Direct observation of ultraslow hyperbolic polariton propagation with negative phase velocity

Time-domain interferometry and near-field scanning microscopy are used to investigate infrared phonon polaritons exhibiting hyperbolic dispersion. Negative phase velocity and group velocity as small as 0.002 c are confirmed. Polaritons with hyperbolic dispersion are key to many emerging photonic technologies, including subdiffraction imaging, sensing and spontaneous emission engineering 1 , 2 , 3 , 4 , 5 , 6 , 7 , 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 0.002 c 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.