  We present a combined study with conventional far-infrared and time-domain
terahertz ellipsometry of the temperature dependent optical response of SrTiO3
thin films (85 and 8.5 nm) that are grown by pulsed-laser deposition on LSAT
substrates. We demonstrate that terahertz ellipsometry is very sensitive to the
optical response of these thin films, in particular, to the soft mode of
SrTiO3. We show that for the 85 nm film the eigenfrequency of the soft mode is
strongly reduced by annealing at 1200 C, whereas for the 8.5 nm film it is
hardy affected. For the latter, after annealing the mode remains at 125 cm-1 at
300 K and exhibits only a weak softening to about 90 cm-1 at 10 K. This
suggests that this ultrathin film undergoes hardly any relaxation of the
compressive strain due to the LSAT substrate.
