Spatiotemporal visualization of long-range anisotropic plasmon polaritons in hyperbolic MoOCl₂

Manipulating light at the nanoscale with minimal loss remains a central challenge for nanophotonic technologies that can be tackled by using the direction-dependent polariton modes supported by anisotropic materials. Although best known for their highly confined polaritons, hyperbolic materials can also host long-range directional polaritons, whose direct observation has remained challenging as it requires experimental techniques that combine nanometer and femtosecond spatial and temporal resolution, respectively. Here, we use time-resolved photoemission electron microscopy for direct nanoscale visualization of long-range anisotropic plasmon polariton (LRAPP) dynamics on a flake of the van der Waals hyperbolic material molybdenum oxydichloride. We directly image plasmon polaritons with propagation lengths larger than 10 μm, exhibiting an approximately three times longer propagation length and intrinsically lower optical loss than short-range polaritons previously reported on the same material. By tracking the spatiotemporal evolution of LRAPPs, we determine their phase and group velocities at the nanoscale and directly observe their reflections at flake edges. These results establish molybdenum oxydichloride as a versatile platform for integrated nanophotonics, supporting both low-loss directional transport and deeply subwavelength field confinement within a single natural material in the visible spectral range.