The methodology for a heterodyned laser-induced transient thermal gratingtechnique for non-contact, non-destructive measurements of thermal transport inopaque material is presented. Phase-controlled heterodyne detection allows usto isolate pure phase or amplitude transient grating signal contributions byvarying the relative phase between reference and probe beams. The phase gratingsignal includes components associated with both transient reflectivity andsurface displacement whereas the amplitude grating contribution is governed bytransient reflectivity alone. By analyzing the latter with the two-dimensionalthermal diffusion model, we extract the in-plane thermal diffusivity of thesample. Measurements on a 5 {\mu}m thick single crystal PbTe film yieldedexcellent agreement with the model over a range of grating periods from 1.6 to2.8 {\mu}m. The measured thermal diffusivity of 1.3 \times 10-6 m2/s was foundto be slightly lower than the bulk value.
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