Femtosecond spectroscopic studies of the ultrafast relaxation process in the charge-transfer state of insulating cuprates

We have investigated the transient absorption spectra of insulating ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{\mathit{y}}$ and ${\mathrm{Nd}}_2$ ${\mathrm{CuO}}_4$ thin films by the pump and probe method from 1.3 to 2.8 eV with a time resolution of 100 fs. For both insulating samples, a bleaching signal with a 0.6–0.9 ps initial decay time was observed in the spectral range of the ${\mathrm{O}}_{2\mathit{p}}$ to ${\mathrm{Cu}}_{3\mathit{d}}$ charge-transfer transition at around 1.8 eV. This initial decay can be explained as a nonradiative transition through the successive emission of magnons having a comparably large energy quantum. In the photon-energy region lower than the bleaching signal, an induced absorption signal is observed. The origin of the induced absorption by pumping pulses has not necessarily been resolved, but the in-gap states induced by the photodoping effect is suggested as a reason.