Direct Mode‐Resolved Measurement of Interfacial Phonon Transport by Acoustic Phonon Reflectometry

ABSTRACT As nanoscale devices continue to shrink in size and increase in complexity, their thermal performance becomes increasingly governed by interfacial phenomena. In particular, thermal boundary resistance (TBR) plays a critical role in controlling heat dissipation. In the conventional phonon Landauer formalism, TBR depends on the interfacial reflection coefficient for each phonon mode, a quantity for which direct experimental probes are limited. Here, we introduce a novel analysis, acoustic phonon reflectometry, based on picosecond acoustics that enables direct experimental determination of the mode‐resolved phonon reflection coefficient at semiconductor interfaces at a single frequency. Applying this approach to longitudinal acoustic phonons in aluminum nitride, we observe excellent agreement with predictions from the acoustic mismatch model across three distinct solid‐solid interfaces. This work establishes a new method for probing phonon transport at interfaces and provides critical insights into the microscopic mechanisms governing interfacial heat transfer at the nanoscale.