Fresnel transmission coefficients for thermal phonons at solid interfaces

TL;DR

This paper reports the first direct measurements of Fresnel transmission coefficients for thermal phonons at a metal-semiconductor interface, revealing that interfaces preferentially transmit low-frequency phonons, which dominate heat transfer.

Contribution

It introduces a novel method combining ab-initio modeling and thermoreflectance to measure microscopic phonon transmission coefficients at interfaces.

Findings

Interfaces act as thermal phonon filters favoring low-frequency phonons.

Low-frequency phonons dominate heat transfer across the interface.

The method provides new insights into microscopic interfacial heat conduction.

Abstract

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.