  Time-domain thermoreflectance (TDTR) is a well-established pump/probe method
for measuring thermal conductivity and interface conductance of multilayers.
Interpreting signals in a TDTR experiment requires a thermal model.In standard
front/front TDTR experiments, both pump and probe beams typically irradiate the
surface of a multilayer. As a result, existing thermal models for interpreting
thermoreflectance experiments assume the pump and probe beams both interact
with the surface layer. Here, we present a frequency-domain solution to the
heat-diffusion equation of a multilayer in response to nonhomogenous laser
heating. This model allows analysis of experiments where the pump and probe
beams irradiate opposite sides of a multilayer. We call such a geometry a
front/back experiment to differentiate such experiments from standard TDTR
experiments. As an example, we consider a 60nm amorphous Si film. We consider
how signals differ in a front/front vs. front/back geometry and compare thermal
model predictions to experimental data.
