  Polaritons, coupled excitations of photons and dipolar matter excitations,
can propagate along anisotropic metasurfaces with either hyperbolic or
elliptical dispersion. At the transition from hyperbolic to elliptical
dispersion (corresponding to a topological transition), various intriguing
phenomena are found, such as an enhancement of the photonic density of states,
polariton canalization and hyperlensing. Here we investigate theoretically and
experimentally the topological transition and the polaritonic coupling of
deeply subwavelength elements in a uniaxial infrared-phononic metasurface, a
grating of hexagonal boron nitride (hBN) nanoribbons. By hyperspectral infrared
nanoimaging, we observe, for the first time, a synthetic transverse optical
phonon resonance (that is, the strong collective near-field coupling of the
nanoribbons) in the middle of the hBN Reststrahlen band, yielding a topological
transition from hyperbolic to elliptical dispersion. We further visualize and
characterize the spatial evolution of a deeply subwavelength canalization mode
near the transition frequency, which is a collimated polariton that is the
basis for hyperlensing and diffraction-less propagation. Our results provide
fundamental insights into the role of polaritonic near-field coupling in
metasurfaces for creating topological transitions and polariton canalization.
