  BaNi$_{2}$As$_{2}$ is a non-magnetic analogue of BaFe$_{2}$As$_{2}$, the
parent compound of a prototype ferro-pnictide high-temperature superconductor.
Recent diffraction studies on BaNi$_{2}$As$_{2}$ demonstrate the existence of
two types of periodic lattice distortions above and below the tetragonal to
triclinic phase transition, suggesting charge-density-wave (CDW) order to
compete with superconductivity. We apply time-resolved optical spectroscopy and
demonstrate the existence of collective CDW amplitude modes. The smooth
evolution of these modes through the structural phase transition implies the
CDW order in the triclinic phase smoothly evolves from the unidirectional CDW
in the tetragonal phase and suggests that the CDW order drives the structural
phase transition.
