Observation of THz phonon-polariton beats inLiTaO3

We report on the impulsive excitation and phase-sensitive detection of terahertz phonon polaritons in the ferroelectric ${\mathrm{LiTaO}}_3$ using femtosecond pulses. The coherent propagation of the polaritons and their dephasing is investigated in detail. Polaritons with frequencies below 1.5 THz clearly show that there is a resonance at 0.95 THz that has not been found in previous Raman and IR reflectivity studies. This resonance allows the coherent excitation of two polaritons of different frequencies. This leads to the first observation of phonon-polariton beats.