Photoinduced insulator-to-metal transition and coherent acoustic phonon propagation in ${\mathrm{LaCoO}}_3$ 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.

Figure 1

(a) Penetration depth and (b) the index of refraction of ${\mathrm{La}}_{1-x}{\mathrm{Sr}}_x{\mathrm{CoO}}_3$ thin films with $x=0$ (black solid line), $x=0.5$ (red solid line), and LSAT substrate (blue solid line). The dashed and dash-dotted lines denote the pump energy and limits of the measurement range, respectively.

Figure 2

(a) Real part of the optical conductivity ${\sigma }_1$ and (b) the real part of the dielectric function ${\varepsilon }_1$ at room temperature of ${\mathrm{La}}_{1-x}{\mathrm{Sr}}_x{\mathrm{CoO}}_3$ films with $x=0$ and a thickness of 100 nm and $x=0.5$ and a thickness of 31 nm. Panels show the equilibrium data (black lines) and the pump-probe data before the pump, i.e., at 0 fs delay (green thick solid lines) and at delays corresponding to the largest pump-induced response of 250 fs ($x=0$) and 150 fs ($x=0.5$) (solid red lines). The red dotted lines represent Kramers-Kronig consistent extrapolations of the $x=0$ spectra at 250 fs to lower energies as detailed in the text. The arrow in (a) denotes the crossing point at $E=E_{\mathrm{CP}}$.

Figure 3

Transient signals relative to the values before the pump (a) and (c) of the real part of the conductivity $\mathrm{\Delta }{\sigma }_1$ and (b) and (d) of the real part of the dielectric function $\mathrm{\Delta }{\varepsilon }_1$ of ${\mathrm{La}}_{1-x}{\mathrm{Sr}}_x{\mathrm{CoO}}_3$ films with (a) and (b) $x=0$ and a thickness 100 nm and (c) and (d) $x=0.5$ and a thickness of 31 nm at selected probe photon energies as shown in the graphs. (e) and (f) $\mathrm{\Delta }{\varepsilon }_1$ transients at 3 eV normalized to the maximum values near 250 fs $\mathrm{\Delta }\varepsilon {}_{1\mathrm{N}}$(3 eV) for several $x=0$ films with thicknesses $d$ specified in the panels. Red circles in (e) represent fits by the biexponential decay model detailed in the text. (g) The delays of the maxima $t_{\max }$, denoted by arrows in (f), versus the film thickness $d$. (h) Transients from (f) as a function of the normalized delay $t_{\mathrm{N}}=t/t_{\max }$. All data were measured at room temperature.

Figure 4

(a) Normalized $\mathrm{\Delta }\varepsilon {}_{1\mathrm{N}}$ transients of a ${\mathrm{LaCoO}}_3$ film with a thickness of 188 nm at selected energies between 1.8 and 3.4 eV (with 0.2 eV steps). The dashed and dash-dotted lines are guides to the eye. (b) Period $\tau$ as a function of ${\lambda }^{\prime }=\lambda /(ncos\beta )$. The line corresponds to Eq. (1) with $v=5.6$ nm/ps. (c) and (d) An overview of the normalized $\mathrm{\Delta }\varepsilon {}_{1\mathrm{N}}$(3 eV) transients of all samples on semilogarithmic and linear scales, respectively. For clarity, the transients are shifted vertically.