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Materials lacking in-plane symmetry are ubiquitous in a wide range ofapplications such as electronics, thermoelectrics, and high-temperaturesuperconductors, in all of which the thermal properties of the materials play acritical part. However, very few experimental techniques can be used to measurein-plane anisotropic thermal conductivity. A beam-offset method based ontime-domain thermoreflectance (TDTR) was previously proposed to measurein-plane anisotropic thermal conductivity. However, a detailed analysis of thebeam-offset method is still lacking. Our analysis shows that uncertainties canbe large if the laser spot size or the modulation frequency is not properlychosen. Here we propose an alternative approach based on TDTR to measurein-plane anisotropic thermal conductivity using a highly elliptical pump(heating) beam. The highly elliptical pump beam induces a quasi-one-dimensionaltemperature profile on the sample surface that has a fast decay along the shortaxis of the pump beam. The detected TDTR signal is exclusively sensitive to thein-plane thermal conductivity along the short axis of the elliptical beam. Byconducting TDTR measurements as a function of delay time with the rotation ofthe elliptical pump beam to different orientations, the in-plane thermalconductivity tensor of the sample can be determined. In this work, we firstconduct detailed signal sensitivity analyses for both techniques and provideguidelines in determining the optimal experimental conditions. We then comparethe two techniques under their optimal experimental conditions by measuring thein-plane thermal conductivity tensor of a ZnO [11-20] sample. The accuracy andlimitations of both methods are discussed.

review of scientific instruments online/review of scientific instruments

A new elliptical-beam method based on time-domain thermoreflectance  (TDTR) to measure the in-plane anisotropic thermal conductivity and its  comparison with the beam-offset method