Femtosecond time-resolved study of the generation and propagation of phonon polaritons inLiNbO3

Using intense femtosecond pulses, we have generated phonon polaritons in the ferroelectric crystal ${\mathrm{LiNbO}}_3$. Phonon-polariton pulses consisting of ≊8 oscillations of the electric field were generated. They were detected in a time-resolved way by diffraction of a probe pulse from the standing wave formed by these phonon polaritons. We determined their dispersion for frequencies up to 130 ${\mathrm{cm}}^{\mathrm{-}1}$. The pulse width of the phonon polaritons was ≊3 ps. In addition, we have studied their propagation in the crystal, by diffracting a probe pulse from one of the traveling phonon polaritons. We demonstrate that in this case, the diffracted signal is sensitive to the phase of the phonon polariton. Analytical calculations show that this can be explained in terms of the interference between the electric fields of the nondiffracted probe beam and the first-order diffracted probe beam.