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.