TIME-RESOLVED STUDY OF PHONON POLARITONS IN LITAO3

We study the temperature dependence of the low-frequency dielectric pr operties of the ferroelectric LiTaO3 by generating phonon polaritons w ith frequencies in the terahertz regime. The phonon polaritons are imp ulsively excited and phase-sensitively detected with 60-fs laser pulse s. The propagation and damping of the polaritons are investigated as a function of frequency and temperature. The experimental results are c ompared with a quantum-mechanical model for the low-frequency dielectr ic response of LiTaO3. In this model the lowest-energy A1 mode is desc ribed with a one-dimensional anharmonic potential in a single unit cel l. We find that at 300 K the polariton dispersion and damping are dete rmined by a strong resonance at 6 THz and a weak resonance at 1 THz. T he latter resonance is due to a tunneling transition and leads to the observation of phonon-polariton beats. The model predicts that with in creasing 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 agreemen t with these theoretical predictions. Thereby we obtain evidence that the ferroelectric phase transition in LiTaO3 takes place without mode softening.