Coherent phonon polaritons as probes of anharmonic phonons in ferroelectrics

In the last decade it has been shown that the low-frequency dielectric response of ferroelectrics can be investigated with great sensitivity via the impulsive excitation and phase-sensitive detection of ultrashort THz phonon-polariton wave packets. In this paper the authors review this technique and the new information obtained with it on the dielectric response and phonon properties of ferroelectrics. The technique is demonstrated with measurements on the perovskite ferroelectrics ${\mathrm{LiTaO}}_3$ and ${\mathrm{LiNbO}}_3.$ The results are compared with those of other time-resolved studies on phonon polaritons and continuous-wave Raman and infrared-reflectivity studies on ${\mathrm{LiTaO}}_3,$ ${\mathrm{LiNbO}}_3$ and other ferroelectric crystals. For several ferroelectrics, the polariton dispersion and polariton damping are observed to be strongly influenced by the presence of resonances at frequencies below the fundamental ferroelectric phonon frequency. The occurrence of these resonances is explained from the intrinsic anharmonicity of the ferroelectric phonon or from cross-anharmonic couplings between different normal-mode lattice vibrations.