Polarization dependence of the nonlinear optical responses of ${\mathrm{KTa}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Nb}}_{\mathit{x}}$${\mathrm{O}}_3$ crystals after picosecond-pulse laser excitation

Fast nonlinear optical signals on picosecond and nanosecond time scales have been studied in the mixed crystal system ${\mathrm{KTa}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Nb}}_{\mathit{x}}$ ${\mathrm{O}}_3$ using transient degenerate four-wave mixing. The two distinct signal components, denoted as an instantaneous response and slow response in previous studies, show the dependence of the polarization configuration of the optical pulses. The results were consistent with a lattice-relaxation model based on the laser-induced dynamic displacement of the Nb ion. In this model the signal variations result from the competition between the dynamic displacement and the intrinsic distortion associated with the spontaneous polarization along the optic axis.