Probing anisotropic heat transport using time-domain thermoreflectance with offset laser spots

An analytic solution is derived for the time-domain thermoreflectance signal that occurs using non-concentric pump and probe beams on multilayer anisotropic materials. When in-plane heat transport is negligible, the experimental signal is the same as for the concentric case. However, for samples where in-plane heat diffusion distances are comparable to the spot size, the signal is sensitive to in-plane heat transport. This sensitivity to in-plane transport can be exploited to measure the in-plane thermal conductivity. Examples with experimental data are given for thin-film Al and the in-plane thermal conductivity of pyrolytic graphite.