Terahertz time domain spectroscopy of phonon-polaritons in ferroelectric lithium niobate crystals

We report the observation of far-infrared A(1) (z) and E(y) symmetry phonon-polaritons of ferroelectric lithium niobate crystals by THz time domain spectroscopy. The obtained lowest polariton limit of 3 cm(-1) is far below the limit of the Raman scattering technique. The intensity and phase of transmittance for propagation along the x-axis have been accurately measured using THz coherent radiation. With an incident polarization plane parallel to the z-axis (E//z), the polariton dispersion curve of A, (z) symmetry has been observed in detail. The dispersion shows no "avoided crossing" predicted by strong anharmonicity. With an incident polarization plane parallel to the y-axis (E//y), the lowest polariton branch of E(x, y) symmetry has been clearly observed for the first time. The complex dielectic constants of epsilon(z) (omega) and epsilon(y) (omega) from 0.1 THz to 2 THz have been also determined. [DOI: 10.1143/JJAP.41.7033].