waves generation and propagation in WSe2 waveguides. Our observations suggest an important interplay between the waveguide EP mode and the tip-supported plasmon. Morever, we observe an intriguing ultrafast change in the EP waveguiding properties of the WSe2 waveguides happening in the first few hundreds of femtoseconds of the EP wave formation. Our method paves the way to in-situ ultrafast coherent control of EPs modes in vdW materials.
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