Sub-Picosecond Optical Dynamics in Perovskite NdNiO₃ Thin Films via Broadband Pump-Probe Ellipsometry

The tunable physical properties of strongly correlated oxides make them interesting for emerging device applications in opto-electronics as well as serve as prototypical systems for condensed matter physics research. Here, we report room-temperature, broadband time-resolved spectroscopic ellipsometry (TRSE) measurements of ultrafast optical dynamics in perovskite nickelate NdNiO₃ (NNO) thin films following femtosecond photoexcitation at three pump wavelengths (266, 400, and 800 nm). By tracking the transient complex dielectric function with sub-picosecond temporal resolution, we disentangle electronic and lattice contributions to the photoinduced response. Within the first picosecond, photoexcitation drives a rapid change in the dielectric function of NNO, attributed to the generation of a nonequilibrium electron population, accompanied by a transient reduction in Drude spectral weight and enhanced carrier scattering consistent with a crossover toward a bad-metal state. Short-lived coherent optical phonon oscillations are observed in the photon energy domain during the first hundreds of fs. At longer time scales (10-100 ps), coherent acoustic strain generates Brillouin oscillations in the transient ellipsometry signal, consistent with time-domain Brillouin scattering from the propagating strain pulse generated at the NNO layer after its transmission into the substrate.