Quasiballistic heat transfer studied using the frequency-dependent Boltzmann transport equation

Quasiballistic heat transfer occurs when there is a temperature gradient over length scales comparable to phonon mean free paths (MFPs). This regime has been of interest recently because observation of quasiballistic transport can lead to useful information about phonon MFPs, knowledge of which is essential for engineering nanoscale thermal effects. Here, we use the Boltzmann transport equation (BTE) to understand how observations of quasiballistic transport can yield information about MFPs. We solve the transient, one-dimensional, frequency-dependent BTE for a double-layer structure of a metal film on a substrate, the same geometry that is used in transient thermoreflectance experiments, using a frequency-dependent interface condition. Our results indicate that phonons with MFPs longer than the thermal penetration depth do not contribute to the measured thermal conductivity, providing a means to probe the MFP distribution. We discuss discrepancies between our simulation and experimental observations which offer opportunities for future investigation.