DYNAMICS OF PHOTOTHERMAL SURFACE EXPANSION AND DIFFUSIVITY USING LASER-INDUCED HOLOGRAPHIC GRATINGS

We report ultrafast measurements of the dynamic thermal expansion of a surface and the temperature dependent surface thermal diffusivity using a two-color reflection transient grating technique. Studies were performed on p-type, n-type, and undoped GaAs(100) samples at several temperatures. By utilizing a 90 fs ultraviolet probe with visible excitation beams, the effects of interband saturation and carrier dynamics become negligible; thus lattice expansion due to heating and subsequent contraction caused by cooling provide the dominant influence on the probe. At room temperature a rise due to thermal expansion was observed, corresponding to a maximum net displacement of almost-equal-to 1 angstrom at 32 ps. The diffracted signal is composed of two components, thermal expansion of the surface and heat flow away from the surface, thus allowing a determination of the rate of expansion as well as the surface thermal diffusivity D(s). By varying the fringe spacing of the grating, this technique has the potential to separate the signal contributions to the expansion of the lattice in the perpendicular and parallel directions. In the data presented here a large fringe spacing was used, thus the dominant contribution to the rising edge of the signal is due to expansion perpendicular to the surface. Comparison of the results with a straightforward thermal model yields good agreement over a range of temperatures (20-300-degrees-K). Values for the surface thermal diffusivity of GaAs are measured and found to be in reasonable agreement with bulk values above 50-degrees-K. Below 50-degrees-K, D(S) was determined to be up to an order of magnitude slower than the bulk diffusivity due to increased phonon boundary scattering. The applicability and advantages of the transient grating technique for studying photothermal and photoacoustic phenomena are discussed.