Far-field imaging of plasmon polaritons in natural van der Waals material

Plasmon polaritons (PPs), hybrids of photons and collective electron oscillations, enable subwavelength confinement and enhanced light–matter interaction. Recently, the natural biaxial van der Waals (vdW) crystal molybdenum oxychloride (MoOCl₂) was shown to host low-loss PPs with lenticular and hyperbolic dispersion across the near-infrared to visible range, offering a promising platform for nanophotonic device applications. Here, we use far-field transient microscopy to image PPs in MoOCl₂ by tracking the interference arising from the interaction between the PPs and the resonant wide-field probe. The plasmonic fringes emerge only when the excitation is aligned with the metallic crystallographic axis. We conduct a systematic study of the PPs as a function of pump/probe energy, polarization, and flake thickness. Our results provide an accessible far-field approach for probing PPs in vdW materials within the visible range.