  BaNi$_{2}$As$_{2}$ is a non-magnetic analogue of BaFe$_{2}$As$_{2}$, the
parent compound of a prototype pnictide high-temperature superconductor,
displaying superconductivity already at ambient pressure. Recent diffraction
studies demonstrated the existence of two types of periodic lattice distortions
above and below the triclinic phase transition, suggesting the existence of an
unconventional charge-density-wave (CDW) order. The suppression of CDW order
upon doping results in a sixfold increase in the superconducting transition
temperature and enhanced nematic fluctuations, suggesting CDW is competing with
superconductivity. Here, we apply time-resolved optical spectroscopy to
investigate collective dynamics in BaNi$_{2}$As$_{2}$. We demonstrate the
existence of several CDW amplitude modes. Their smooth evolution through the
structural phase transition implies the commensurate CDW order in the triclinic
phase evolves from the high-temperature unidirectional incommensurate CDW, and
may indeed trigger the structural phase transition. Excitation density
dependence reveals exceptional resilience of CDW against perturbation, implying
an unconventional origin of the underlying electronic instability.
