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 YBa₂Cu₃O_y and Nd₂CuO₄ 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 O_2p to Cu_3d 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.