Dynamics of photothermal surface expansion and diffusivity using laser-induced holographic gratings

The transient thermal expansion of a GaAs surface was measured directly using a two color reflection transient grating technique. Isolation of the expansion signal requires the selection of wavelengths which pump and probe different regions of the absorption bands in the sample. The surface thermal diffusivity was also determined to be equivalent to the bulk value above 50 K. Below 50 K, D_s was determined to be up to an order of magnitude slower than the bulk diffusivity due to increased phonon boundary scattering. It is demonstrated that the expansion signal perpendicular to the surface can be modeled with moderate success using a straightforward one-dimensional analytic theory. The work reported is restricted to expansion perpendicular to the surface, validating the applicability of this technique for measuring the perpendicular expansion signal and surface diffusivity. However, the true power of the reflection transient grating technique lies in its ability to measure phenomena in and out of the plane of the surface simultaneously by varying the fringe spacing of the grating