Multidimensional quasiballistic thermal transport in transient grating spectroscopy

TL;DR

This paper develops a parameter-free analytical method to analyze two-dimensional quasiballistic thermal transport in transient grating spectroscopy, enabling extraction of phonon mean free paths in opaque bulk materials.

Contribution

It introduces a Green's function approach for 2D quasiballistic heat conduction analysis, extending MFP spectroscopy to opaque bulk materials without fitting parameters.

Findings

Analytic Green's function for 2D quasiballistic transport

Method to extract phonon MFPs from measurements

Extension of MFP spectroscopy to opaque bulk materials

Abstract

Transient grating spectroscopy has emerged as a useful technique to study thermal phonon transport because of its ability to perform thermal measurements over length scales comparable to phonon mean free paths (MFPs). While several prior works have performed theoretical studies of quasiballistic heat conduction in transient grating, the analysis methods are either restricted to one spatial dimension or require phenomenological fitting parameters. Here, we analyze quasiballistic transport in a two-dimensional transient grating experiment, in which heat conduction can occur both in- and cross-plane, using an analytic Green's function of the Boltzmann equation we recently reported that is free of fitting parameters. We demonstrate a method by which phonon MFPs can be extracted from these measurements, thereby extending the MFP spectroscopy technique using transient grating to opaque bulk materials.