  Interfaces play an essential role in phonon-mediated heat conduction in
solids, impacting applications ranging from thermoelectric waste heat recovery
to heat dissipation in electronics. From a microscopic perspective, interfacial
phonon transport is described by transmission and reflection coefficients,
analogous to the well-known Fresnel coefficients for light. However, these
coefficients have never been directly measured, and thermal transport processes
at interfaces remain poorly understood despite considerable effort. Here, we
report the first measurements of the Fresnel transmission coefficients for
thermal phonons at a metal-semiconductor interface using ab-initio phonon
transport modeling and a thermal characterization technique, time-domain
thermoreflectance. Our measurements show that interfaces act as thermal phonon
filters that transmit primarily low frequency phonons, leading to these phonons
being the dominant energy carriers across the interface despite the larger
density of states of high frequency phonons. Our work realizes the
long-standing goal of directly measuring thermal phonon transmission
coefficients and demonstrates a general route to study microscopic processes
governing interfacial heat conduction.
