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The efficiency of phonon-mediated heat transport is limited by the intrinsicatomistic properties of materials, seemingly providing an upper limit to heattransfer in materials and across their interfaces. The typical speeds ofconductive transport, which are inherently limited by the chemical bonds andatomic masses, dictate how quickly heat will move in solids. Given thatphonon-polaritons, or coupled phonon-photon modes, can propagate at speedsapproaching 1 percent of the speed of light - orders of magnitude faster thantransport within a pure diffusive phonon conductor - we demonstrate thatvolume-confined, hyperbolic phonon-polariton(HPhP) modes supported by manybiaxial polar crystals can couple energy across solid-solid interfaces at anorder of magnitude higher rates than phonon-phonon conduction alone. Usingpump-probe thermoreflectance with a mid-infrared, tunable, probe pulse withsub-picosecond resolution, we demonstrate remote and spectrally selectiveexcitation of the HPhP modes in hexagonal boron nitride in response toradiative heating from a thermally emitting gold source. Our work demonstratesa new avenue for interfacial heat transfer based on broadband radiativecoupling from a hot spot in a gold film to hBN HPhPs, independent of the broadspectral mismatch between the pump(visible) and probe(mid-IR) pulses employed.This methodology can be used to bypass the intrinsically limiting phonon-phononconductive pathway, thus providing an alternative means of heat transfer acrossinterfaces. Further, our time-resolved measurements of the temperature changesof the HPhP modes in hBN show that through polaritonic coupling, a material cantransfer heat across and away from an interface at rates orders of magnitudefaster than diffusive phonon speeds intrinsic to the material, thusdemonstrating a pronounced thermal transport enhancement in hBN viaphonon-polariton coupling.

Ultrafast evanescent heat transfer across solid interfaces via hyperbolic phonon polaritons in hexagonal boron nitride