Soft-mode studies in ${\mathrm{KTa}}_{0.93}$${\mathrm{Nb}}_{0.07}$${\mathrm{O}}_3$ with use of the time-resolved third-order optical susceptibility ${\mathrm{\chi }}^3$

Determinations of the third-order optical susceptibility have been used to investigate the dynamical properties of the ${\mathit{A}}_1$(TO) soft-polariton mode in ${\mathrm{KTa}}_{0.93}$ ${\mathrm{Nb}}_{0.07}$ ${\mathrm{O}}_3$ as a function of temperature. Saturation of the soft-polariton frequency as T→${\mathit{T}}_{\mathit{c}}$ indicates an interaction with the relaxation mode and evidence is found for the presence of low-frequency excitations that significantly increase the observed damping rate. Also, with the use of the polariton dispersion and self-energy formulas, frequency and damping are compared to the TO Raman-active phonon parameters.