  Anisotropic thermal properties are of both fundamental and practical
interests, but remain challenging to characterize using conventional methods.
In this work, a new metrology based on asymmetric beam time-domain
thermoreflectance (AB-TDTR) is developed to measure three-dimensional
anisotropic thermal transport by extending the conventional TDTR technique.
Using an elliptical laser beam with controlled elliptical ratio and spot size,
the experimental signals can be exploited to be dominantly sensitive to measure
thermal conductivity along the cross-plane or any specific in-plane directions.
An analytic solution for a multi-layer system is derived for the AB-TDTR signal
in response to the periodical pulse, elliptical laser beam, and heating
geometry, to extract the anisotropic thermal conductivity from experimental
measurement. Examples with experimental data are given for various materials
with in-plane thermal conductivity from 5 W/mK to 2000W/mK, including isotropic
materials (silicon, boron phosphide, boron nitride), transversely isotropic
materials (graphite, quartz, sapphire) and transversely anisotropic materials
(black phosphorus). Furthermore, a detailed sensitivity analysis is conducted
to guide the optimal setting of experimental configurations for different
materials. The developed AB-TDTR metrology provides a new approach to
accurately measure anisotropic thermal phenomena for rational materials design
and thermal applications.
