Direct visualization of visible-light hyperbolic plasmon polaritons in real space and time

Abstract: Hyperbolic materials support exotic polaritons with hyperbolic dispersion that enable subdiffraction focusing and enhanced light-matter interactions. Visible-frequency hyperbolic plasmon polaritons (HPPs) offer significant advantages over hyperbolic phonon polaritons, which operate in the infrared frequency range - namely lower losses and greater technological relevance. However, these HPPs remained experimentally inaccessible until the recent identification of molybdenum(IV) oxychloride (MoOCl$_2$). Here we achieve the first direct real-space and real-time visualization of hyperbolic plasmon polaritons in natural materials using time-resolved photoemission electron microscopy with femtosecond time resolution and nanometer spatial resolution. Our direct imaging enables measurement of HPP propagation velocities and lengths, real-time observation of plasmon-material edge interactions, experimental validation of hyperbolic dispersion through polarization-dependent experiments, and direct visualization of hyperbolic focusing phenomena. This spatiotemporal visualization validates theoretical predictions while establishing an experimental foundation for exploiting these unusual light-matter states in fundamental studies of hyperbolic media and nanophotonics.