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It is challenging to characterize thermal conductivity of materials withstrong anisotropy. In this work, we extend the time-domain thermoreflectance(TDTR) method with a variable spot size approach to simultaneously measure thein-plane (Kr) and the through-plane (Kz) thermal conductivity of materials withstrong anisotropy. We first determine Kz from the measurement using a largerspot size, when the heat flow is mainly one-dimensional along the through-planedirection, and the measured signals are sensitive to only Kz. We then extractthe in-plane thermal conductivity Kr from a second measurement using the samemodulation frequency but with a smaller spot size, when the heat flow becomesthree-dimensional, and the signal is sensitive to both Kr and Kz. By choosingthe same modulation frequency for the two sets of measurements, we can avoidpotential artifacts introduced by the frequency-dependent Kz, which we havefound to be non-negligible, especially for some two-dimensional layeredmaterials like MoS2. After careful evaluation of the sensitivity of a series ofhypothetical samples, we provided a guideline on choosing the most appropriatelaser spot size and modulation frequency that yield the smallest uncertainty,and established a criterion for the range of thermal conductivities that can bemeasured reliably using our proposed variable spot size TDTR approach. We havedemonstrated this variable spot size TDTR approach on samples with a wide rangeof in-plane thermal conductivity, including fused silica, rutile titania (TiO2[001]), zinc oxide (ZnO [0001]), molybdenum disulfide (MoS2), hexagonal boronnitride (h-BN), and highly ordered pyrolytic graphite (HOPG).

Time-domain thermoreflectance (TDTR) measurements of anisotropic thermal conductivity using a variable spot size approach