  Selective optical excitation of a substrate lattice can drive phase changes
across hetero-interfaces. This phenomenon is a non-equilibrium analogue of
static strain control in heterostructures and may lead to new applications in
optically controlled phase change devices. Here, we make use of time-resolved
non-resonant and resonant x-ray diffraction to clarify the underlying physics,
and to separate different microscopic degrees of freedom in space and time. We
measure the dynamics of the lattice and that of the charge disproportionation
in NdNiO3, when an insulator-metal transition is driven by coherent lattice
distortions in the LaAlO3 substrate. We find that charge redistribution
propagates at supersonic speeds from the interface into the NdNiO3 film,
followed by a sonic lattice wave. When combined with measurements of magnetic
disordering and of the metal-insulator transition, these results establish a
hierarchy of events for ultrafast control at complex oxide hetero-interfaces.
