  Phonon polaritons (PPs) in van der Waals (vdW) materials can strongly enhance
light-matter interactions at mid-infrared frequencies, owing to their extreme
infrared field confinement and long lifetimes. PPs thus bear potential for
achieving vibrational strong coupling (VSC) with molecules. Although the onset
of VSC has recently been observed spectroscopically with PP nanoresonators, no
experiments so far have resolved VSC in real space and with propagating modes
in unstructured layers. Here, we demonstrate by real-space nanoimaging that VSC
can be achieved between propagating PPs in thin vdW crystals (specifically
h-BN) and molecular vibrations in adjacent thin molecular layers. To that end,
we performed near-field polariton interferometry, showing that VSC leads to the
formation of a propagating hybrid mode with a pronounced anti-crossing region
in its dispersion, in which propagation with negative group velocity is found.
Numerical calculations predict VSC for nanometer-thin molecular layers and PPs
in few-layer vdW materials, which could make propagating PPs a promising
platform for ultra-sensitive on-chip spectroscopy and strong coupling
experiments.
