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 1 ( 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 ε z (ω) and ε y (ω) from 0.1 THz to 2 THz have been also determined.