Ultrafast dynamical response of the lower exciton-polariton branch in CdZnTe

We investigate the transient optical response in high-quality ${\mathrm{Cd}}_{0.88}{\mathrm{Zn}}_{0.12}\mathrm{Te}$ crystals in the regime of slow light propagation on the lower exciton-polariton branch. Femtosecond photoexcitation leads to very substantial transmission changes in a $\ensuremath{\sim}10$-meV broad spectral range within the transparency window of the unexcited semiconductor. These nonlinear optical signatures decay on picosecond time scales governed by carrier thermalization and recombination. The temporal and spectral dependence indicate the dynamical optical response as arising from excitation-induced dephasing and perturbed free induction decay. Model simulations for the optical response taking into account the actual exciton-polariton dispersion and excitation-induced dephasing of a nonlinearly driven two-level system support this interpretation.