Coherent phonon-polaritons as a probe of anharmonic lattice vibrations

We investigate the dielectric properties of LiTaO₃ and LiNbO₃ in the THz regime by measuring the dynamics of coherently generated phonon-polaritons. The dielectric response of these ferroelectric crystals is dominated by the lowest-energy lattice vibration of A₁ symmetry. This so-called ferroelectric mode is associated with a strong absorption peak at 6 THz (200 cm^-1) in LiTaO3 and at 7.5 THz (250 cm^-1) in LiNbO ₃. The properties of these vibrations are strongly affected by the displacement of ions which occurs when the lattice temperature approaches the ferroelectric phase transition temperature. The dielectric properties of LiTaO₃ and LiNbO₃ can be described by modelling these modes with a microscopic vibrational potential within one unit cell. The structure of this potential determines critically the polariton response. Therefore, the experimental determination of the phonon-polariton dispersion and damping provides information about the microscopic properties of LiTaO ₃ and LiNbO₃