  The efficiency of phonon-mediated heat transport is limited by the intrinsic
atomistic properties of materials, seemingly providing an upper limit to heat
transfer in materials and across their interfaces. The typical speeds of
conductive transport, which are inherently limited by the chemical bonds and
atomic masses, dictate how quickly heat will move in solids. Given that
phonon-polaritons, or coupled phonon-photon modes, can propagate at speeds
approaching 1 percent of the speed of light - orders of magnitude faster than
transport within a pure diffusive phonon conductor - we demonstrate that
volume-confined, hyperbolic phonon-polariton(HPhP) modes supported by many
biaxial polar crystals can couple energy across solid-solid interfaces at an
order of magnitude higher rates than phonon-phonon conduction alone. Using
pump-probe thermoreflectance with a mid-infrared, tunable, probe pulse with
sub-picosecond resolution, we demonstrate remote and spectrally selective
excitation of the HPhP modes in hexagonal boron nitride in response to
radiative heating from a thermally emitting gold source. Our work demonstrates
a new avenue for interfacial heat transfer based on broadband radiative
coupling from a hot spot in a gold film to hBN HPhPs, independent of the broad
spectral mismatch between the pump(visible) and probe(mid-IR) pulses employed.
This methodology can be used to bypass the intrinsically limiting phonon-phonon
conductive pathway, thus providing an alternative means of heat transfer across
interfaces. Further, our time-resolved measurements of the temperature changes
of the HPhP modes in hBN show that through polaritonic coupling, a material can
transfer heat across and away from an interface at rates orders of magnitude
faster than diffusive phonon speeds intrinsic to the material, thus
demonstrating a pronounced thermal transport enhancement in hBN via
phonon-polariton coupling.
