  We investigate the non-equilibrium insulator-metal transition driven in a
SmNiO3 thin film by coherent optical excitation of the LaAlO3 substrate
lattice. By probing the transient optical properties over a broad frequency
range (100 - 800 cm^-1), we analyze both the time dependent metallic plasma and
the infrared optical phonon line shapes. We show that the light-induced
metallic phase in SmNiO3 has the same carrier density as the equilibrium
metallic phase. We also report that the LaAlO3 substrate acts as a transducer
only at the earlier time delays, as the vibrations are driven coherently. No
long-lived structural rearrangement takes place in the substrate. Finally, we
show that the transient insulator-metal transition occurs both below and above
the N\'eel temperature. We conclude that the supersonic melting of magnetic
order measured with ultrafast x-rays is not the driving force of the formation
of the metallic phase. We posit that the insulator metal transition may origin
from the rearrangement of ordered charges at the interface propagating into the
film.
