Femtosecond Reflection Spectroscopy in $La_{1.5}Sr_{0.5}CoO_4$

Ultrafast optical response has been investigated using fs laser system on a perovskite-type cobalt oxide, $La_{1.5}Sr_{0.5}CoO_4$. After the photoirradiation at room temperature, the time profile of relative change of reflectance (Δ R/R) shows a sudden change within the pulse duration ( ≈ 150 fs) and decays with a lifetime of ≈ 330 fs. The sign of Δ R/R after the photoexcitation is positive in the mid-infrared region (at 0.50 eV) while negative in the visible energy region (at 2.0 eV), implying photoinduced change of the electronic structure after the photoexcitation.