Direct observation of ultraslow hyperbolic polariton propagation with negative phase velocity

Polaritons with hyperbolic dispersion are key to many emerging photonic technologies, including subdiffraction imaging, sensing and spontaneous emission engineering(1-8). Fundamental to their effective application are the lifetimes of the polaritons, as well as their phase and group velocities(7,9). Here, we combine time-domain interferometry(10) and scattering-type near-field microscopy(11) to visualize the propagation of hyperbolic polaritons in space and time, allowing the first direct measurement of all these quantities. In particular, we study infrared phonon polaritons in a thin hexagonal boron nitride(8,12,13) waveguide exhibiting hyperbolic dispersion and deep subwavelength-scale field confinement. Our results reveal-in a natural material-negative phase velocity paired with a remarkably slow group velocity of 0.002c and lifetimes in the picosecond range. While these findings show the polariton's potential for mediating strong light-matter interactions and negative refraction, our imaging technique paves the way to explicit nanoimaging of polariton propagation characteristics in other two-dimensional materials, metamaterials and waveguides.