TIME-RESOLVED STUDY OF PHONON POLARITONS IN LITAO3

We study the temperature dependence of the low-frequency dielectric properties of the ferroelectric LiTaO3 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 LiTaO3. In this model the lowest-energy A1 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 LiTaO3 takes place without mode softening.