  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 the microscopic perspective,
interfacial phonon transport is described by transmission coefficients that
link vibrational modes in the materials composing the interface. However,
direct experimental determination of these coefficients is challenging because
most experiments provide a mode-averaged interface conductance that obscures
the microscopic detail. Here, we report a metrology to extract thermal phonon
transmission coefficients at solid interfaces using ab-initio phonon transport
modeling and a thermal characterization technique, time-domain
thermoreflectance. In combination with transmission electron microscopy
characterization of the interface, our approach allows us to link the atomic
structure of an interface to the spectral content of the heat crossing it. Our
work provides a useful perspective on the microscopic processes governing
interfacial heat conduction.
