  We visualized negative refraction of phonon polaritons, which occurs at the
interface between two natural crystals. The polaritons - hybrids of infrared
photons and lattice vibrations - form collimated rays that display negative
refraction when passing through a planar interface between the two hyperbolic
van der Waals materials: molybdenum oxide ($MoO_3$) and isotopically pure
hexagonal boron nitride ($h^{11}BN$). At a special frequency $\omega_0$, these
rays can circulate along closed diamond-shaped trajectories. We have shown that
polariton eigenmodes display regions of both positive and negative dispersion
interrupted by multiple gaps that result from polaritonic level repulsion and
strong coupling.
