  Strong coupling between molecular vibrations and microcavity modes has been
demonstrated to modify physical and chemical properties of the molecular
material. Here, we study the much less explored coupling between lattice
vibrations (phonons) and microcavity modes. Embedding thin layers of hexagonal
boron nitride (hBN) into classical microcavities, we demonstrate the evolution
from weak to ultrastrong phonon-photon coupling when the hBN thickness is
increased from a few nanometers to a fully filled cavity. Remarkably, strong
coupling is achieved for hBN layers as thin as 10 nm. Further, the ultrastrong
coupling in fully filled cavities yields a cavity polariton dispersion matching
that of phonon polaritons in bulk hBN, highlighting that the maximum
light-matter coupling in microcavities is limited to the coupling strength
between photons and the bulk material. The tunable cavity phonon polaritons
could become a versatile platform for studying how the coupling strength
between photons and phonons may modify the properties of polar crystals.
