  The methodology for a heterodyned laser-induced transient thermal grating
technique for non-contact, non-destructive measurements of thermal transport in
opaque material is presented. Phase-controlled heterodyne detection allows us
to isolate pure phase or amplitude transient grating signal contributions by
varying the relative phase between reference and probe beams. The phase grating
signal includes components associated with both transient reflectivity and
surface displacement whereas the amplitude grating contribution is governed by
transient reflectivity alone. By analyzing the latter with the two-dimensional
thermal diffusion model, we extract the in-plane thermal diffusivity of the
sample. Measurements on a 5 {\mu}m thick single crystal PbTe film yielded
excellent agreement with the model over a range of grating periods from 1.6 to
2.8 {\mu}m. The measured thermal diffusivity of 1.3 \times 10-6 m2/s was found
to be slightly lower than the bulk value.
