EFFECT OF NIOBIUM DOPING ON THE PROPERTIES OF PICOSECOND LASER-INDUCED TRANSIENT GRATINGS IN KTA1-XNBXO3

A study of the properties of laser-induced transient gratings produced in KTa(1-x)Nb(x)O3 Crystals by picosecond-pulse, two-photon excitation was carried out with use of degenerate-four-wave-mixing techniques. The grating properties were found to depend on the Nb concentration. The diffraction efficiency of the gratings exhibited a maximum at an intermediate Nb concentration while the signal decay rate increased uniformly with x. A lattice-relaxation model was developed to explain the experimental results that includes the intrinsic lattice distortion due to the niobium ions in the normally cubic perovskite crystal and the strong electron-phonon interaction causing a lattice distortion around photoionized ions. The predictions of the theoretical model agree with the experimental data. The results of grating decay rate measurements show that the excited states produced by two-photon laser excitation act as localized quasiparticles (excitons or polarons) having a hopping rate that increases with Nb concentration x. Strong electron-phonon coupling produces phonon-assisted, incoherent hopping migration.