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 µm thick single crystal PbTe film yielded excellent agreement with the model over a range of grating periods from 1.6 to 2.8 µm. The measured thermal diffusivity of 1.3 × 10 --6 m 2 /s was found to be slightly lower than the bulk value.

A general technique has been demonstrated at microwave and submillimeter-wave frequencies for photoconductive sampling in the frequency domain using photomixers and continuous-wave laser diodes. A microwave version in which two photomixers were coupled by a transmission line was developed to quantitatively test the concept from 0.05 to 26.5 GHz. A quasioptical version using antenna-coupled photomixers was demonstrated from 25 GHz to 2 THz. Such a system can outperform systems based on time-domain photoconductive sampling in frequency resolution, spectral brightness, system size, and cost.

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We report the through-thickness lattice thermal conductivity Λl of (PbTe)1−x/(PbSe)x nanodot superlattices (NDSLs) over a wide range of periods 5 nm≤h≤50 nm, compositions 0.15≤x≤0.25, growth temperatures 550 K≤Tg≤620 K, and growth rates 1 μm h−1≤R≤4 μm h−1. All of our measurements approach Λl of bulk homogenous PbTe1−xSex alloys with the same average composition. For 5 nm≤h≤50 nm, Λl is independent of h; a result we attribute to short mean-free paths of phonons in PbTe and small acoustic impedance mismatch between PbTe/PbSe. We alloyed the PbTe layers of four NDSLs with SnTe up to a mole fraction y=18%; Λl is reduced by <25%.