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Interfaces play an essential role in phonon-mediated heat conduction insolids, impacting applications ranging from thermoelectric waste heat recoveryto heat dissipation in electronics. From a microscopic perspective, interfacialphonon transport is described by transmission and reflection coefficients,analogous to the well-known Fresnel coefficients for light. However, thesecoefficients have never been directly measured, and thermal transport processesat interfaces remain poorly understood despite considerable effort. Here, wereport the first measurements of the Fresnel transmission coefficients forthermal phonons at a metal-semiconductor interface using ab-initio phonontransport modeling and a thermal characterization technique, time-domainthermoreflectance. Our measurements show that interfaces act as thermal phononfilters that transmit primarily low frequency phonons, leading to these phononsbeing the dominant energy carriers across the interface despite the largerdensity of states of high frequency phonons. Our work realizes thelong-standing goal of directly measuring thermal phonon transmissioncoefficients and demonstrates a general route to study microscopic processesgoverning interfacial heat conduction.

Fresnel transmission coefficients for thermal phonons at solid interfaces