Time-resolved study of phonon polaritons inLiTaO3

We study the temperature dependence of the low-frequency dielectric properties of the ferroelectric ${\mathrm{LiTaO}}_3$ by generating phonon polaritons with frequencies in the terahertz regime. The phonon polaritons are impulsively excited and phase-sensitively detected with 60-fs laser pulses. The propagation and damping of the polaritons are investigated as a function of frequency and temperature. The experimental results are compared with a quantum-mechanical model for the low-frequency dielectric response of ${\mathrm{LiTaO}}_3$. In this model the lowest-energy ${\mathit{A}}_1$ mode is described with a one-dimensional anharmonic potential in a single unit cell. We find that at 300 K the polariton dispersion and damping are determined by a strong resonance at 6 THz and a weak resonance at 1 THz. The latter resonance is due to a tunneling transition and leads to the observation of phonon-polariton beats. The model predicts that with increasing temperature the strength of the resonance at 6 THz decreases and that a new broad resonance at 3 THz and a central mode arise. The measured polariton dispersion and damping are in quantitative agreement with these theoretical predictions. Thereby we obtain evidence that the ferroelectric phase transition in ${\mathrm{LiTaO}}_3$ takes place without mode softening.