Femtosecond spectroscopic studies of photodoping effects in insulating YBa2Cu3O6+x and Nd2CuO4 thin films

We have investigated photodoping effects in insulating TdBa(2)Cu(3)O(6.2) and Nd2CuO4, which are parent materials of high Tc superconductors, by means of femtosecond pump-probe spectroscopy using white continuum light. The absorption changes are observed in the wide energy region after the irradiation of the femtosecond laser pulse. The spectral features of the absorption change are in good agreement with the change by the doping with changing the chemical composition, and this indicates that the electronic structures in the CuO2 planes are drastically changed by femtosecond laser pulses. The photo-induced change in the electronic structure suggests the formation of the optically induced in-gap stare with the decay time of similar to 1 ps. Furthermore, we have investigated the photodoping effect by changing the photon energies of the pumping pulse. (C) 1997 Elsevier Science B.V.