  Materials lacking in-plane symmetry are ubiquitous in a wide range of
applications such as electronics, thermoelectrics, and high-temperature
superconductors, in all of which the thermal properties of the materials play a
critical part. However, very few experimental techniques can be used to measure
in-plane anisotropic thermal conductivity. A beam-offset method based on
time-domain thermoreflectance (TDTR) was previously proposed to measure
in-plane anisotropic thermal conductivity. However, a detailed analysis of the
beam-offset method is still lacking. Our analysis shows that uncertainties can
be large if the laser spot size or the modulation frequency is not properly
chosen. Here we propose an alternative approach based on TDTR to measure
in-plane anisotropic thermal conductivity using a highly elliptical pump
(heating) beam. The highly elliptical pump beam induces a quasi-one-dimensional
temperature profile on the sample surface that has a fast decay along the short
axis of the pump beam. The detected TDTR signal is exclusively sensitive to the
in-plane thermal conductivity along the short axis of the elliptical beam. By
conducting TDTR measurements as a function of delay time with the rotation of
the elliptical pump beam to different orientations, the in-plane thermal
conductivity tensor of the sample can be determined. In this work, we first
conduct detailed signal sensitivity analyses for both techniques and provide
guidelines in determining the optimal experimental conditions. We then compare
the two techniques under their optimal experimental conditions by measuring the
in-plane thermal conductivity tensor of a ZnO [11-20] sample. The accuracy and
limitations of both methods are discussed.
