Limits to Fourier theory in high thermal conductivity single crystals

We report the results of time-domain thermoreflectance (TDTR) experiments that examine the ability of Fourier theory to predict the thermal response in single crystals when heater dimensions are small. We performed TDTR measurements on Al-coated diamond, 6H-SiC, GaP, Ge, MgO, GaAs, and GaSb single crystals with a wide range of laser spot size radii, 0.7 μm < w0 < 12 μm. When the laser spot-size is large, w0 ≈ 12 μm, TDTR data for all crystals are in agreement with predictions of Fourier theory with bulk thermal conductivity values. When the laser spot-size is small, w0 < 2 μm, there are significant differences between the predictions of Fourier theory and TDTR data for all crystals except MgO.