  We present a combined ultrafast optical pump-probe and ultrafast x-ray
diffraction measurement of the CDW dynamics in SmTe$_3$ at 300 K. The ultrafast
x-ray diffraction measurements, taken at the Linac Coherent Light Source reveal
a $\sim 1.55$ THz mode that becomes overdamped with increasing fluence. We
identify this oscillation with the lattice component of the amplitude mode.
Furthermore, these data allow for a more clear identification of the
frequencies present in the optical pump-probe data. In both, reflectivity and
diffraction, we observe a crossover of the response from linear (for small
displacements) to quadratic in the amplitude of the order parameter
displacement. Finally, a time-dependent Ginzburg-Landau model captures the
essential features of the experimental observations.
