Photo-induced insulator-to-metal transition and coherent acoustic phonon propagation in LaCoO$_3$ thin films explored by femtosecond pump-probe ellipsometry

TL;DR

This study investigates ultrafast photo-induced insulator-to-metal transition and coherent acoustic phonon propagation in LaCoO$_3$ thin films using femtosecond pump-probe ellipsometry, revealing spectral weight transfer, acoustic strain pulse dynamics, and heat diffusion effects.

Contribution

It provides new insights into the ultrafast dynamics and phonon propagation in LaCoO$_3$ thin films following photo-excitation, combining spectral and temporal analysis.

Findings

Spectral weight transfer indicates insulator-to-metal transition.

Propagation of acoustic strain pulse observed in transient signals.

Heat diffusion modeled with bi-exponential decay over 100 ps.

Abstract

We have studied ultrafast dynamics of thin films of LaCoO$_3$ and La$_{0.5}$Sr$_{0.5}$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 LaCoO$_3$ that corresponds to an insulator-to-metal transition. The photo-induced metallic state initially relaxes via a fast process with a decay constant of about 200 fs. Both LaCoO$_3$ and La$_{0.5}$Sr$_{0.5}$CoO$_3$ exhibit a significant secondary transient structure 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 modelled with a bi-exponential decay.