Ultrafast evanescent heat transfer across solid interfaces via hyperbolic phonon-polariton modes in hexagonal boron nitride.

Thermal transport across solid-solid interfaces is vital for advanced electronic and photonic applications, yet conventional conduction pathways often restrict performance. In polar crystals, hybridized vibrational modes called phonon polaritons offer a promising avenue to overcome the limitations of intrinsic phonon heat conduction. Here our work demonstrates that volume-confined hyperbolic phonon polariton (HPhP) modes can transfer energy across solid-solid interfaces at rates far exceeding phonon-phonon conduction. Using pump-probe thermoreflectance with a mid-infrared, tunable probe pulse with subpicosecond resolution, we remotely and selectively observe HPhP modes in hexagonal boron nitride (hBN) via broadband radiative heating from a gold source. Our measurements ascertain that hot electrons impinging at the interface radiate directly into the HPhPs of hBN in the near field, bypassing the phonon-phonon transport pathway. Such polaritonic coupling enables thermal transport speeds in solids orders of magnitude faster than possible through diffusive phonon processes. We thereby showcase a pronounced thermal transport enhancement across the gold-hBN interface via phonon-polariton coupling, advancing the limits of interfacial heat transfer.