Asymmetric-beam steady-state thermoreflectance for three-dimensional anisotropic thermal conductivity measurements

The accurate measurement of anisotropic thermal conductivity is essential for understanding heat transport and enabling practical applications, such as thermal management, yet reliable characterization remains challenging, particularly for in-plane anisotropic materials with low thermal conductivity. Here, we present an asymmetric-beam steady-state thermoreflectance (AB-SSTR) technique that integrates an elliptical laser beam into a conventional steady-state thermoreflectance framework. By controlling the beam aspect ratio and spot size, the elliptical geometry modifies the thermoreflectance sensitivity to thermal conductivity along the three principal axes. Thermal conductivities are extracted by combining two measurements with orthogonally exchanged major and minor beam axes and analyzing the resulting response using a steady-state three-dimensional heat conduction model. We validate the approach using various samples, and the measured values are in good agreement with the literature. These results establish AB-SSTR as a robust, non-contact method for probing anisotropic heat transport in diverse materials.