  A recently developed enhanced Fourier law is applied to the problem of
extracting thermal properties of materials from frequency-domain
thermoreflectance (FDTR) experiments. The heat transfer model comprises
contributions from two phonon channels; one a high-heat-capacity diffuse
channel consisting of phonons of mean free path (MFP) less than a threshold
value, and the other a low-heat-capacity channel consisting of phonons with MFP
higher than this value that travel quasi-ballistically over length scales of
interest. The diffuse channel is treated using the Fourier law, while the
quasi-ballistic channel is analyzed using a second-order spherical harmonic
expansion of the phonon distribution function. A recent analysis of FDTR
experimental data suggested the use of FDTR in deriving large portions of the
MFP accumulation function; however, it is shown here that the data can
adequately be explained using our minimum-parameter model, thus highlighting an
important limitation of FDTR experiments in exploring the accumulation function
of bulk matter.
