Ultrafast real space dynamics of photoexcited state in a layered perovskite-type spin crossover oxide La_1.5Sr_0.5CoO ₄

Ultrafast optical responses were investigated using a fs laser system on a layered perovskite-type cobalt oxide, La_1.5Sr_0.5CoO ₄ with a checkerboard-type charge ordering. To investigate the spatial variation of the photoexcited state, we measured the transient changes of three independent optical quantities, i.e., transmittance, reflectivity, and backside reflectivity, as well as their fluence dependence at 0.25 eV. Using the numerical calculations based on Maxwell's equations, we calculated not only the dielectric constant but also the decay length of the photoexcited region (L) and analyzed the real-space dynamics of the photoexcited area in terms of the fluence and time dependence of L. With the above information on the time dependence of L and Kramers-Kronig analysis, we numerically calculated the transient optical conductivity spectra, which showed instantaneous formation of a polaron-like absorption peak in the midinfrared region. A 10 fs pump-probe reflection measurement revealed that the photoexcited state occurs within the time resolution (≈10 fs). The time profile of the reflectivity is accompanied with an oscillation assigned as a breathing phonon mode in the CoO₂ layer observed in Raman spectroscopy. Analyses of the decay time and the frequency revealed that the photoirradiation melted the charge ordering in La_1.5Sr_0.5CoO₄.