  Spatiotemporal dynamics of electron-hole pairs locally excited in a
silicon-on-insulator structure by indirect interband absorption are studied by
measuring differential transmission caused by free-carrier absorption of a
probe pulse tuned below the bandgap, with 200-fs temporal and 3-micrometer
spatial resolution. From sample temperatures of 250 K to 400 K, the ambipolar
diffusivity decreases, and is similar to reported values of bulk silicon.
Cooling the sample from 250 K to 90 K, a decrease of ambipolar diffusivity is
observed, indicating important influences of defects and residual stress on
carrier diffusion. No detectable density dependence of ambipolar diffusivity is
observed.
