Photoinduced insulator-to-metal transition and coherent acoustic phonon propagation in LaCoO3 thin films explored by femtosecond pump-probe ellipsometry

We have studied ultrafast dynamics of thin films of ${\mathrm{LaCoO}}₃$ and ${\mathrm{La}}_0.5{\mathrm{Sr}}_0.5{\mathrm{CoO}}₃$ with femtosecond pump-probe ellipsometry in the energy range of 1.6--3.4 eV. We have observed a large pump-induced transfer of spectral weight in ${\mathrm{LaCoO}}₃$ that corresponds to an insulator-to-metal transition. The photoinduced metallic state initially relaxes via a fast process with a decay constant of about 200 fs. Both ${\mathrm{LaCoO}}₃$ and ${\mathrm{La}}_0.5{\mathrm{Sr}}_0.5{\mathrm{CoO}}₃$ exhibit a significant secondary peak in the 1--30 ps range. Results of measurements on films with different thicknesses demonstrate that it corresponds to a propagation of an acoustic strain pulse. On timescales longer than 100 ps, heat diffusion to the substrate takes place that can be modeled with a biexponential decay.