  We use ultrafast x-ray pulses to characterize the lattice response of SrTiO3
when driven by strong terahertz (THz) fields. We observe transient changes in
the diffraction intensity with a delayed onset with respect to the driving
field. Fourier analysis reveals two frequency components corresponding to the
two lowest energy zone-center optical modes in SrTiO3. The lower frequency mode
exhibits clear softening as the temperature is decreased while the higher
frequency mode shows slight temperature dependence.
